Bioenergetics of the Archaea - Microbiology and Molecular Biology

51 Pages·1999·1.17 MB·English

MICROBIOLOGYANDMOLECULARBIOLOGYREVIEWS,Sept.1999,p.570–620 Vol.63,No.3
1092-2172/99/$04.0010
Copyright©1999,AmericanSocietyforMicrobiology.AllRightsReserved.
Bioenergetics of the Archaea
GU¨NTERSCHA¨FER,1*MARTINENGELHARD,2ANDVOLKERMU¨LLER3
Institutfu¨rBiochemie,MedizinischeUniversita¨tzuLu¨beck,Lu¨beck,1MaxPlanckInstitutfu¨rMolekularePhysiologie,
Dortmund,2andLehrstuhlfu¨rMikrobiologie,Ludwigs-MaximiliaUniversita¨t,Munich,3Germany
INTRODUCTION.......................................................................................................................................................571
CHEMIOSMOSISINARCHAEA.............................................................................................................................572
ENERGETICSOFMETHANOGENESIS...............................................................................................................573
ProtonMotiveElectronTransportChainsinMethanogens............................................................................574
ComponentsoftheElectronTransportChain...................................................................................................575
Heterodisulfidereductase..................................................................................................................................575 D
Hydrogenases.......................................................................................................................................................576 o
F dehydrogenase.............................................................................................................................................576 w
420 n
MPoesmsibbrleanMe-eIcnhtaegnrisamlEsloefctDromnHC1aFrorrimerast.i..o..n...C...o..u..p..l..e.d....t.o....E..l..e.c..t.r..o..n....T..r..a..n..s..p..o..r..t...R..e..a..c..t.i..o..n..s..................................................................557777 loa
SodiumBioenergeticsofMethanogenesis...........................................................................................................578 d
e
ATPSynthesisinMethanogens............................................................................................................................579 d
BioenergeticsoftheAcetyl-CoAPathwayinArchaeaandBacteria:DifferencesandSimilarities...............579 f
r
ENERGETICSOFRESPIRATION.........................................................................................................................579 o
m
AerobiosisandOtherRespirationFormsinArchaea........................................................................................579

ComponentsofAerobicElectronTransfer..........................................................................................................580 h
t
Membrane-residingquinonereductases..........................................................................................................581 tp
(i)NADHdehydrogenases.............................................................................................................................581 :/
/
(ii)SDHsandanovelcomplexII................................................................................................................581 m
Membrane-integralquinol-oxidizingcomplexes.............................................................................................582 m
(i)TheSoxABCDcomplex.............................................................................................................................582 b
r
(ii)TheSoxMcomplex...................................................................................................................................583 .
a
(iii)Otherarchaealterminaloxidases........................................................................................................583 s
m
Mobileelectroncarriers,hemes,andsmallmetalproteins.........................................................................585
.
(i)Quinones.....................................................................................................................................................585 o
r
(ii)Hemes........................................................................................................................................................585 g
/
(iii)Ferredoxins..............................................................................................................................................586
o
(iv)Rieskeiron-sulfurproteins....................................................................................................................587 n

(v)Smallcopperproteins..............................................................................................................................587 A
OrganizationofArchaealRespiratoryChains...................................................................................................588 p
r
Oxygenrespiration..............................................................................................................................................588 il
Alternatetypesofrespiration...........................................................................................................................589 1
3
ProtonPathwaysinTerminalOxidasesofArchaea...........................................................................................590 ,

LIGHT-DRIVENENERGETICS..............................................................................................................................592 2
0
GeneralOverview....................................................................................................................................................592 1
BR.............................................................................................................................................................................592 9

StructureofBR...................................................................................................................................................592 b
y
Mechanismofprotonpumping.........................................................................................................................594
g
HR.............................................................................................................................................................................595 u
e
StructureofHR..................................................................................................................................................595
s
Mechanismofchloridepumping......................................................................................................................596 t
SRs............................................................................................................................................................................597
SRI........................................................................................................................................................................597
SRII.......................................................................................................................................................................597
ProtonTransportinArchaealRhodopsins:aCommonPropertyofIonPumpsandPhotoreceptors.......598
PhototaxisandChemotaxis...................................................................................................................................598
Signaltransductionchain..................................................................................................................................599
Flagellaandtheflagellarmotor.......................................................................................................................600
SECONDARYENERGYCONVERTERS................................................................................................................600
TheFamilyofATPases..........................................................................................................................................600
TheATPasesofSulfolobus.....................................................................................................................................601
*Corresponding author. Mailing address: Institut fu¨r Biochemie,
MedizinischeUniversit¨atzuLu¨beck,23538Lu¨beck,Germany.Phone:
49 451 500-4060. Fax: [phone]. E-mail: schaefer@biochem
.mu-luebeck.de.
570
VOL.63,1999 BIOENERGETICS OF THE ARCHAEA 571
TheHalobacterialATPases...................................................................................................................................601
TheATPasesofMethanogens...............................................................................................................................602
CellularfunctionoftheA A ATPasefrommethanogens............................................................................602
1 o
FeaturesofATPasesfrommethanogens.........................................................................................................603
GeneticOrganizationofKnownA A ATPases..................................................................................................603
1 o
PropertiesandFunctionsofthePolypeptidesInvolvedinATPaseFunctionandAssembly......................604
AStructuralModel.................................................................................................................................................605
ThenovelfeaturesofarchaealATPsynthases...............................................................................................606
AuxiliaryEnergyTransducers...............................................................................................................................606
Theuniquestructureofarchaealadenylatekinases.....................................................................................607
ArchaealPPases..................................................................................................................................................607
CONCLUSIONSANDPERSPECTIVES.................................................................................................................608
ACKNOWLEDGMENTS...........................................................................................................................................608
ADDENDUMINPROOF..........................................................................................................................................608
REFERENCES............................................................................................................................................................608
D
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INTRODUCTION well as organotrophic species. In addition to obligate anaer- n
obes such as the methanogens, a second group that performs lo
Eversincearchaeahavebeenstudied,theirabilitytothrive various types of aerobic or anaerobic respiration can be dis- ad
inunusualhabitatsunderextremelyharshconditionshasstim- tinguished.Further,forsomehalobacteriawehavetoconsider e
ulated interest in the molecular mechanisms that confer heat archaeal phototrophic energy transformation in addition to d
stabilityonproteinsattemperaturesabove100°C,toleranceof respiratorymechanisms. fr
o
extremepHvaluesandsaltconcentrations,anduniquemeta- In contrast to this diversity, archaeal membrane structures m
bolicfunctionsnotfoundinbacteria,suchasmethanogenesis reveal a comparatively homogeneous phenotype, significantly
h
or rhodopsin-linked energy and signal transduction. Actually, differentfromthatofotherprokaryotes.Dietherandtetraether t
t
eversinceArchaeawasidentifiedasathirdevolutionaryking- lipidsareuniformlyusedasbuildingblocksforarchaealplasma p
:
dom (606–608) presumably located relatively near the hypo- membranes(266).Obviously,thelowionpermeabilityofmem- //
m
thetical root of the evolutionary tree, it has been speculated branes formed from these bipolar monolayer-forming lipids
m
that the structural organization and metabolic pathways of (136,571)contributessignificantlytothestabilityofchemios-
b
archaea might reflect more ancestral organisms whose essen- motic charge separation in archaea, particularly at high tem- r
.
tialpropertiesdifferfromthoseofbacteriaandeucarya.Inthis peraturesand/oratextremelylowpHvalues. a
s
regard, one has to realize with respect to biological energy Interestingly,neitheroxygenicnoranoxygenic“green”pho- m
conservationthatallexistingformsofliferelyontheuniversal tosynthesishasbeenfoundinthearchaealkingdom.Thislatter .
o
principle of chemiosmotic energy transduction (366, 367), observationservedasanargumentinfavoroftherespiration- r
g
whichinphylogenetictermsshouldhaveevolvedveryearly.In first hypothesis, suggesting that the formation of the basic /

