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Molecular Biochemistry I

Electron Transfer Chain

Contents of this page:

Electron transfer reactions

Electron carriers

Respiratory chain

Electron Transfer is discussed on p. 555-556, 571-574 and 802-820 of Biochemistry, 3rd Edition, by

Voet & Voet.

Aox + Bred � Ared + Box

Aox is the oxidized form of A (the oxidant in the reaction shown)

Bred is the reduced form of B (the reductant).

For such an electron transfer, one may consider two half-cell reactions:

1. Aox + n e- � Ared ......e.g., Fe+++ + e- � Fe++

2. Box + n e- � Bred

For each half reaction:

E = E�' - RT/nF (ln [reduced]/[oxidized])

e.g., for the first half reaction:

E = E�' - RT/nF (ln [Ared]/[Aox])

E = voltage, R = Gas Constant, F = Faraday Constant, n = number of e- transferred.

When [Ared] = [Aox], .. E = E�'.

E�' is the mid-point potential, or standard redox potential. It is the potential at which [oxidant] = [reductant]for the half reaction.

For an electron transfer:

DE�' = E�'(oxidant) - E�'(reductant) = E�'(e- acceptor) - E�'(e- donor)

DGo' = - nFDEo'

An electron transfer is spontaneous (negative DG) if E�' (mid-point potential) of the e- donor is more negative

than E�' of the e- acceptor, i.e., when there is a positive DE�'.

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Consider, for example, transfer of 2 electrons from NADH to oxygen:

a. �O2 + 2H+ + 2 e- � H2O E�' = + 0.815 V

b. NAD+ + 2H+ + 2 e- � NADH + H+ E�' = - 0.315 V

Subtracting reaction b from reaction a:

�O2 + NADH + H+ � H2O + NAD+ DE�' = + 1.13 V

DGo' = - nFDEo' = - 2(96485 Joules/Volt � mol)(1.13 V) = - 218 kJ/mol

Electron carriers:

NAD+/NADH and FAD/FADH2 were introduced in the class on bioenergetics.

FMN (Flavin MonoNucleotide) is a prosthetic group of some flavoproteins. It is similar in structure to FAD

(Flavin Adenine Dinucleotide), but lacking the adenine nucleotide.

FMN (like FAD) can accept 2 e- + 2 H+ to yield FMNH2. When bound at the active site of some enzymes,

FMN can accept 1 e-, converting it to the half-reduced semiquinone radical. The semiquinone can accept a

second e- to yield FMNH2.

Role of FMN:Since it canaccept/donate

either 1 or 2 e-,FMN has an

important role inmediating

electron transferbetween carriers

that transfer 2 e-

(e.g., NADH)and carriers that

can only accept

1 e- (e.g.,

Fe+++). See

discussion ofcomplex I

below.

Coenzyme Q (also called CoQ, Q or ubiquinone) is veryhydrophobic. It dissolves in the hydrocarbon core of a membrane.

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The structure of CoQ includes a long isoprenoid tail, with multiple units

having a carbon skeleton comparable to that of the compoundisoprene. In human cells, most often the number of isoprene units (n) is

10.

The isoprenoid tail of Q10 is longer than the width of a lipid bilayer.

The isoprenoid moiety of CoQ may be folded to yield a more compact

structure, and is postulated to reside in the central hydrophobic domainof a membrane, between the two lipid monolayers.

The quinone ring of coenzyme Q can

be reduced to a quinol in a 2e-

reaction:

Q + 2e- + 2H+ � QH2

When bound to special sites inrespiratory chain complexes, CoQ can

accept a single electron to form a

semiquinone radical (Q�-). ThusCoQ, like FMN, can mediate

between one-electron and two-electron donors/acceptors.

Coenzyme Q functions as a mobile

electron carrier within themitochondrial inner membrane. Its role

in transmembrane H+ transportcoupled to electron transfer (the QCycle) will be discussed in the section

on oxidative phosphorylation.

