Electron Transfer An electron transfer reaction: A ox + B red A red + B ox A ox is the oxidized form of A (the oxidant) B red is the reduced form of B (the reductant). For such an electron transfer, one may consider two half-cell reactions: A ox + n e - A red e.g., Fe e - Fe ++ B ox + n e - B red
A ox + n e - A red B ox + n e - B red For each half reaction: E = E°' – RT/nF (ln [reduced]/[oxidized]) e.g., for the first half reaction: E = E°' – RT/nF (ln [A red ]/[A ox ]) E = voltage, R = gas const., F = Faraday, n = # of e . When [A red ] = [A ox ], E = E°'. E°' is the mid-point potential, or standard redox potential, the potential at which [oxidant] = [reductant] for the half reaction.
For an electron transfer: E°' = E°' (oxidant) – E°' (reductant) = E°' (acceptor) – E°' (donor) G o ' = – nF E°' (E°' is the mid-point potential) An electron transfer reaction is spontaneous (negative G) if E°' of the donor is more negative than E°' of the acceptor, i.e., when there is a positive E°'.
Consider transfer of 2 electrons from NADH to oxygen: a. ½ O 2 + 2H + + 2e - H 2 O E°' = V b. NAD + + 2H + + 2e - NADH + H + E°' = V Subtracting reaction b from a: c. ½ O 2 + NADH + H + H 2 O + NAD + E°'= V G = nF E o ' = – 2(96494)(1.13) = – 218 kJ/mol
Electron Carriers NAD + /NADH and FAD/FADH 2 were introduced earlier. 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 H + to form FMNH 2.
FMN, when bound at the active site of some enzymes, can accept 1 e to form the half-reduced semiquinone radical. The semiquinone can accept a 2 nd e to yield FMNH 2. Since it can accept/donate 1 or 2 e , FMN has an important role mediating e transfer between carriers that transfer 2e (e.g., NADH) & those that can accept only 1e (e.g., Fe +++ ).
Coenzyme Q (CoQ, Q, ubiquinone) is very hydrophobic. It dissolves in the hydrocarbon core of a membrane. It includes a long isoprenoid tail, with multiple units having a carbon skeleton comparable to that of isoprene. In human cells, most often n = 10. Q 10 ’s isoprenoid tail is longer than the width of a bilayer. It may be folded to yield a more compact structure, & is postulated to reside in the central domain of a membrane, between the 2 lipid monolayers.
The quinone ring of coenzyme Q can be reduced to the quinol in a 2e reaction: Q + 2 e + 2 H + QH 2.
When bound to special sites in respiratory complexes, CoQ can accept 1 e to form a semiquinone radical (Q· ). Thus CoQ, like FMN, can mediate between 1 e & 2 e donors/acceptors.
Coenzyme Q functions as a mobile e carrier within the mitochondrial inner membrane. Its role in trans-membrane H + transport coupled to e transfer (Q Cycle) will be discussed later.
Heme is a prosthetic group of cytochromes. Heme contains an iron atom in a porphyrin ring system. The Fe is bonded to 4 N atoms of the porphyrin ring.
Hemes in the 3 classes of cytochrome (a, b, c) differ slightly in substituents on the porphyrin ring system. A common feature is 2 propionate side-chains. Only heme c is covalently linked to the protein via thioether bonds to cysteine residues.
Heme a is unique in having a long farnesyl side-chain that includes 3 isoprenoid units.
The heme iron can undergo a 1 e transition between ferric and ferrous states: Fe e Fe ++ In the RasMol display of heme c at right, the porphyrin ring system is displayed as ball & sticks, while Fe is displayed as spacefill.
Axial ligands may be S or N atoms of amino acid side-chains. Axial ligands in cyt c are Met S (yellow) and His N (blue). A heme that binds O 2 may have an open (empty) axial ligand position. The porphyrin ring is planar. The heme Fe is usually bonded to 2 axial ligands, above & below the heme plane (X,Y) in addition to 4 N of porphyrin.
Cytochromes are proteins with heme prosthetic groups. They absorb light at characteristic wavelengths. Absorbance changes upon oxidation/reduction of the heme iron provide a basis for monitoring heme redox state. Some cytochromes are part of large integral membrane complexes, each consisting of several polypeptides & including multiple electron carriers. Individual heme prosthetic groups may be separately designated as cytochromes, even if in the same protein. E.g., hemes a & a 3 that are part of the respiratory chain complex IV are often referred to as cytochromes a & a 3. Cytochrome c is instead a small, water-soluble protein with a single heme group.
Positively charged lysine residues (in magenta) surround the heme crevice on the surface of cytochrome c. These may interact with anionic residues on membrane complexes to which cyt c binds, when receiving or donating an e .
Iron-sulfur centers (Fe-S) are prosthetic groups containing 2, 3, 4 or 8 iron atoms complexed to elemental & cysteine S. 4-Fe centers have a tetrahedral structure, with Fe & S atoms alternating as vertices of a cube. Cysteine residues provide S ligands to the iron, while also holding these prosthetic groups in place within the protein. Fe-S spacefill; cysteine ball & stick. Fe orange; S yellow. PDB 2FUG
E.g., a 4-Fe center might cycle between redox states: Fe +++ 3, Fe ++ 1 (oxidized) + 1 e Fe +++ 2, Fe ++ 2 (reduced) Electron transfer proteins may contain multiple Fe-S 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.