fact,theoriginofcellularlifemusthavebeenconnectedwith structure of terminal oxidase complexes preceded the occur- o
n
the permanent manifestation of mechanisms allowing the renceofchlorophyll-basedwater-splittingandcharge-separat-
A
transductionofenergybetweenexergonicandendergonicpro- ingsystems(93,94). p
ceensesregsyasntodrewsi.th the development of transitory and long-term chaIteaislbthioeenaeimrgeotfictshbisyraecvrieitwicatlostianttero-odfu-tchee-tahretrreepadoertratnodatro- ril 1
ThedefinitionofArchaeaasaseparatedomainoforganisms demonstrate similarities and distinguishing features by con- 3
,
wasbasedonthecomparativeanalysisof16SrRNAsequences trasttobacterialandeucaryalsystems.Itwillnotbepossiblein 2
(405), which led to a result different from classical taxonomy. all cases to give an unambiguous answer to the question of 0
1
ThetermArchaeareflectsanearlierideathattheseorganisms what is typical or genuine for the archaeal domain, because, 9
descendedfromlifeformsthatexistedpriortothedivisioninto especially within respiratory electron transport, we will find a b
the bacterial and eukaryal domains. However, based on the number of chimeric functional complexes. In fact, one has to y

sequences of universally present proteins (176), Archaea has assume that during early evolution, i.e., prior to the division g
u
beenplacedonthebranchalsoleadingtoEucarya.Afeature intothreeurkingdoms,thebarriersagainstlateralgenetrans- e
that distinguishes Archaea from Bacteria is the structure of ferweremuchlowerthantheyarenow(605). s
t
archaeal ribosomes (435), which in halophiles were first rec- Of the various bioenergetic mechanisms in archaeal organ-
ognized to contain acidic rather than basic proteins (42). In isms, the present review focuses specifically on the primary
addition, the transcriptional machinery is unique as to the energy conservation that involves membrane-residing chemi-
structure of DNA-dependent RNA polymerases (133, 626). osmotic processes. Therefore, purely fermentative energy
With respect to subunit structure, a closer relationship to eu- transductionbysubstrate-levelphosphorylationaswellassec-
karyotes than to bacteria was found (300). Another feature ondary active-transport systems for solutes will not be dis-
distinguishing Archaea from Bacteria is the specific composi- cussed.Asanexceptionamongsecondaryenergytransducers,
tion of archaeal surface layers (266), which do not contain the ATP synthase complexes will be dealt with because they
peptidoglycans. Their glycoprotein surface layers can form apparentlypossessauniqueandubiquitouslyconservedmech-
quasicrystallinestructures(97,360)thatarefirmlyattachedto anism,irrespectiveoftheprimaryenergyconverterwhichpro-
theplasmamembrane,thusleavingpracticallynoperiplasmic videstheelectrochemicaliongradienttobeusedasthedriving
space(39). force for high-energy bond formation during ADP phosphor-
Nevertheless, although archaea are located on a distinct ylation.
evolutionarybranchasdepictedinFig.1,theyrepresent,with Anotherlimitationisthegreatdiversityofthearchaealdo-
regardtotheirprimaryenergy-transducingmechanisms,avery main, much greater than that suggested by Fig. 1. What we
heterogeneous domain comprising chemolithoautotrophic as presently know about diversity within the archaeal domain is
572 SCHA¨FER ET AL. MICROBIOL.MOL.BIOL.REV.
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FIG. 1. Phylogenetictree.TheschemedemonstratesthedivisionintoCrenarchaeotaandEuryarchaeotaandshowsthepositionofthemajorarchaealgenera.The o
treewasredrawnaccordingtoreferences88,175,and608.Starsdenotearchaealspeciesforwhichspecificbioenergeticinformationhasbeenfound. n

A
p
probably only the tip of the iceberg. By means of molecular osmotic theory, at the expense of such ion gradients. These r
studiesbasedonrRNA-directedprobes,newarchaeaarecon- experimentswereofsignificancebecauseminimalsystemssuch il
1
stantlybeingdiscoverednotonlyindeep-seaventsorsolfataric as inverted plasma membrane vesicles or spheroplasts which 3
fields (37, 89, 379) but also in mesophilic and even low-tem- areeasilypreparedfromseveralbacterialorganismsareessen- ,
2
perature environments. Unfortunately, only a few of these tially inaccessible in the case of Archaea. The rigid structure 0
isolates or new species identified by DNA hybridization will and extremely tight adhesion or interdigitation of the glyco- 1
9
prove amenable to cultivation. Thus, the diversity of bioener- proteincellwallscoveringarchaealplasmamembranes(39,98,
b
geticsystemsmaywellexceedthenumberofclassesreviewed 181, 270, 514) represent an invincible obstacle. For the same y