Heme is a prosthetic group of cytochromes.

Heme contains an iron atom embedded in a

porphyrin ring system, shown at right & below.

The Fe is bonded to 4 N atoms of theporphyrin ring.

Hemes in the three classes of cytochrome (a,

b, c) differ slightly in substituents on theporphyrin ring system (see p. 813). A common

feature is two propionate side-chains.

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Only heme c is covalently linked to the protein

via thioether bonds to cysteine residues, as

shown at right.

Heme a is unique in having a long farnesyl side-

chain that includes three isoprenoid units.

Synthesis of heme is discussed separately.

In the RasMol display of heme c at right, the porphyrin ring is displayed as ball& stick, while the iron is shown as spacefill. Iron is colored gold and nitrogen

blue.

The heme iron atom can undergo a 1 e- transition between ferric and ferrousstates:

Fe+++ + e- � Fe++

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The porphyrin ring structure is planar. The iron atom of heme is usually bonded

to two axial ligands, above and below the heme plane (X & Y in the diagram

at right), in addition to the 4 N of the porphyrin ring system. Axial ligands maybe sulfur or nitrogen atoms of amino acid side-chains of the protein.

Axial ligands in cytochrome c are a methionine S (yellow) and a histidine N(blue), as shown at right. A heme that binds O2 may have an open (empty)

axial ligand position.

Cytochromes are proteins with heme prosthetic groups. They absorb light

at characteristic wavelengths. Changes in light absorbance upon oxidation or

reduction of the heme iron provide a basis for monitoring the redox state of the

heme.

Some cytochromes are part of large integral membrane complexes,

each consisting of several polypeptides and including multiple electron

carriers. Individual heme prosthetic groups may be separatelydesignated as cytochromes, even if associated with the same protein.

For example, hemes a and a3 that are part of the respiratory chain

complex IV are often referred to as cytochromes a and a3.

Cytochrome c is instead a small, water-soluble protein, with a single

heme group.

Positively charged lysine residues surround the heme crevice on the

surface of cytochrome c. These may interact with anionic residues onmembrane complexes to which cytochrome c binds, when it is receiving or

donating an electron (diagram below).

In the image at right, Lys residues are colored magenta; all atoms aredisplayed as spacefill except for the porphyrin ring of heme which is in ball

and stick display.

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Explore the structure of cytochrome c at right.

Cytochrome c

Iron-sulfur centers (Fe-S) are prosthetic groups containing 2, 3, 4, or

8 iron atoms, complexed to a combination of elemental and cysteine sulfuratoms.

4-Fe centers have a tetrahedral structure, with Fe and S atoms alternating as

vertices of a cube, as depicted at right. See also diagrams p. 808 & 809.

The cysteine residues provide sulfur ligands to the iron, while also holding

these prosthetic groups in place within the protein.

Electron transfer proteins may contain multiple iron-sulfur centers.

Iron-sulfur centers transfer only one electron even if they contain two

or more iron atoms, because of the close proximity of the iron atoms.

For example a 4-Fe center might cycle between the redox states:

Fe+++3, Fe++

1 (oxidized) + 1 e- � Fe+++2, Fe++

2 ( reduced)

Iron-sulfur centers in spacefill display; cysteines in ball & stick display.

Iron is red-orange; sulfur is yellow. Data from PDB file 2FUG.

Respiratory chain

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Most constituents of the respiratory chain are embedded inthe inner mitochondrial membrane (or in the cytoplasmic

membrane of aerobic bacteria). The inner mitochondrial

membrane has infoldings called cristae that increase the

membrane area.

Electrons are transferred from NADH to O2 via multi-

subunit inner membrane complexes I, III, & IV, plus

coenzyme Q and cytochrome c. Within each complex,

electrons pass sequentially through a series of electron

carriers.