Most constitutents of the respiratory chain are embedded in the inner mitochondrial membrane (or in the cytoplasmic membrane of aerobic bacteria). The inner mitochondrial membrane has infoldings called cristae that increase the membrane area. Respiratory Chain:
Within each complex, electrons pass sequentially through a series of electron carriers. CoQ is located in the lipid core of the membrane. There are also binding sites for CoQ within protein complexes with which it interacts. Cytochrome c resides in the intermembrane space. It alternately binds to complex III or IV during e transfer. Electrons are transferred from NADH O 2 via multisubunit inner membrane complexes I, III & IV, plus CoQ & cyt c.
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. Individual respiratory chain complexes have been isolated and their composition determined.
Composition of Respiratory Chain Complexes Complex Name No. of Proteins Prosthetic Groups Complex INADH Dehydrogenase 46FMN, 9 Fe-S cntrs. Complex IISuccinate-CoQ Reductase 5FAD, cyt b 560, 3 Fe-S cntrs. Complex IIICoQ-cyt c Reductase 11cyt b H, cyt b L, cyt c 1, Fe-S Rieske Complex IVCytochrome Oxidase 13cyt a, cyt a 3, Cu A, Cu B
Mid-point potentials of constituent e carriers are consistent with the e transfers shown being spontaneous. Respiratory chain inhibitors include: Rotenone (a rat poison) blocks complex I. Antimycin A blocks electron transfer in complex III. CN & CO inhibit complex IV. Inhibition at any of these sites will block e transfer from NADH to O 2.
NADH + H + + Q NAD + + QH 2 Transmembrane H + flux associated with this reaction will be discussed in the section on oxidative phosphorylation. An atomic-level structure is not yet available for the entire complex I, which in mammals includes at least 46 proteins, along with prosthetic groups FMN & several Fe-S centers. Complex I catalyzes oxidation of NADH, with reduction of coenzyme Q:
The peripheral domain, containing the FMN that accepts 2e from NADH, protrudes into the mitochondrial matrix. Iron-sulfur centers are also located in the hydrophilic peripheral domain, where they form a pathway for e transfer from FMN to coenzyme Q. A binding site for coenzyme Q is thought be close to the interface between peripheral and intra-membrane domains. Complex I is L-shaped.
The initial electron transfers are: NADH + H + + FMN NAD + + FMNH 2 FMNH 2 + (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 QH 2.
This bacterial complex I contains fewer proteins than the mammalian complex I, but includes the central subunits found in all prokaryotic & eukaryotic versions of complex I. The prosthetic groups are found to be all in the peripheral domain, that in the mammalian complex would protrude into the mitochondrial matrix. An X-ray structure has been determined for the hydrophilic peripheral domain of a bacterial complex I
N2, the last Fe-S center in the chain, passes e one at a time to the mobile lipid redox carrier coenzyme Q. A proposed binding site for CoQ is close to N2 at the interface of peripheral & membrane domains. Iron-sulfur centers are arranged as a wire, providing a pathway for e transfer from FMN through the protein.
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 tunneling through 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.animation For more diagrams see A review by U. Brandt (requires Annual Reviews subscription).review The Complex I Home PageComplex I Home Page
FAD is reduced to FADH 2 during oxidation of succinate to fumarate. FADH 2 is then reoxidized by transfer of electrons through a series of 3 iron-sulfur centers to CoQ, yielding QH 2. The QH 2 product may be reoxidized via complex III, providing a pathway for transfer of electrons from succinate into the respiratory chain. Succinate Dehydrogenase of the Krebs Cycle is also called complex II or Succinate-CoQ Reductase. FAD is the initial e acceptor.
FAD FeS center 1 FeS center 2 FeS center 3 CoQ In this crystal structure oxaloacetate (OAA) is bound in place of succinate. X-ray crystallographic analysis of E. coli complex II indicates a linear arrangement of electron carriers within complex II, consistent with the predicted sequence of electron transfers:
Complex III accepts electrons from coenzyme QH 2 that is generated by electron transfer in complexes I & II. The structure and roles of complex III are discussed in the class on oxidative phosphorylation. Cytochrome c 1, 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: O H e 2 H 2 O 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 -helices.
Metal centers of cytochrome oxidase (complex IV): heme a & heme a 3, Cu A (2 adjacent Cu atoms) & Cu B. O 2 reacts at a binuclear center consisting of heme a 3 and Cu B.
Metal center ligands in complex IV: Heme axial ligands are His N atoms. Heme a is held in place between 2 transmembrane -helices by its axial His ligands.
Heme a 3, which sits adjacent to Cu B, has only one axial ligand. Cu ligands consist of His N, & in the case of Cu A also Cys S, Met S, & a Glu backbone O. Electrons enter complex IV one at a time from cyt c to Cu A. They then pass via heme a to the binuclear center where the chemical reaction takes place. Electron transfers: cyt c → Cu A → heme a → heme a 3 /Cu B O 2 binds at the open axial ligand position of heme a 3, adjacent to Cu B.
The open axial ligand position makes heme a 3 susceptible to binding each of the following inhibitors: CN , CO, and the radical signal molecule ·NO. ·NO may regulate cell respiration through its inhibitory effect, & can induce a condition comparable to hypoxia. O H e 2 H 2 O Details of the reaction sequence are still debated. A Tyr-His complex adjacent to the binuclear center is postulated to have a role in O-O bond splitting.