inthiscomprehensivestudy. reason, the preparation of intact complexes of energy-trans- g
u
ducingmembraneproteinsisquitedifficult.Inaddition,other e
CHEMIOSMOSISINARCHAEA factorsarefrequentlyresponsibleforthefailuretopurifycat- st
alytically active complexes, such as ATP synthase or terminal
Primaryenergyconservationbymembrane-residingsystems oxidases. Such factors can be the absence of the high pH
ischaracterizedbytheformationofanelectrochemicalpoten- gradient to which membrane proteins are exposed in vivo,
tialofhydrogenionsorsodiumions.Accordingtotheworkof extreme salt concentrations, hypersensitivity toward oxygen,
Mitchell (367), the free energy stored in this gradient is de- and cold dissociation even at room temperature. Also, the
scribedbyequations1and2fortheprotonmotiveforce: determinationofenergeticparameterssuchasDpHorDCby
direct monitoring or by distribution of diffusible molecular
DmH15RTzln([H1i]/[H1o])1FzDC (1) probesisverylimitedatambientpHvaluesbelow3,thephys-
Dp 5 Dm 1/F5DC2ZDpH (2) iologicalenvironmentalpHformanyextremeacidophiles.
H Extremely acidophilic organisms, including the archaeon
Theprimarypumpsmaybedrivenbyredoxsystems,bymethyl Thermoplasma, were shown to create an inverted membrane
transferreactionsasinmethanogenesis,orbylightasinpho- potential (30, 364) in order to prevent acidification of the
tophosphorylatinghalobacteria. cytosol by influx of H1 at the prevailing DpH. This does not
As illustrated by some representative examples below, generally apply to all acidophiles, however. The membrane
whole-cell experiments with various genera of Archaea have potentialofthermoacidophilicarchaeasuchasSulfolobusmay
proven that ATP synthesis is driven, according to the chemi- beratherlow(approximately30mV),andmostoftheproton
VOL.63,1999 BIOENERGETICS OF THE ARCHAEA 573
motiveforceismaintainedbyalargepHgradientof.3(335,
370,467).ChemiosmoticH1cycling(370)withH1/Oratiosof
3 and a strict correlation of proton motive force (Dp) with
cellularATPlevelscouldbeestablishedforSulfolobus.Dissi-
pationofDpbyprotonophorescausedanimmediatecollapse
of ATP synthesis; at Dp > 0, a persisting residual DpH was
counterbalancedbyaninvertedmembranepotential,inwhich
theinsidewaspositive(335).Inthesameexperiments,external
protonpulsesthatloweredthepHfrom6.1to3.4producedan
increaseofDpfrom294to2170mVwithaconcomitantrise
of intracellular ATP. For experimental reasons, the reported
datawasdeterminedat45°CatanambientpHof3.5andthus
may assume slightly different values at the optimal growth
temperature of the cells. A review of strategies to cope with
extremelylowpHvaluesisgiveninreference465.
D
WithHalobacteriumhalobium,thecouplingofeitheralight-
o
orarespiration-inducedelectrochemicalprotongradientwith w
intracellular ATP has been established (361–363). Interest- n
ingly, by cation counter transport considerable energy can be lo
a
stored in the form of a potassium gradient also. Photophos- d
phorylation is a backup system under oxygen limitation in e
d
extremely halophilic archaea. This is corroborated by recent
f
studies of the haloalkaliphile Natronobacterium pharaonis r
o
(604)demonstratingfullrecoveryofDpunderoxygen-limiting m
conditions during illumination. Actually, in these latter ar-
h
chaea the main contribution to the proton motive force is t
t
madebythemembranepotentialofDC52225to2280mV, p
:
and DpH is influenced only marginally by oxygen limitation. //
m
Under these conditions, the high membrane potential is gen-
m
eratedbyanoutwardlydirectedchloridegradientproducedby
b
the light-activated chloride pump halorhodopsin (HR); it is r
.
insensitivetoprotonophoresanduncouplersandcanevenbe a
s
increasedbytheCl2/OH2exchangertriphenyltin(604). m
Themembranepotentialcancontributeapproximately90% .
o
totheprotonmotiveforce(56)inmethanogenicarchaeaalso, r
g
as shown with Methanosarcina barkeri. Evidence for H1- and /

Na1-mediatedchemiosmoticenergytransductioninmethano- o
n
genshasbeencompiledpreviously(124);thereby,thesodium
A
and proton gradients may be linked by Na1-H1 antiporters p
(sa3r8c2i)n.aAmmaezethi)a,nisogtheneiconstlyrakinn,oGwo¨n1c(ansoewinclwahssiicfihedthaessMucecthesasnfou-l meFmIbGr.an2e. Pbrioimenaeryrgeetnicesr.gyT-threantsodpucsicnhgempreociellsussetsrataensdthceoupprloincgesspersinfcoipulneds iinn ril 1
preparation of archaeal vesicular membrane systems has pro- archaeathatcontributetotheformationofeitherprotonorsodiumionpoten- 3
,
vided a useful experimental model for the study of energy tialsacrosstheplasmamembrane.Detailsarediscussedthroughoutthisreview. 2
transduction(58,59). Thebottomschemesillustratethreemechanismsbywhichaniongradientcanbe 0
produced:(a)chemicalchargeseparation(onlyelectronsaretransferredthrough 1
The coexistence of proton- and sodium ion-coupled energy themembrane);(b)amobilemembrane-integralcofactorlikethequinonesor 9
converters in anaerobes as well as the branching of electron methanophenazine functioning as proton transporter (examples are bc1 com- b
transport pathways in aerobic archaea is difficult to resolve plexes);and(c)redox-drivenpumpslikecytcoxidase.Allschemesaredrawnfor y
because these organisms either lack or are insensitive to the anH1/e2ratioof1.Schemedillustratestheproton-drivenATPsynthaseofthe g
FF orAA typeasanexampleforasecondaryenergytransducer.D,electron u
site-specific inhibitors known to function in bacteria or euca- doono1r;Ac1,eolectronacceptor. e
rya. In addition, genetic systems for directed mutagenesis or s
t
genedisruptioninarchaeahavescarcelybeendevelopedorare
unavailable.
TheschemeofFig.2illustratesthegenerationofiongradi- able to grow by the conversion of a small number of com-
entsbyprimarypumpsandtheirutilizationbysecondarypro- poundstomethane.Thisrathersimplepathwayisnotcoupled
cesses. In the following sections, molecular properties of the tosubstrate-levelphosphorylationbut,instead,tothegenera-
known functional complexes are discussed separately for tion of ion gradients across the membrane that are used to
methanogenic, respiring, or photophosphorylating archaea; it drivethesynthesisofATP.Interestingly,thepathwayofmeth-
should be noted, however, that current complete genome ane formation is coupled to the simultaneous generation of
projects have predicted the existence of additional functional primarygradientsofbothprotonsandsodiumions.Although
complexes which have not yet been verified at the protein or methanogensarenutritionallyrathersimilarandemployiden-
mRNAlevel. ticalpathways,theydiffersignificantlywithrespecttothecom-
ponentsinvolvedintheprotonmotiveelectrontransportchain
and, therefore, most likely employ different mechanisms to
ENERGETICSOFMETHANOGENESIS
generate the proton gradient. For example, methylotrophic
Methanogensareaphylogeneticallydiversebutnutritionally methanogens, such as M. mazei Go¨1, contain cytochromes,
rather uniform group of strictly anaerobic archaea. They are whereashydrogenotrophicmethanogens,suchasMethanobac-
574 SCHA¨FER ET AL. MICROBIOL.MOL.BIOL.REV.
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FIG. 3. Pathwaysofmethanogenesis.Reactionsinvolvedinenergyconservationareboxed.Thereductionofmethyl-CoM(reactions6and7)iscommontoall
methanogenicsubstrates.DuringmethaneformationfromH2plusCO2,reactions1to5proceedinthedirectionofCO2reduction.Themethylgroupsofmethanol .o
andacetateenterthecentralpathwayatthelevelofH4MPT.Duringmethanogenesisfrommethanol,one-fourthofthemethanolisoxidizedtoCO2bythereversal rg
ofreactions1to5;thesixreducingequivalentsgainedareusedtoreduce3molofmethanoltomethane.Duringmethanogenesisfromacetate,thecarboxylgroupis
/
oxidizedtoCO2andtheelectronsgainedareusedtoreducethemethylgrouptoacetate.F420,oxidizedformofcoenzymeF420;F420H2,reducedformofF420;HS-CoM, o
CoM(2-mercaptoethanesulfonate);HS-CoB,CoB(7-mercaptoheptanoylthreoninephosphate);CoM-S-S-CoB,heterodisulfideofHS-CoMandHS-CoB.Enzymes:1, n
formyl-MFdehydrogenase;2,formyl-MF:H4MPTformyltransferaseandmethenyl-H4MPTcyclohydrolase;3,F420-dependentmethylene-H4MPTdehydrogenase;4, A
F420-dependentmethylene-H4MPTreductase;5,methyl-H4MPT:CoM-methyltransferase;6,methyl-CoMreductase;7,heterodisulfidereductasesystem(different p
electrondonorsystemsareindicated). r
il
1
3
,
teriumthermoautotrophicum,donot.Sincemostofourcurrent genaseisemployedduringgrowthonH2plusCO2,COdehy- 2
knowledge derives from studies using methylotrophic meth- drogenase(orreducedferredoxin:heterodisulfideoxidoreduc- 0
1
afoncougseensso,ninthpeasertoicruglaanrisMm.s.bFaorkrearimaonrdeMth.ormouazgehi,dtihscisussseioctnioonf t(aFs4e2)0,isau5s9e-ddedauzraiflnagvginro,iwstthheonunaicveetrastael,ealnedctFro42n0cdaerhriyedrriongmeneatshe- 9 b
the pathways and the biochemistry of methanogenesis, the anogens) and formylmethanofuran (formyl-MF) dehydroge- y