Coenzyme Q is located within the lipid core of the inner

membrane. There are also binding sites for coenzyme Qwithin protein complexes with which it interacts.

Cytochrome c resides in the intermembrane space

(within the lumen of the cristae). It alternately binds to

Complex III or Complex IV during electron transfer.

Individual respiratory chain complexes have been isolated and their composition determined. There is also

evidence for the existence of stable supramolecular aggregates containing multiple complexes. E.g., complex

I, which transfers electrons to coenzyme Q, may associate with complex III, which reoxidizes the reduced

coenzyme Q, to provide a pathway for direct transfer of coenzyme Q between them.

The composition of each of the respiratory chain complexes is shown below and in Table 22-1 p. 806.

Complex Name No. of

Proteins

Prosthetic Groups

Complex I NADH Dehydrogenase 46 FMN, 9 Fe-S centers

Complex II Succinate-CoQ Reductase 5 FAD, cyt b560, 3 Fe-S centers

Complex III CoQ-cyt c Reductase 11 cyt bH, cyt bL, cyt c1, Fe-SRieske

Complex IV Cytochrome Oxidase 13 cyt a, cyt a3, CuA, CuB

The approximate mid-point potentials of constituent electron carriers is represented in the diagram on p. 803

and in table 22-1 on p. 806. The mid-point potentials are consistent with the electron transfers shown above

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being spontaneous.

Respiratory chain inhibitors include the following:

Rotenone (a common rat poison) blocks electron transfer in complex I.

Antimycin A blocks electron transfer in complex III.

Cyanide and carbon monoxide inhibit complex IV.

Inhibition at any of these sites will block electron transfer from NADH to oxygen.

Complex I catalyzes oxidation of NADH, with reduction of coenzyme Q:

NADH + H+ + Q � NAD+ + QH2

Transmembrane H+ flux associated with this reaction is discussed in the section on oxidative phosphorylation.

An atomic-level structure is not yet available for the

entirecomplex I, which in mammals includes at least 46

proteins, along with prosthetic groups FMN and several iron-

sulfur centers.

Complex I is L-shaped. For a low-resolution structure

determined by electron microscopy see diagram in the textbookp. 810.

The peripheral domain of the complex, containing the FMN

that accepts 2 electrons from NADH, protrudes into the

mitochondrial matrix. Iron-sulfur centers are also located in the

hydrophilic peripheral domain, where they form a pathway for

electron transfer from FMN to coenzyme Q. A binding site for

coenzyme Q is thought be close to the interface betweenperipheral and intra-membrane domains.

The initial electron transfers are:

NADH + H+ + FMN � NAD+ + FMNH2

FMNH2 + (Fe-S)ox � FMNH� + (Fe-S)red + H+

After Fe-S is reoxidized by transfer of the electron to the next iron-sulfur center in the pathway:

FMNH� + (Fe-S)ox � FMN + (Fe-S)red + H+

Electrons pass through a series of iron-sulfur centers, and are eventually transferred to coenzyme Q. Coenzyme

Q accepts 2 e- and picks up 2 H+ to yield the fully reduced QH2.

An X-ray structure has been determined for the

hydrophilic peripheral domain of a bacterial complexI. This bacterial complex I contains fewer proteins than

the mammalian complex I, but includes the central

subunits found in all prokaryotic and eukaryotic versions

of complex I.

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The prosthetic groups are found to be all in the

peripheral domain, that in mammalian complex I would

protrude into the mitochondrial matrix.

Iron-sulfur centers are arranged as the a wire,

providing a pathway for electron transfer from FMN

through the protein. The last iron-sulfur center in the

chain, designated N2, passes electrons one at a time tothe mobile lipid redox carrier coenzyme Q. A proposed

binding site for coenzyme Q is close to N2 at the

interface of peripheral & membrane domains.

