Biological Electrochemistry

551 Pages·1982·7.918 MB·English

by  Glenn Dryhurst (Auth.)

BIOLOGICAL
ELECTROCHEMISTRY
VOLUME I
GLENN DRYHURST
Department of Chemistry
University of Oklahoma
Norman, Oklahoma
KARL M. KADISH
Department of Chemistry
University of Houston
Houston, Texas
FRIEDER SCHELLER
REINHARD RENNEBERG
Akademie der Wissenschaften der DDR
Zentralinstitut fur Molekularbiologie
Bereich Angewandte Enzymologie
Berlin, German Democratic Republic
1982
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Library of Congress Cataloging in Publication Data
Main entry under title:
Biological electrochemistry.
Includes bibliographical references and index.
1. Bioelectrochemistry. I. Dryhurst, Glenn,
Date.
QP517.B53B56 574.19'283 82-1711
ISBN 0-12-222401-9 (v. 1) AACR2
PRINTED IN THE UNITED STATES OF AMERICA
82 83 84 85 9 8 7 6 5 4 3 2 1
Alcron was a beginning.
There will never be an end.
PREFACE
The genesis of this book began with an invitation from Professor A. Vlcek
of the Heyrovsky Institute of Physical Chemistry and Electrochemistry in
Czechoslovakia to me to organize a symposium on the Elucidation of Bio-
logical Redox Mechanisms in Terms of Electrochemical Concepts as part of
the J. Heyrovsky Memorial Congress on Polarography held in Prague in Au-
gust 1980. Three keynote speakers, Karl Kadish, Frieder Scheller, and my-
self, prepared talks designed to present an overview of the electrochemistry
of small organic molecules, large organic molecules, and inorganic mole-
cules, and how electrochemical information helps to understand some of the
biological redox reactions of such systems.
Anyone who has been called on to deliver such lectures will appreciate
the time and effort required in preparation. It seemed logical, therefore, that
this effort should be utilized and expanded by preparing a book concerned
with the electrochemistry of biologically significant compounds. This is the
first of two volumes which will be published. Volume I is largely concerned
with the electrochemistry of small and macromolecular organic com-
pounds. Volume II will deal with various inorganic and organometallic com-
pounds of biological interest.
The work reviewed in this book presents a reasonably complete summary
of material published through 1980. We anticipate the material will be of
use to electrochemists, biochemists and biologists, and other scientists
working in various fields of biotechnology.
I would like to acknowledge the help of Sushma Goyal who prepared
most of the figures and Nancy Burnett who typed the entire manuscript and
handled much of the extensive correspondence involved.
Glenn Dryhurst
xi
1
QUINONES
I. INTRODUCTION
Quinones occur extensively in nature and appear to play a variety of roles
in the life cycles of living organisms. Thus, a basic interest in the biological
role and function of these compounds has stimulated a significant amount of
chemical and biochemical research into their properties and behavior.
A very detailed and comprehensive review of the occurrence, structure,
and properties of naturally occurring quinones has been prepared by Thom-
son [7]. Bentley and Campbell [2] have comprehensively reviewed the bio-
logical reactions of quinones in terms of their biosynthesis and functional
significance. Most of the chemistry of quinones has been reviewed in the
treatise edited by Patai [3]. The role of quinones in biological electron trans-
port has been reviewed by Crane [4].
The quinones of particular interest in this chapter are those associated
with the electron-transport or respiratory systems in living organisms. Such
quinones are based on 1,4-benzoquinone (I) and 1,4-naphthoquinone (II).
The naturally occurring benzoquinone
ο
ο ο
ι II
1
2 1. Quinones
or naphthoquinone derivatives are generally lipid-soluble, because of a long
hydrocarbon chain attached to the quinone nucleus. There are four main
groups of naturally occurring quinones of this type. These are the ubiqui-
none or coenzyme Q, vitamin K, plastoquinone, and tocopherylquinone
groups. All of these groups are thought to participate in electron-transport
systems and/or energy coupling processes, which are supported by electron
flow. Because of the involvement of these molecules in biological electron-
transport processes, it would seem natural that electrochemists would be in-
timately involved in the elucidation of their redox mechanisms and in re-
search to understand their biological role in electron transport. This, how-
ever, is only true to a rather limited extent.
