Biochemistry

148 Pages·1980·6.597 MB·English

by  J D Dunitz

STRUCTURE
AND BONDING
Volume 40
Editors:
J. D. Dunitz, Ziirich • J. B. Goodenough, Oxford
P. Hemmerich, Konstanz J.A. Ibers, Evanston
C. K. Jorgensen, Gen6ve • J. B. Neilands, Berkeley
D. Reinen, Marburg • R.J.P. Williams, Oxford
With 35 Figures and 14 Tables
Springer-Verlag
Berlin Heidelberg New York 1980
ISBN 3-540-09816-X Springer-Verlag Berlin Heidelberg New York
ISBN 0-387-09816-X Springer-Verlag New York Heidelberg Berlin
Library of Congress Catalog Card Number 67-11280
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In the preface to the first volume of Structure and Bonding it was stated that one of
the fundamental objectives of the series was to bridge the gaps between modem
inorganic chemistry, chemical physics and biochemistry. As we have now reached
volume 40 it is pertinent to ask how well we have fulfilled this goal.
We have published a total of about 170 articles, several of which are in purely
biochemical topics. Although we devote single volumes to specific topics, we hope
that all chemists and biochemists with an interest in metal ions and bonding will
continue to consult the entire series.
Looking ahead, there is every reason to believe that the field of bit-
coordination chemistry will continue to expand. As analytical techniques are
refined and made more sensitive, additional metal ions from the periodic table will
be assigned roles in biology. Nickel, tin and vanadium have recently been
implicated in human nutrition; chromium is by now well established as a
bioorganic ion. Studies on the basic coordination properties of these ions will
enable the biochemist to determine the nature of their protein ligands as well as
the mechanism of action of the bound metal ion. The publication of compre-
hensive reviews on these subjects in Structure and Bonding will facilitate progress
in these fields. Such reviews should be a source of information for both researcher
and teacher. In the years which have elapsed since the appearance of Vol. 1 of
Structure and Bonding, the discipline of bioorganic chemistry has truly come of
age. This is evidenced by the increasing number of symposia, monographs and
textbooks on the subject.
The editors are grateful to the several hundred authors who have contributed
to our series and we look to continued fruitful collaboration with the inorganic-
biochemical community of scientists.
The Editors
Contents
Metal-Metal Interactions in MetaUoporphyrins,
Metalloproteins and Metalloenzymes
I. A. Cohen . . . . . . . . . . . . . . . . . . . . . . . . . . .
Non-Heine Iron Dioxygenases. Structure and
Mechanism
L. Que, Jr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
The Bleomycins : Antitumor Copper-Binding
Antibiotics
H. Umezawa, T. Takita . . . . . . . . . . . . . . . . . . . . . . 73
Phytochrome, A Light Receptor of Plant
Photomorphogenesis
W. Riidiger . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Author Index Volumes 1-40 . . . . . . . . . . . . . . . . . . . . 141
Metal-Metal Interactions ni Metalloporphyrins,
Metalloproteins and Metalloenzymes
Irwin A. Cohen
City University of New York, Brooklyn College, Department of Chemistry, Brooklyn,
New York 11210, U.S.A.
elbaT of Contents
Introduction ....... ........................................ 2
Oxo and Nitrido Bridged Hemin Dimers ................................ 6
Hemerythrin ............................................... 14
Hemocyanin ............................................... 18
Laccase .................................................. 23
Cytochrome c Oxidase ......................................... 72
References ................................................ 33
Recent years have seen the identification of multimetal active sites in a considerable number of
proteins and enzymes In some of these, the metal ions interact with each other through a ligand
bridge with substantial modification of their properties They act in concert, often binding or
reducing dioxygen in a multi-electron process Although the mechanistic implications of the me-
tal-metal interactions are most important, the evidence for the interactions usually come from
magnetic and spectroscopic studies. Five systems have been selected in order to review the metho-
dology and results found in this ares. The oxo and nitrido bridged hemin dimers are very well bi-
nuclear bridged iron and copper active sites and show strong antiferromagnetic properties. Laccase
and eytochrome oxidase are multi-copper and iron containing enzymes which reduce dioxygen
and are much studied but less understood than the others.