readerisreferredtorecentreviews(58,124,381,561). nase are used during growth on methyl group-containing C g
1 u
substrates. e
ProtonMotiveElectronTransportChainsinMethanogens byHin2v-edretpeednvdeesnictlreesdoufcMtio.nmoafzetihGeho¨e1t,ewroasdiascuclofimdep,aansiecdatbaylyzHe1d st
Central to all pathways of methane formation is the inter- translocationintothelumenofthevesicles(Fig.4).Protono-
mediate methyl coenzyme M (2-methylthioethanesulfonate; phoresinhibitedATPformationbutstimulatedelectrontrans-
CoM),theultimateprecursorofmethane(Fig.3).Itisreduc- port, i.e., heterodisulfide reduction. Electron transport and
tively demethylated by the methyl-CoM reductase with elec- ATP synthesis were inhibited by the ATPase inhibitor N,N9-
trons derived from reduced CoB (7-mercaptoheptanoylthreo- dicyclohexylcarbodiimide(DCCD),butinhibitionwasrelieved
ninephosphate),togiverisetomethaneandaheterodisulfide by the addition of protonophores. These effects are clearly
of CoM and CoB (CoM-S-S-CoB; henceforth referred to as reminiscentofrespiratorycontrolasobservedinmitochondria
the heterodisulfide), in a reaction involving the cofactor F andcanbetakenasevidencethattheDm 1 generateddrives
430 H
(reaction 6 in Fig. 3). To complete the cycle, the heterodisul- the synthesis of ATP from ADP and P. Washed everted ves-
i
fide is reduced by the heterodisulfide reductase complex (re- icles exhibited stringent coupling between heterodisulfide re-
action7inFig.3);thisreactionismostimportantintermsof duction and ATP synthesis, with maximal stoichiometries of
energy conservation (561). The heterodisulfide reductase is 1H1translocated/e2and1ATPsynthesized/4e2(120).
membrane bound and operates as the final limb of several The F H -dependent heterodisulfide reduction was also
420 2
membrane-boundelectrontransportchains(124).Depending showntodriveprotontranslocationintothelumenofeverted
on the substrate, the electron donor used is different. Hydro- vesiclesofM.mazeiGo¨1,resultinginthegenerationofaDm 1
H
VOL.63,1999 BIOENERGETICS OF THE ARCHAEA 575
lation of the F H -dependent heterodisulfide reduction by
420 2
ADP indicate stringent coupling between electron transport
andATPsynthesis.TheF H -dependentheterodisulfidere-
ductase system displayed42s0toi2chiometries of 1 H1 translo-
cated/e2and0.8ATPsynthesized/4e2(122).
EvidencethattheconversionofCOtoCO andH (DG895
220 kJ/mol) by resting cells of M. barkeri i2s coupl2ed to the
synthesisofATPhasbeenpresented(70,71).Thecleavageof
acetyl-CoA as catalyzed by carbon monoxide dehydrogenase
yieldsenzyme-boundCOandanenzyme-boundmethylgroup
(150, 308). The latter is transferred via a corrinoid protein to
tetrahydromethanopterin (H MPT). Enzyme-bound CO un-
4
dergoes ferredoxin-dependent oxidation to carbon dioxide,
catalyzedbycarbonmonoxidedehydrogenase(151,560).Ina
reconstitutedsystemconsistingofpurifiedCOdehydrogenase,
D
heterodisulfide reductase, and ferredoxin, CO oxidation was
o
coupled to heterodisulfide reduction. However, the rate of w
heterodisulfidereductionwasincreased10-foldbyadditionof n
membranes, indicating a membrane-bound electron transport lo
a
chainfromferredoxintotheheterodisulfide(Fig.6)(420,512). d
Methyl group oxidation proceeds via the reversal of CO e
2 d
reduction(reactions1to5ofFig.3intheoxidativedirection).
f
FIG. 4. Tentativeschemeofelectronflowandprotontranslocationduring There are indications that formyl-MF oxidation is accompa- ro
heterodisulfidereductionwithH aselectrondonor.Thisreactionsequenceis nied by the generation of an electrochemical ion potential m
2
partofmethanogenesisfromH2-CO2.Thisschemeisvalidformethylotrophic acrossthemembrane,eitherprotonsorsodiumions(262,603). h
methanogens only, for hydrogenotrophic methanogens do not contain cyto-
Theformyl-MF-dependentheterodisulfidereductionisassoci- t
chherteormoedsisaunlfiddteherepdruescetansceeiosfnmoettihnadnicoaptheedntaozibneea(MprPo)thoanspnuomtbpe,ebnutvetrhiifisecda.nTnhoet atedwithalargeDG89of258kJ/mol.Incontrast,DG89ofthe tp:
/
bsteoicruhlieodmeoturyt oafp3ritoori.4THh1istsrcahneslmoceatiesdb/maseetdhyolngrothueperxepdeurciemde.nPta,lplyerdipelraivsmed; eerleacbtlryonsmtraallnesrfe(r2f2ro9mkJF/m42o0Hl).2Stoinctheethheetperhoydsiiosulolfigdicealisecleocntsriodn- /m
m
CM,cytoplasmicmembrane;C,cytoplasm.Forexplanations,seethetext.
acceptor employed in the oxidation of formyl-MF to CO is
unknown, the DG89 of the formyl-MF oxidation cannot2be br
.
calculated.However,themidpointpotentialatpH7(E )of a
(Fig. 5) Protonophores stimulated the heterodisulfide reduc- theCO –formyl-MFcoupleof2500mVindicatesthatma,7low- sm
AtioTnPbsuytntphraesveeninthedibiDtomrHD1CfoCrDmadteiocnreaanseddAthTePrasytentohfesFis. THhe- potentia2lelectroncarriercanbereduced(47). .o
420 2 r
dependent heterodisulfide reduction. The reversal of this g
DCCD-mediated inhibition by protonophores and the stimu- ComponentsoftheElectronTransportChain o/
n
Since little is known about the formyl-MF-dependent het-
A
erodisulfide reduction, only the F420, the H2-, and the CO- p
deHpeentedreondtissuyslfitedmesrwedilulcbteasceo.nTsihdeereredahcteiroen. catalyzed by the ril 1
heterodisulfide reductase resembles a polysulfide reduction 3
,
catalyzed by some bacteria and archaea. The S-S bonds of 2
polysulfide can be reduced by H as external electron donor, 0
2 1
andthisreactioniscoupledwithenergyconservation(480). 9
Heterodisulfide reductase was first purified from H -CO - b
2 2
grown M. thermoautotrophicum. It contained three subunits y