II. STRUCTURES OF BIOLOGICALLY SIGNIFICANT QUINONES
Only the four groups of naturally occurring quinones mentioned above
will be discussed. Details on other naturally occurring quinones are given by
Thomson [7 ].
Probably the most widely distributed quinones are those in the ubiqui-
none or coenzyme Q group. The structure of these compounds is based on a
2,3-dimethoxy-5-methylbenzoquinone nucleus with an unsaturated terpe-
noid sidechain (Fig. 1). Clearly, the terpenoid sidechain substituted^ the
C(6)-position consists of a number of five-carbon methylbutenyl units. Indi-
vidual ubiquinones are designated either by the number of methylbutenyl
units in the sidechain or by the number of carbon atoms in the sidechain.
Thus, ubiquinone-20 is the same compound as coenzyme Q , and ubiqui-
4
none-30 is the same compound as coenzyme Q . The most common ubi-
6
quinones are those that have a terpenoid chain that contains 30 to 50 carbon
atoms, i.e., Q to Q . There are a few variations of the structure shown in
6 10
Fig. 1, but these are quite rare and have been discussed by Crane [4],
Members of the vitamin Κ group of quinones are also found very extensively
in nature and may occur even more commonly than the ubiquinone group
[4]. The vitamin Κ group of compounds has a naphthoquinone nucleus and,
ο L -In
Fig. 1. Basic structure and numbering system for the ubiquinone or coenzyme Q group of
quinones.
II. Structures of Biologically Significant Quinones 3
ο
Fig. 2. Structures of (A) vitamin Κ, ; (B) vitamin K ; (C) vitamin K (menadione).
2 3
usually, a methyl group at C(2) (Fig. 2). Vitamin K, (A, Fig. 2) is 2-methyl-3-
phytyl-1,4-naphthoquinone and hence possesses a monounsaturated phytyl
sidechain or a 20-carbon chain. Sometimes this compound is designated as
vitamin M20) for obvious reasons. Vitamin K-, is probably the most common
form of this vitamin. Vitamin K (B, Fig. 2) has a terpenoid side chain that is
2
very similar to the coenzyme Q group. These compounds are often referred
to as menaquinones and have sidechains ranging from π = 0 to 12 [2]. In
certain instances, the vitamin K or menaquinones are designated MK-n,
2
where the η refers to the number of unsaturated isopentenyl units (B, Fig. 2).
When η = 0, the resulting compound is usually referred to as vitamin K or
3
menadione (C, Fig. 2). This compound is obviously 2-methyl-1 ^-naphtho-
quinone.
The plastoquinones are a group of 2,3-dimethylbenzoquinones found in
all oxygen-producing photosynthetic organisms. The basic structure of the
plastoquinone group of quinones is shown in Fig. 3. The plastoquinones are
ο
Fig. 3. Basic structure of the plastoquinone group of quinones.
4 1. Quinones
Fig. 4. Structure and numbering system for tocopherylquinones: a-tocopherylquinone,
R3 = R5 = Re = CH3;0form, R3 = Re = CH3, R5 = H; y form, R5 = R6 = CH3, R3 = Η; δ form.
=
Re CH R = R = H.
3/ 3 5
often designated PQ-n where η is the number of isopentenyl units in the
sidechain. The common values of η are 3, 4, 8, and 9 [/]. Detailed discus-
sions of the origins of various plastoquinones and structural variations have
been presented by Crane [4] and Thomson [/].
The fourth major group of naturally occurring quinones is the tocopheryl-
quinones, which are structurally similar to the plastoquinones. Thus, the to-
copherylquinones are a series of Ί ,4-benzoquinones with one, two, or three
methyl groups substituted on the quinone nucleus and a hydroxylated phytyl
sidechain substituted at the C(2) position. The most extensively studied com-
pound is α-tocopherylquinone, 3,5,6-trimethyl-2-(3-hydroxy)phytyl-1,4-
benzoquinone. The structures of the latter compound and of the β-, y-, and
δ-tocopherylquinones are shown in Fig. 4.