I.A. nehoC
Introduction
The study of the role of metal ions in the function of metalloenzymes usually bene-
fits by the classification of enzymes and model compounds into groups according to
several important features. Most often the categories are based on the type of sub-
strate, product, reaction, metal ion or prosthetic group. There is now a sufficient
body of information available in "bioinorganic" chemistry so that other factors may
be identified as significant and common for a group of enzymes. It is well known that
the properties of metal ions are affected by the presence and types of bound ligands
(and vice versa). In particular, ligands which allow two or more metal ions to interact
lead to substantial changes in some properties of these metal ions. Truly cooperative
phenomena, extending over large numbers of atoms, leading to ferromagnetism and
electrical conductivity are impressive examples.
A considerable amount of recent work on several enzymes and model systems
has begun to show that intermetallic interactions involving just a few ions can be
functionally important and observable. As a matter of fact, there has been so much
work in some areas, that this review can not even attempt to be exhaustive. Some
systems, such as the iron sulfur proteins have been very well studied and reviewed
and will not be included again here. Enzymes which show only long range intermetal-
lic effects, such as the famous allosteric home-heine interaction of hemoglobin, will
not be considered. Nor will the iron storage proteins, which show strong intennetallic
interactions, but those interactions are not intrinsic to the enzyme function, more a
consequence of it. Instead, five systems will be discussed as illustrative of the meth-
odology and results found in the literature, representing this area. The heroin dimers
are included because of their relative simplicity and somewhat complete characteriza-
tion. Hemerythrin has been rather well studied and the coordination geometry about
the metals is almost known with certainty. Next is hemocyanin, where the mode of
the intermetallic effect is understood but the details of the coordination geometry
are not. Laccase presents an example where the intermetallic effect is proven but the
exact role in the en£yme function and any information about the coordination
geometry is lacking. Finally, cytochrome c oxidase is included because the very exist-
ence and functional role of the metal-metal interaction is not universally accepted.
The study of magnetic properties is, at present, one of the most useful techniques
in the identification and evaluation of metal-metal interactions. Fortunately, there
are many excellent sources which cover various aspects of magnetochemistry in-
cluding intermetallic effectst-8). Therefore only a selective discussion is warranted
here. Cooperative magnetic phenomena usually include interactions between pairs
or small clusters of atoms as well as interactions which involve many atoms in for
example, a large chain or a crystal lattice. Although the principles are the same, 0nly
interactions within discrete pairs of ions need be considered for the systems included
here. It is useful to compare the magnetic properties of interacting and isolated
systems.
lateM-lateM Interactions in ,sniryhpropoUateM snietorpoUateM dna semyzneollateM
In a magnetic field the ground state of high spin d s Fe(III) will be split by the
Zeeman effect into levels with atomic magnetic dipoles aligned with or against the
field. For non-interacting ions, the population aligned with the field is lower in
energy by an amount proportional to the field intensity. Thus at constant temperature
there is a Boltzman distribution amongst the allowed states and the net alignment
of the atomic dipoles with the field, called the magnetization (M), is proportional
only to the field intensity (H). The ratio of magnetization to field is eaUed the sus-
ceptibility )X( so that X = M/H. On the other hand thermal randomization of the
collection of the magnetic dipoles tends to decrease the magnetization at constant
field. At infinite temperature no field can produce net alignment and the suscepti-
bility goes to zero, while at low temperature (ignoring saturation effects) the suscep-
tibility becomes very large due to population of only low energy, field aligned states.
That is the Curie law, characteristic of simple paramagnetic systems, i.e.; X = c/T.
N# 2
Accordingly, the effective magnetic moment (Pen)" sx defined such that. . × _ - 3--~--#~ 2
and/aeer = 2.84 vr~ X( in egs units) or/zoer = 789 x/~(X in SI units) where N is
Avogadro's number,/3 is the Bohr magneton and k is Boltzman's constant.
Since it is the susceptibility that is usually measured, the data are often presented
as a plot of ×/1 vs T. For isolated high spin Fe(III) that is a straight line with zero
intercept and a positive slope which produces a value of/a m = 5.9 Bohr magnetons.
When adjacent pairs of metal ions can interact, the magnetic result depends on
some details of the mechanism of the interaction, to be pointed out later. However,
regardless of the mechanism, two paramagnetie ions earl interact to produce one of
two extremes. Ferromagnetism results when the electron spins of one ion tend to
align with those of its neighbor. That stabilizes the alignment of the magnetic dipoles
with the field and greatly increases the susceptibility. Ferromagnetic behavior does
not appear to have been reported in enzymatic studies.