withapparentmolecularmassesof80(HdrA),36(HdrB),and g
u
21(HdrC)kDaand(permolofheterotrimer)approximately1 e
molofflavinadeninedinucleotide(FAD),20molofnonheme s
t
iron,and20molofacid-labilesulfur(205,508).Theencoding
geneshavebeenclonedandsequenced(206).Sequencecom-
parisons indicated that HdrA harbors four [4Fe-4S] clusters
andbindsFAD.HdrCisconsideredtobeanelectroncarrier
protein with two [4Fe-4S] clusters and a short stretch of hy-
drophobic amino acids that could anchor the complex to the
membrane. Interestingly, HdrB is similar to subunit C of the
succinate dehydrogenase (SDH) of Acidianus ambivalens and
Sulfolobusacidocaldarius.
From membranes of acetate-grown M. barkeri, a heterodi-
sulfide reductase complex which also contained the electron
FIG. 5. Tentativeschemeofelectronflowandprotontranslocationduring donor, the F -nonreactive hydrogenase, was purified. This
420
heterodisulfidereductionwithF420H2aselectrondonor.Thisreactionsequence complex contained nine subunits of 46, 39, 28, 25, 23, 21, 20,
is part of methanogenesis from methanol, methylamines, and formate. This 16, and 15 kDa, three of which are subunits of the F -non-
schemeisvalidformethylotrophicmethanogensonly(seethelegendtoFig.4). 420
reactive hydrogenase. The monomeric heterodisulfide reduc-
F , coenzyme F ; MP, methanophenazine; P, periplasm; CM, cytoplasmic
420 420
membrane;C,cytoplasm.Forexplanations,seethetext. tasecontained0.7molofcytochromeb(cytb)and18molof
576 SCHA¨FER ET AL. MICROBIOL.MOL.BIOL.REV.
D
o
w
n
lo
a
d
e
d

f
r
o
m

h
t
t
p
FIG. 6. TentativeschemeofelectronflowandprotontranslocationcoupledtoheterodisulfidereductionwithCOaselectrondonor.Thisreactionsequenceispart :
/
ofmethanogenesisfromacetate.Thepresenceofmethanophenazine(MP)inacetate-growncellshasnotbeenverified.Fd,ferredoxin;P,periplasm;CM,cytoplasmic /m
membrane;C,cytoplasm.Forexplanations,seethetext.
m
b
r
.
nonhemeironandacid-labilesulfur.The23-kDasubunitcar- dyes only. The latter enzyme is therefore often referred to as a
s
riedcytb(211). methyl viologen-reactive hydrogenase. The function of the m
Heterodisulfide reductase itself was purified from mem- F -reactivehydrogenaseinenergymetabolismisstillamat- .
420 o
branesofmethanol-grownM.barkeri,andtheencodinggenes ter for debate (14, 78, 338, 385). On the other hand, there is r
g
were cloned and sequenced (212, 296). The reductase was clear evidence that the F -nonreactive hydrogenase is the /
composed of only two subunits with apparent molecular electron donor for a me4m20brane-bound electron transport o
n
masses of 46 (HdrD) and 23 (HdrE) kDa. The enzyme con- chain. In methylotrophic methanogens, the F -nonreactive
420 A
tained 0.6 mol of cyt b and 20 mol of nonheme iron and hydrogenase is found in the particulate fraction. The enzyme p
amcoidle-lcaublialredsautlafurrevpeearledmtohlatoHf dhreEteirsoadibm-teyrp.eBciyotcohcehmroimcaelwainthd sausbpuunritifisecdonfrtaoimninMg.rmedaozxe-iacGtio¨v1e wNaisacnodmiproosne-dsulofufronclluysttewros ril 1
five potentially membrane-spanning helices. HdrD contains (123). A molecular analysis revealed that M. mazei Go¨1 con- 3
,
two [4Fe-4S] clusters, and its N and C termini are similar to tains two operons encoding isoenzymes designated vho for 2
HdrC and HdrB from M. thermoautotrophicum, respectively, viologen-reactive hydrogenase 1 and vht for viologen-reactive 0
1
indicating that HdrD of M. barkeri and HdrC and HdrB of hydrogenase2.Bothoperonsencodethestructuralsubunitsof 9
M.thermoautotrophicumarefunctionallyequivalent.Although thehydrogenase(VhoG,VhtG,VhoA,andVhtA)andagene b
small amounts of FAD were found in the heterodisulfide re- codingforcytb(VhoCandVhtC);thisindicatesthattheseb y