P. L. Dutton and coworkers have called attention to the

relevance of conserved distances between redox

carriers within respiratory chain complexes with regard to

the energy barrier at each step for electron tunnelingthrough the protein. They have modeled electron transfers

through the respiratory chain complexes, and provide an

animation of the time course of electron transfer through

Complex I. (For details see the article by Moser et al.,

also included in the reference list.)

For more diagrams and additional information see:

A review by U. Brandt (requires Annual Reviews

subscription.)

The Complex I Home Page

Peripheral domain of complex I from T. thermophilus. A. Protein in cartoon display; FMN & FeS centers spacefill.

B. Same but with protein hidden.Structure published by Sazanov & Hinchcliffe in 2006.

Succinate Dehydrogenase of the Krebs Cycle is also called complex

II or Succinate-CoQ Reductase.

FAD is the initial electron acceptor.

FAD is reduced to FADH2 during oxidation of succinate to fumarate.

FADH2 is then reoxidized by transfer of electrons through a series of three

iron-sulfur centers to Coenzyme Q, yielding QH2.

The QH2 product may then be reoxidized via complex III, providing a

pathway for transfer of electrons from succinate into the respiratory chain.

X-ray crystallographic analysis of E. coli complex II indicates a linear

arrangement of electron carriers within complex II, consistent with the

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predicted sequence of electron transfers:

FAD � FeScenter 1 � FeScenter 2 � FeScenter 3 � CoQ

In the crystal structure at right, oxaloacetate (colored black) is bound at the

active site in place of succinate.

See also diagram p. 811.

Complex III accepts electrons from coenzyme QH2 that is generated by electron transfer in complexes I

and II. The structure and roles of complex III are discussed in the section on oxidative phosphorylation.

Cytochrome c1, a prosthetic group within complex III, reduces cytochrome c, which is the electron donor to

complex IV.

Cytochrome oxidase (complex IV) carries out

the following irreversible reaction:

O2 + 4 H+ + 4 e- � 2 H2O

The four electrons are transferred into the complex

one at a time from cytochrome c.

Intramembrane domains of cytochrome oxidase

(complex IV) consist mainly of transmembrane a-

helices.

Metal centers of cytochrome oxidase (complex IV) include heme a, heme a3, CuA

(consisting of 2 adjacent Cu atoms) and CuB.

O2 reacts at a binuclear center, consisting of heme a3 and CuB.

In the diagram at right, the iron atom of heme a3 in gold, and copper atom in dark

red, are displayed as spacefill, with the heme displayed as sticks.

Metal center ligands:

Axial ligands of the hemes in complex IV are histidine N atoms. Note at right

how heme a is held in place within the complex, between 2 transmembrane a-

helices, by its axial histidine ligands.

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Heme a3 , which sits adjacent to CuB, has only one such axial ligand (diagram

below right).

Ligands of Cu atoms consist of histidine N, and in the case CuA also cysteine

S, a methionine S, and a glutamate backbone O.

Electrons enter complex IV one at a time by transfer from cytochrome c to

CuA. They then pass via heme a to the binuclear center consisting of heme

a3 and CuB, where the chemical reaction takes place.

Order of e- transfers: cyt c → CuA → heme a → heme a3/CuB

O2 binds at the open axial ligand position of heme a3, adjacent to CuB. A possible

reaction sequence is depicted on p. 819. Details of the reaction are still debated. A

tyrosine-histidine complex adjacent to the binuclear center is postulated to have arole in O-O bond splitting. Diagram p. 818.

Proton pumping linked to electron transfer in complex IV will be discussed

separately.

The open axial ligand position of the iron atom in heme a3 makes it susceptible to

binding each of the following inhibitors: CN-, CO, and the radical signal molecule

�NO (nitric oxide).

�NO may regulate cellular respiration through its inhibitory effect, and can induce a

condition comparable to hypoxia.

Explore at right the structure of cytochrome oxidase (complex IV).

Cytochrome Oxidase

Copyright � 1998-2007 by Joyce J. Diwan. All rights reserved.

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