When the interaction between a pair of paramagnetie ions leads to opposition of
the magnetic dipoles on adjacent ions, that produces a ground state with less electron
spin than for a non-interacting pair. For high spin Fe(III) the result is a diamagnetic
(no spin) ground state. That is antfferromagnetie coupling, and it is often revealed by
the temperature dependence of susceptibility. Whereas low temperature leads to high
magnetization for isolated paramagnets, it allows the population of the diamagnetic
state and produces a low susceptibility for antiferromagnetic materials. Increasing
the temperature, at first causes the population of more energetic states, with greater
magnetic moments, and the susceptibility increases. At some temperature, called
the Ned point, the available thermal energy is sufficient to overcome the coupling
of spins and the susceptibility reaches a maximum. At still higher temperature the
collection of magnetic dipoles simply undergoes thermal randomization, and the
susceptibility begins to decrease as for a normal paramagnet. In the so-called paramag-
netic region, the susceptibility remains lower than for a normal paramagnetic material
and now follows the Curie-Weiss law; × = e/(T + O). A plot of 1× vs T at high tem-
perature is a straight line which extrapolates to an intercept at - O degress where @ is
the Weiss constant. The observation of this type of Curie-Weiss behavior is not by
I. .A nehoC
itself adequate proof of the existence of an antiferromagnetic interaction )2,1 but
strongly supports other evidence such as spectral studies. In addition the Curie-Weiss
law does not allow for the quantitative evaluation of the interaction between the
pairs of ions but such an analysis is available.
By the methods developed in several sources 1-s,7,8) the temperature dependence
of the magnetic susceptibility of antiferromagnetic dimers can be analyzed in terms
of an spin exchange coupling constant. J. When J is positive the interaction is fer-
romagnetic; if J is negative the effect is antfferromagnetic. The conventional Hamil-
tonian for the exchange interaction is - 2JSt $2 where St and $2 are the spin
angular momentum quantum numbers for each ion in the pair. For two identical ions
(St = $2 = S) the dimer will have a new quantum number S' which can take 2 S + 1
different values with S' = 0, ,1 2... 2 S and each with an energy above the ground
state of J(S'(S' + 1)). For two antiferromagnetieaUy coupled high spin Fe(III) ions
this corresponds to states with energies of- 30 J, - 20 J, - 12 J, - 6 J, - 2 J and 0.
The derivation of the resultant susceptibility equations for various combinations of
ions are illustrated in several sources t-s,7,s) and tabulated for 51 different pairs
including those of inequivalent ions9). For the cases including Fe(III) and Cu(I) the
results are given.
When St = $2 = 1/2
1
2Ng2~2
M× = kT 3 + exp (- 2 J/kT)
When 1$ = $2 = 5/2
55 + 30 exp (- 01 J/kT) + 14 exp (- 81 J/kT)
2 Ng 2 2~ + 5 exp (- 24 J/kt) + exp (- 28 J/kT) I
MX = kT 11 + 9 exp (- 10 J/kT) + 7 exp (- 81 J/kT)
+ 5 exp (- 24 J/kT) + 3 exp (- 28 J/kT)
+ exp (- 30 J/kT) J
and when 1$ = 1/2, S 2 = 5/2

MX 2gN 2~
10 exp (- 6 J/kT) + 8_27
5 exp (- 6 J/kT) +
Where MX is the molar corrected susceptibility per dimer, J is the coupling constant
and the other terms are as before. The calculated curves for isolated and antiferromag-
neticaUy interacting high spin Fe(III) ions are shown for comparison in Fig. .1 The
strength of the interactions vary, but a J of- 100 cm- 1 produces a first excited state
at 200 cm-1 ~ 600 cal/mole ~ 2.4 J/mole or about %1 of common bond energies
and ligand field splittings.
4
lateM-lateM Interactions in Metaltoporphyrins, Metalloproteins and semyzneollateM
c80 ,- 6,4 X 10 6
SI
1 B "
X
0 0 T,K 003 I
Fig. .1 Reciprocal magnetic susceptibility (per gram atom) sv temperature. A) Isolated S = 5/2
system. )B Two interacting S = 5/2 systems with J = - 5 cm -1. Note that two interacting S =
5/2 systems with J = - 001 cm -1 si off scale with ×/1 smoothly increasing from about 007 )sgc(
at 003 K to 22 x 1014 at 01 K
Spin exchange between metal ions has been described s-s) mainly in terms of
two mechanisms, direct orbital contact between adjacent metals and superexchange.