ductasefromM.barkeri,itwasshownlaterthatheterodisulfide cytochromes are the natural electron acceptors of the two g
u
reductiondidnotdependonFAD.Moreover,noFADbinding F -nonreactive isoenzymes. The small subunit contains a e
site was found in the deduced amino acid sequence of the le4a2d0er peptide, which suggests that the catalytic part of the s
t
enzyme(296),whichisincontrasttotheenzymefromhydrog- enzymefacestheperiplasm(121).Interestingly,theCtermini
enotrophicmethanogens. ofthetwobcytochromesarenothomologous,indicatingthat
From membranes of acetate-grown Methanosarcina ther- they interact with different proteins. Indeed, Northern blot
mophila, a two-subunit heterodisulfide reductase (53 and 27 analysisrevealedthattheexpressionoftheisoenzymesissub-
kDa)wasisolated.Thesmallsubunitcontained2molofcyto- stratedependent.vhowasapparentlyconstitutivelyexpressed,
chrome;thelargesubunitcontainedtwodistinct[Fe S ]21/11 whereas vht was expressed only during growth on H -CO or
4 4 2 2
clusters. One heme is a high-spin heme with a midpoint po- methanol(119).Therefore,itwasspeculatedthatthevhogene
tentialof223mV,whereasthelow-spinhemehasamidpoint products are part of the heterodisulfide reductase system
potential of 2180 mV. The midpoint potentials for the two whereasthevhtgeneproductsareinvolvedinelectronflowto
clustersare2100and2400mV(512). andfromCO inthecourseoftheformyl-MFdehydrogenase
2
Hydrogenases. The hydrogenases are the entry point for reaction(124).
electronsderivedfrommolecularhydrogen.Ofthefourtypes F dehydrogenase.TheF H dehydrogenaseistheentry
420 420 2
of hydrogenases isolated from methanogens to date, one was point for the electrons derived from F H oxidation. The
420 2
clearlyshowntobeinvolvedinenergyconservation.TheF - enzyme was first isolated from Methanolobus tindarius after
420
reactive hydrogenase reacts with F and viologen dyes, solubilizationfrommembraneswithdetergents(182).Theap-
420
whereastheF -nonreactivehydrogenasereactswithviologen parent molecular mass of the native enzyme was 120 kDa; it
420
VOL.63,1999 BIOENERGETICS OF THE ARCHAEA 577
consistedoffivedifferentsubunitsof45,41,22,18,and17kDa.
Theenzymecontained16molofnonhemeironand16molof
acid-labile sulfur per mol, but flavin was not detected. The
gene encoding the 40-kDa subunit (ffdB) was cloned (600).
Sequenceanalysis,primerextension,andreversetranscription-
PCR indicated that ffdB is part of an operon harboring three
additional genes (ffdA, ffdC, and ffdD). FfdA is similar to the
F -dependent methylene-H MPT reductase. The first 90
420 4
aminoacidsofFfdBaresimilartonumerousferredoxins,sug-
gesting the likely presence of at least two iron-sulfur centers.
FfdCandFfdDaresimilartoproteinsofunknownfunctionof
Methanococcus jannaschii and Archaeoglobus fulgidus. FfdD
appears to be very hydrophobic and is likely to be the mem-
brane anchor. Recently, F dehydrogenases were purified
420
fromM.mazeiGo¨1andthesulfate-reducingarchaeonA.fulgi-
D
dus(3,297).Flavinwasdetectedinbothenzymes.Therefore,
o
it is likely that flavin is also present in the enzyme from M. w
tindariusbutlostduringpurification. n
lo
a
Membrane-IntegralElectronCarriers d
e
FIG. 7. Structureandreactivityofmethanophenazine,amembrane-integral d
With respect to their membrane-integral electron carriers, electronandhydrogencarrierofmethanogens.
f
and thus probably with respect to the mechanism of proton r
o
translocation, methanogens can be divided into two groups: m
the methylotrophic organisms, in which a variety of b- and fide (2, 40). In a reconstituted system consisting of purified h
c-type cytochromes were found, and the hydrogenotrophic F dehydrogenase and heterodisulfide reductase, methano- tt
420 p
methanogens,whicharedevoidofcytochromes(260,295).In phenazine mediated the electron transfer from F to the :
420 /
hydrogenotrophic methanogens, the situation is far from set- heterodisulfide (40). Methanophenazine was isolated from /m
tled;polyferredoxinsdescribedaboveandarecentlydescribed methanol-grownM.mazeiGo¨1,butitisprobablyalsoinvolved m
flavoprotein encoded by the gene fpaA (394) are the only inelectrontransporttotheheterodisulfidefromotherdonors, b
electroncarriersidentifiedsofar. i.e., formyl-MF and CO. The most interesting question, r.
In methylotrophic methanogens, there are several lines of whether methanophenazine is also present in hydrogenotro- as
evidencefortheinvolvementofcytochromesinelectrontrans- phicmethanogens,remainstobesolved. m
port from the F -nonreactive hydrogenase to the heterodis- .
420 o
ulfide in methylotrophic methanogens. First, membranes of PossibleMechanismsofDm 1FormationCoupledto rg
acetate-growncellscatalyzeanH -dependentreductionofcy- H /
tochromes(275,560),andsecond2,hdrE(designatedcytb )is ElectronTransportReactions o
2 n
partoftheheterodisulfidereductaseoperonandwasexpressed In methylotrophic methanogens, the F -nonreactive hy-
420 A
duringgrowthonH2-CO2(296).Thevhooperonencodingcyt drogenase is localized in the periplasm, as inferred from its p
bh1ydarloognegnawsiethwtahseastlsroucteuxrparlesssuebdundiutsrionfgthgreowF4th20-onnonHrea-cCtiOve lcehardoemres-ecqounetanicneinagndbaitcstehroiamlohloygdyrotgoemnaesmesb.raFnoer-mboautinodn, coyftoa- ril 1
2 2
(119). Therefore, an electron flow from the F -nonreactive protonpotentialcanbeeasilyenvisaged,becausetheuptakeof 3
420 ,
hydrogenase via cyt b and b to the heterodisulfide can be H andtransferofelectronstoanelectronacceptorwouldlead 2
1 2 2
envisaged(Fig.4). to the liberation of scalar protons on the outside of the cyto- 0
ExperimentsperformedwithM.mazeiGo¨1stronglysuggest plasmicmembrane.However,theH1/CH stoichiometryof3 19
4
that one or several cytochromes also participate in electron to 4 (measured in M. barkeri during methanogenesis from b
transport from F H to the heterodisulfide (264). Mem- methanol-H [57]) cannot be accounted for by scalar protons y
branes of M. maze4i20Go¨21 contain two b- and two c-type cyto- only. That l2eaves us with the question of the nature of the g
u
chromes with midpoint potentials (E ) of2135 and 2240 vectorial proton pump. Electron flow from F H to meth- e
mV (b-type cytochromes) and 2140ma,7nd 2230 mV (c-type anophenazine,aswellasfromthereducedmet4h2a0no2phenazine s
t
cytochromes). The cytochromes were reduced by F H and to the heterodisulfide, is coupled to proton translocation, in-
420 2
oxidized by the heterodisulfide at high rates. Addition of the dicating the presence of two coupling sites (2). The F H
heterodisulfide to reduced cytochromes and subsequent low- dehydrogenaseandtheH1-translocatingbacterialNADH420de2-
temperature spectroscopy showed the oxidation of cyt b . hydrogenase have in common a complex structure and the
564
This indicates the involvement of cytochromes in electron presence of flavins and iron-sulfur centers. Therefore, it is
transportfromF viacytbtotheheterodisulfide(Fig.5). temptingtospeculatethattheF H dehydrogenase,likethe
420 420 2
A different class of membrane-bound electron carriers was NADH dehydrogenase, is a proton pump. It is not known
discovered recently (1). Membranes of methanogens do not whethertheheterodisulfidereductaseitselfisaprotonpump.
containtypicalquinonesfoundinbacteriaoraerobicarchaea. The genes encoding the hydrogenase, the heterodisulfide re-
However, extraction of membranes from methanol-grown ductase, and part of the F dehydrogenase are known, but
420
M. mazei Go¨1 with isooctane yielded a fraction containing a thesimilaritiesofthededucedproteinstosubunitsofNADH
redox-active, low-molecular-weight compound identified as a dehydrogenases or cytochrome oxidases are too low to allow
phenazine derivative, methanophenazine. The structure and identificationofpolypeptidesinvolvedinprotontransport.
reactivity of methanophenazine are given in Fig. 7. Methano- Withthediscoveryofmethanophenazine,anotherpossibility
phenazine is reduced by F dehydrogenase or hydrogenase, hasemerged.Byanalogywiththeubiquinonecycleinthebc
420 1
andreducedmethanophenazinethenreducestheheterodisul- complex,itislikelythatelectrontransferfromcytb tometh-
1
578 SCHA¨FER ET AL. MICROBIOL.MOL.BIOL.REV.
D
o
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a
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e
d