III. OCCURRENCE AND BIOLOGICAL SIGNIFICANCE
OF QUINONES
A. Ubiquinone or Coenzyme Q
The coenzyme Q or ubiquinone group of quinones was discovered about
25 years ago [5,6]. Its members are found in bacteria and fungi, higher
plants and algae, invertebrates, and vertebrates, including man [7,8]. They
are found in the liver, heart, spleen, pancreas, and other tissues. The name
ubiquinone [9] was first used to indicate its wide occurrence in Nature.
However, it is, in fact, not ubiquitous [/].
Ubiquinones have a major role in respiratory electron-transport processes
[4,10,1 /]. They are normally found in mitochondria, the bulk of cellular ubi-
quinones being found in the inner mitochondrial membrane, which is the
subcellular site of electron transport. The ubiquinones appear to undergo cy-
clic oxidation and reduction during the oxidation of substrates such as pyru-
vate, isocitrate, α-ketoglutarate, and malate, i.e., substrates of the citric acid
cycle. In support of its participation in the respiratory chain is the fact that
the ubiquinones found in the mitochondrial membrane may be extracted
III. Occurrence and Biological Significance of Quinones 5
with solvents such as acetone and chloroform. This results in the loss of en-
zymatic activity, which may be restored on addition of ubiquinone. How-
ever, as noted by Thomson [12], it can be shown that electron transport can
still occur in mitochondrial particles from which all ubiquinone has been
removed, suggesting that electron transfer proceeds through a branched-
chain system: one pathway depending on ubiquinone, another in which
ubiquinone does not participate. Ubiquinone also appears to function as an
electron carrier in photosynthetic bacteria, taking the place occupied by
plastoquinone in higher plants.
It is, perhaps, worthwhile reviewing the respiratory or electron-transport
chain so that the position and role of ubiquinone can be fully appreciated. In
the electron-transport chain, a number of oxygen-dependent dehydrogena-
tions (oxidations) occur that involve intermediate electron carriers, which
intervene in the flow of electrons between the initial electron donor—e.g.,
pyruvate, malate—and the ultimate electron acceptor, which is molecular
oxygen. The process of electron transport involves the successive interaction
of carriers capable of undergoing a reversible conversion between its re-
duced and oxidized states. Thus each intermediate carrier first participates
in its oxidized state as an acceptor of electrons and is converted to its re-
duced state. In the reduced state, the carrier then functions as a donor and
transfers electrons to the next carrier in its oxidized state, and in doing so is
reconverted back to the original oxidized state. The final carrier transfers
electrons to oxygen, the ultimate acceptor, which is reduced to water.
A moderately detailed schematic representation of the respiratory chain is
presented in Fig. 5. The oxidized and reduced forms of the various electron
carriers are presented in Fig. 6. The reduction potentials shown in Fig. 5 are
those presented by Bohinski [13]. It should be noted that the system (redox
couple) with the more positive potential (Ε°') will spontaneously tend to gain
electrons and undergo reduction. It is clear in Fig. 5 that, with the exception
of the CoQ/CoQH couple, the E°' values become progressively more posi-
2
tive from nicotinamide adenine dinucleotide (NAD+) to oxygen. Thus, the
carriers are arranged in order of an increasing tendency to undergo reduc-
tion. Oxidation of a substrate such as L-malate in the presence of NAD+
and a suitable NAD+-dependent dehydrogenase gives oxalacetate and
NADH. The latter species primes the electron-transport chain, and a transfer
of electrons to flavin mononucleotide (FMN) occurs, giving back NAD+ and
forming FMNH . The FMNH then reduces coenzyme Q (CoQ) to the cor-
2 2
responding hydroquinone species, CoQH , etc., until ultimately a reduced
2
(Fe2+) cytochrome reduces molecular oxygen to give an oxidized (Fe3+)
cytochrome and water (Fig. 5). Obviously coenzyme Q does not properly
fit into the scheme shown in Fig. 5 because its £°' value is more positive
than that for the cytochrome b couple. However, the quoted E°' was deter-
6
Oxalacetate NADH(H"r) 1/202
E°=-0.32V E°=+0.25V Y E°=+0.29V Y E°*=+0.82V
H90
succinate fumarate
Fig. 5. Schematic representation of the respiratory chain. E°' values are reduction potentials in volts vs. the standard hydrogen electrode
at pH 7 and at 20-30°C [13].