In the extreme, the former is a metal-metal bond such as in Mn(CO)s2 with a very
stable diamagnetic ground state. Super-exchange is the interaction of unpaired
electrons on two metal ions thru a bridging ligand which contains a full set of valence
orbitals (eg. s 2 p6 for an oxide bridge).
Hatfield )4 illustrates the mode of exchange by considering the symmetry rela-
tionships between the bridge and metal orbitals. In general it appears that if the
unpaired electrons of the two metals are in orbitals which can both overlap with a full
ligand orbital, the Pauli exclusion principle requires spin pairing and antiferromagnet-
ism results. If the unpaired electrons on the two metals axe in orbitals which can only
interact through an orthogonal orbital then, similarly to Hund's rule, parallel spin
results and produces ferromagnetism. This is true for either o or r7 overlap. If a series
of orbital overlaps are involved in one exchange pathway, then only one orthogonal
pair is sufficient to cause ferromagnetic behavior for that pathway.
In any particulax bridge there may be multiple pathways for exchange of the
same or opposite kinds and Hatfield has discussed the relative magnitude of these
interactions. In the case of a linear oxo bridge between two high spin Fe(III) ions,
an antiferromagnetic interaction is reasonable, and all oxo bridged Fe(III) dimers
show low magnetic moments 4). As pointed out by Murray in his review of oxo bridged
Fe(III) dirners °) the observed J values axe generally about - I00 cm -I .
Aside from providing a convenient and interesting technique for the study of
metalloproteins, magnetic coupling is most likely not biologically significant by itself.
But the formation of such an intermetallic bridge has another profound effect upon
5
I.A. nehoC
an extremely important biological role of metal ions; that is, electron transfer proces-
ses. Furthermore, the very same conditions which allow for intermetallic spin ex-
change, are those that allow for intermetaUic electron transfer.
The role of bridging ligands in the inner sphere pathway for metal ion oxidation-
reduction reactions is reasonably well understood .)31-11 Recent studies of mixed
valence state compounds with bridges between Ru(II) and Ru(III) have provided
further insigh t14). The availability of either G or rl orbitals on a bridge which have the
proper symmetry to overlap with the metal orbitals on each side, facilitate electron
transfer in essentially the same manner as spin exchange. The rate of intramolecular
electron transfer in (bpy) 2 C ~ Ru(II) (pyz) Ru(III) C R (bpy) 2 3 + Copy = bipyridine,
pyz = pyrazine) has been estimated to be ~ 101° s -1 at 25 C° .)41 In some caseslS),
such as (N H 3)s Ru L Ru (N H 3)4 s + (L = NCCN, pyrazine) metal-bridge-metal
overlap is so extensive that the metals become equivalent and the system can only be
described as delocalized. A similar result will be described for a metaUoporphyrin
.retal
In addition to providing an orbital route for electron transfer, the formation of
a stable bridge species is analogous to the formation of the precursor complex required
during an inner sphere process12). The overall result is that for a bridged complex, both
metals can participate in electron transfer with an external reactant with no additional
barriers due to intradimer electron transfer. The advantages for two electron oxida-
tion-reduction reactions are clear.
Oxo and Nitrido Bridged Hemin Dimers
Iron porphyrins, the prosthetic group of the heme proteins are of course of great
interest. Tetraphenylporphine, (TPPH2) (Fig. 2) is not a naturally occurring porphy-
rin. But because of its ease of preparation and close similarity to other porphyrins,
tetraphenylporphinatoiron (TPPFe) derivatives have been extensively studied. Heroes
are Fe(II) compounds such as TPPFe(II), and heroins are five coordinate Fe(III)
derivatives such as TPPFe(III) X (X = halide, pseudohalide, carboxylate, alkoxide,
etc.)16). Six coordinate Fe(III) prophyrins are known as hemichromes, such as
TPPFe(III) L2 X (L = pyridine, piperidine, imidazole, etc.)Xs). Hemichromes are
usually low spin, S = 1/2, systems (~eff ~ 1.9 B.M.) and heroins are of the high spin,
S = 5/2 type (/Jeff ~ 5.9 B.M.) )71 although there are some exceptions with S = 3/2
e.g. TPPFeC(CN)a is).
About ten years ago, on the basis of kinetic studies, an oxo bridge betweentwo
Fe(III) porphyrins was proposed as an intermediate in the oxidation of an Fe(II) heine
by 0219) and on the basis of analytical data as a possible product of that reaction 2°). in
1969 solid (porphyrin) Fe(III)-O-Fe(III)(porphyrin) dimers such as ((TPPFe(III) 2 O),
6