Cau truc mang

When bound to special sites in respiratory complexes, CoQcan accept 1 e− to form a semiquinone radical Q·−... Coenzyme Q functions as a mobile e− carrier within the mitochondrial inner

Electron Transfer Chain

Copyright © 1999-2006 by Joyce J Diwan

All rights reserved.

Molecular Biochemistry I

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 + n e -  B

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:

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 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 2nd e− to yield FMNH2

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+++)

C

C

C H C C

H C

C C C H C C

H C

C

H N

C C C H C C

H C

C

H N

N H

e−+ H+ e−+ H+

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,

The quinone ring of

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 1e− & 2 e−

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

N

N N

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

N

N N

Heme a is unique in having a long farnesyl side-chain that includes 3 isoprenoid units

N

N N

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

PDB file 5CYT

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 O2 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

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 & a3 that are part of the respiratory chain complex IV are often referred to as cytochromes a & a3

 Cytochrome c is instead a small, water-soluble protein

with a single heme group

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

S

S

S S

S

Cys

Cys Cys

Cys

Iron-Sulfur Centers

Fe-S spacefill;

E.g., a 4-Fe center might cycle between redox states:

S

S

S S

S

Cys

Cys Cys

Cys

Iron-Sulfur Centers

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

matrix

inner membrane membrane outer

inter- membrane space

mitochondrion

cristae

Respiratory

Chain:

Electron transfer from NADH to O 2 involves multi-subunit inner membrane complexes I, III & IV, plus CoQ & cyt c.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

Cytochrome c resides in the intermembrane space It

Composition of Respiratory Chain Complexes

No of Proteins

Prosthetic Groups

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

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

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 2e− and picks up 2H+ 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

For more diagrams see

 A review by U Brandt (requires Annual Reviews subscription).

 The Complex I Home Page

FAD is the initial electron receptor

FAD is reduced to FADH 2 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,

FAD → FeS center 1 → FeS center 2 → FeS center 3 → CoQ

In this crystal structure oxaloacetate (OAA) is bound in

membrane domain

CoQ

FeS

FAD OAA

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 c1, a prosthetic group within complex III,

reduces cytochrome c, which is the electron donor to

Cytochrome oxidase (complex IV) carries out the

following irreversible reaction:

O 2 + 4 H + + 4 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

α-helices

membrane

Metal centers of cytochrome oxidase (complex IV):

heme a & heme a3,

CuA (2 adjacent Cu atoms) & CuB

O2 reacts at a binuclear center consisting of heme a3 and

Cu

heme a3 CuB

Complex IV binuclear center

PDB 1OCC

Metal center ligands in

Heme a 3, which sits adjacent to

CuB, has only one axial ligand

Cu ligands consist of His N, & in

the case of CuA 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 → CuA → heme a → heme a3/CuB

O 2 binds at the open axial ligand position of heme a 3,

adjacent to Cu

The open axial ligand position makes heme a3 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 2 + 4 H + + 4 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