f
r
o
FIG. 8. TentativeschemeofthereactionmechanismoftheNa1-translocatingmethyl-HMPT:CoM-methyltransferase.Theenzymeisamultisubunitenzyme m
consistingofeightnonidenticalsubunitsinunknownstoichiometry.Thereactioncanbediv4idedintotwopartialreactions,methylationanddemethylationofan h
enzyme-bound corrinoid cofactor. The demethylation reaction is apparently coupled to Na1 transport. Co(I) and Co(III) denote different valence states of the t
t
enzyme-boundcorrinoidcofactor.HS-CoM,CoM(2-mercaptoethanesulfonate);P,periplasm;CM,cytoplasmicmembrane;C,cytoplasm.Forexplanations,seethetext. p
:
/
/
m
anophenazineiscoupledtoprotonuptakefromthecytoplasm. mazei Go¨1, the methyltransferase was identified as a Na1 m
b
The reduced methanophenazine then donates its electrons to pump (44); this was later corroborated with the purified en- r
.
cyt b , and the protons are liberated into the periplasm. The zymereconstitutedintoliposomes.Theseproteoliposomescat- a
H1/e22 stoichiometry of such a mechanism would be fixed at alyzed an electrogenic Na1 transport with a stoichiometry of sm
1H1/e2. 1.7molofNa1permolofmethyl-H MPTdemethylated(317). .
The methyltransferase contains th4e cofactor Coa–[a-(5-hy- or
g
SodiumBioenergeticsofMethanogenesis droxybenzimidazolyl)]-cobamide(factorIII),whichisinvolved /
in methyl transfer (163, 164, 427). The cofactor in its super- o
n
Apart from the proton motive electron transport chain, reduced Co(I) form accepts the methyl group from methyl-
A
methanogens have a primary sodium ion pump, the methyl- H4MPT, giving rise to a methyl-Co(III) intermediate. In the p
Hpa4rMt PofT:tCheoMcemnterathlypltartahnwsfaeyr,aasend(4t4h,e1r4e9f,o3re84N).aT1hitsraennszpyomret iiss sneuccolenodpphailrictiaalttraecakc,tpiorno,bathbilsymbyetthhyel-tChioo(lIaItIe)ainsisounbojefcCteodMto,toa ril 1
obligatoryformethaneformation.Thisenzymerepresentsthe giverisetomethyl-CoMandregeneratedCo(I)(149,164): 3
,
fiacrrsotsesxaammpelmeborfanae.mBeethcaylutsreantshfeercaesnetrcaaltpaalytzhiwngayioisnretrvaenrssipbolert, CH3-H4MPT1E:Co(I)3H4MPT1E:CH3-Co(III) (3) 20
1
this enzyme functions as generator of a sodium ion potential E:CH -Co(III)1HS-CoM3CH -S-CoM1E:Co(I) (4) 9
duringmethanogenesisfromCO oracetatebutasanender- 3 3 b
gonicreactiondrivenbythesodiu2mionpotentialinthecourse Reaction 3 has a free-energy change of 215 kJ/mol and was y

ofmethylgroupoxidation,whichhastobecarriedoutduring notstimulatedbysodiumions.Ontheotherhand,demethyl- g
u
methanogenesisfrommethylgroup-containingC compounds ation of the enzyme-bound corrinoid (reaction 4) is also ac- e
(380).Unlikethecytochromesandtheresultingd1ifferencesin companied by a free-energy change of 215 kJ/mol, and this s
t
theelectrontransportchains,themethyltransferaseisfoundin reactionwassodiumiondependent,withhalf-maximalactivity
everymethanogen,andthereisnoreasontoassumedifferent obtainedatapproximately50mMNa1.Thisfindingindicates
reactionmechanisms. thatthedemethylationoftheenzyme-boundcorrinoidiscou-
The energetics of the methyltransferase was first investi- pledtosodiumiontranslocation(599)(Fig.8).
gatedbyusingcellsuspensionsofM.barkeriandthesubstrate The methyltransferase was purified from M. thermoautotro-
combination H -HCHO. Upon addition of the substrate, so- phicumandM.mazeiGo¨1(163,317).Inthelatter,sixsubunits
2
diumionswereactivelyextrudedfromthecytoplasm,resulting werefound,withapparentmolecularmassesof34,28,20,13,
in the generation of a transmembrane Na1 gradient of 260 12, and 9 kDa; it contains a [4Fe-4S] cluster with an E 9 of
mV.Na1translocationwasnotinhibitedbyprotonophoresor 2215 mV and a base-on cobamide with a standard reduc0tion
inhibitors of the Na1-H1 antiporter, indicating a primary potentialof2426mVfortheCo21/11couple(324).InM.ther-
mechanism.Thisprocessresultedinthegenerationofaprot- moautotrophicum,eightsubunitswerefound,withapparentmo-
onophore-resistant membrane potential of 260 mV; corre- lecularmassesof34(MtrH),28(MtrE),24(MtrC),23(MtrA),21
spondingly, protonophores elicited formation of a reversed (MtrD),13(MtrG),12.5(MtrB),and12(MtrF)kDa.Thepuri-
DpH(insideacidic)ofthesamemagnitudeastheDC(384).A fiedenzymecontains2molofcorrinoid,8molofnonhemeiron,
Na1-formaldehyde stoichiometry of 3 to 4 was determined and 8 mol of acid-labile sulfur (163, 192). The encoding genes
withcellsuspensions(261).BytheuseofevertedvesiclesofM. havebeensequencedfromanumberofmethanogens;theyare
VOL.63,1999 BIOENERGETICS OF THE ARCHAEA 579
organizedinanoperonintheordermtrEDCBAFGH.Hydro- the methanogens. The structure and function of the A A
1 o
phobicity plots indicate that all of the subunits except MtrA ATPasesarediscussedbelow(“SecondaryEnergyConverters”).
andMtrHarehydrophobicandpotentiallymembranebound.
Very recently, the membrane localization of MtrD was con- BioenergeticsoftheAcetyl-CoAPathwayinArchaeaand
firmedexperimentallyforM.mazeiGo¨1,M.thermoautotrophi- Bacteria:DifferencesandSimilarities
cum, and M. jannaschii (456). This subunit may be directly
involvedinNa1transport(318). Oneofthemajordifferencesbetweentheanaerobicbacteria
MtrAwasoverexpressed,purifiedfromEscherichiacoli,and andthearchaeathatemploytheacetyl-CoApathwayistheway
successfully reconstituted with cobalamin. Electron paramag- that CO2 is activated. In bacteria, this requires the action of
neticresonance(EPR)spectroscopicstudiesindicatethatthe formate dehydrogenase and formyltetrahydrofolate synthase,
cobalaminisinthebase-offformandthattheaxialligandisa at the expense of ATP hydrolysis. In the reverse reaction,
histidine residue of MtrA (191). From this observation, a hy- oxidationofformyltetrahydrofolateiscoupledtoATPsynthe-
potheticalmechanismwasformulatedforcouplingthemethyl sis by substrate-level phosphorylation (320). In methanogens,
ttiroannaslfecrharnegaectiinonthteopiroontetinra(n1s9p1o)r.tItviisakanolownngt-hraantgcoebc(oIIn)faolarmmain- pthleeolofwaprpedrooxximpoatteenlyti2al5o0f0tmheV[C(4O72)1isoMveFr]c/[ofmorem,ynlo-MtbFy]AcoTuP-
and cob(III)alamin, but not cob(I)alamin, carry an axial ligand. hydrolysis,butbyareversedelectronflowdrivenbythetrans- D
Methylationofcob(I)alamingivesrisetoamethylcob(III)alamin, membraneion(H1orNa1)potential(262). ow
whichisthenabletoligatethehistidineresidue;demethylation Whereas all methanogens tested so far require Na1 for n
leadstoareversalofthisreaction.Itiseasilyconceivablethat growthandmethaneformation(422,423),homoacetogenscan lo
bindinganddissociationofthehistidineresiduewiththecor- bedividedintotwogroupswithrespecttotheirenergymetab- ad
rinoidleadtoaconformationalchangeinthehydrophilicpart olism,theprotonorganismsandthesodiumionorganisms.In e
of the enzyme. This change is then transmitted to the mem- the latter, an as yet unidentified primary sodium ion pump is d
brane-bound subunits, giving rise to Na1 transport. Work on operative. Since these organisms have membrane-bound cor- fr
o
the structure and function of this interesting enzyme is just rinoids, it is speculated that the methyltransferase is the m
emergingbutisapparentlywellonitsway. sodium ion pump (382). If this is the case, this would allow
a study of the evolution of Na1-translocating methyltrans- ht
t
ferases. p
ATPSynthesisinMethanogens TheNa1gradientestablishedinmethanogensiscoupledto ://
m
Methanogens are the only microorganisms known to pro- ATP synthesis, but the mechanisms involved are still contro-
m
dtiumcee.twTohepyriamraer,ytihoenregfroardei,enctosn,fDromnNtae1danwdithDmthHe1,patrothbleemsamoef vheormsioaalcaentodgemnas,yHd1iff-etrranasmloocnagtintgheAvTaPraiosuessamreetfhoaunnodgeinnsp.rIon- br
cthoiuspilsinagchboietvheidonisgsrtaidllieantmsatottethrefosyrndtheebsaitseo.fTAhTerPe(h12a4ve).bHeoewn stoynnthoargseanwisamsfso(u1n1d2,in11th3e),Nbau1t-adeNpae1n-dteranntshloocmaotiancgetFo1gFeonAATceP- .asm
tchoenflhiycdtirnoggerneoptorrotpshriecgaarrcdhianegonDmMN.at1h-edrrmivoeanutAoTtroPphsyicnuthme.sSismiin- thoobmacotaecrieutmogwenososdtirie(n2g1t4h,e4n3s9t)h.eTahsesufimndpitniognotfhNataN1a-A1T-APTasPeasseins .or
g
ganandcoworkers(515,516)hadindicationsofaNa1-ATPase arealsopresentinmethanogens. /

along with a H1-ATPase, whereas Kaesler and Scho¨nheit fa- o
n
vored a mechanism in which the DmNa1 established by the ENERGETICSOFRESPIRATION A
methyltransferasereactionisconvertedtoasecondaryproton p
gcaratidnigenAt tAhatAtThePnasder(iv2e6s2)s.yTnthheeslaisttoerfhAyTpPothvieasiasiHss1u-ptrpaonrstleod- AerobiosisandOtherRespirationFormsinArchaea ril 1
1 o
bythefindingthatthegenomesofthehydrogenotrophicmeth- Obligate aerobes are relatively uncommon among the ar- 3
,
anogens M. jannaschii and M. thermoautotrophicum contain chaea. Given the phylogenetic position of Archaea, this may 2
genes that encode the A A ATPase but lack those of F F reflect the prevalence of anaerobic energy-transducing reac- 0
1 o 1 o 1
ATPase (87, 517). On the other hand, differential inhibitor tions at early stages of evolution; likewise, all organisms 9
studiesindicatedthesimultaneouspresenceofbothA A and branching off at the bottom of the phylogenetic tree are hy- b
1 o
F F ATPsynthasesinM.mazeiGo¨1(43).TheA A enzyme perthermophiles, in conformity with the assumption that life y
m1ayobe coupled to H1 transport, whereas the F1FoATPase originatedinhotenvironments(6,540). g
may be Na1 coupled. However, no F F ATPa1seocould be Table1givesanoverviewofthearchaeathatgrowobligately ue
purifiedfromM.mazeiGo¨1,norhaveth1eeoncodinggenesbeen or facultatively with oxygen or other high-potential terminal s
t
detected. In another organism, M. barkeri MS, a gene cluster electronacceptors.Onlythoseforwhichsufficientdataisavail-
encoding an F F ATPase has been identified in addition to able are included. The genus Acidianus displays obligate
1 o
thearchaealA A ATPasegenes(549);however,thededuced chemolithoautotrophic growth with CO as the sole carbon
g subunit is ve1ryounusual and presumably nonfunctional, and source. Most other species are facultativ2e or obligate hetero-
nogeneencodingsubunitdwasfound.SinceanmRNAtran- trophs.TheautotrophicgrowthofS.acidocaldariuswithsulfur
script could not be detected in cells grown on methanol, it is aselectrondonor,asreportedfortheoriginalisolates(82),has
doubtful that the F F -like genes are expressed in M. barkeri to be questioned since the deposited type strains (DSM 639
1 o
(312).ThepresenceofbothF F andA A wasalsoproposed andATCC33909)areincapableofsuchgrowth.
1 o 1 o
for halobacteria (227, 230). Most likely, the F F ATPase Members of the genus Sulfolobus are obligate aerobes. In-
1 o
genes definitely present at least in M. barkeri MS have arisen terestingly,someSulfolobusisolateswerefoundtoreducefer-
fromhorizontalgenetransfer.Inlinewiththisargumentisthe ricionsormolybdateasterminalacceptorsunderlowoxygen
discovery of V V ATPases in bacteria (433, 620). However, tension(80,83).Inaddition,SulfolobusandAcidianusstrains
1 o
the presence of the F F ATPases in Methanosarcina species have been shown to grow aerobically by the oxidation of mo-
1 o
stillhastobeprovenbiochemically,and,ifpresent,theircon- lecularhydrogen(Knallgasreaction)(236)atlowoxygencon-
tributiontoenergymetabolismhastobeclarified.Apparently, centrations(0.2to0.5%).
themechanismforDm 1-drivenATPsynthesisdiffersamong Alternatively,Acidianusspeciescanderiveenergyfromthe
Na