Electron Transport Chain

Electron Transport Chain

An electron transport chain or system is a series of coenzymes and cytochrome that take part in the passage of electron from a chemical to its ultimate acceptor. The inner mitochondrial membrane contains groups of electron and proton transporting enzymes.  In each group, the enzymes are arranged in a specific series termed as, electron transport chain (ETC) .  It is also termed as mitochondrial respiratory chain or electron transport system (ETS).

The passage of electrons from one enzyme or cytochrome to the next result in loss of energy at each step.  At each step, the electron carriers include flavins, iron sulfur complexes, quinones, and cytochromes. Quinones are highly mobile electron carrier.  Most of them are prosthetic group of proteins.

A prosthetic group is a non-protein molecule required for the activity of a protein. Prosthetic groups are organic or inorganic, non-peptide molecules bound to a protein that facilitate its function.  They also include coenzymes.

Enzyme Complexes

Electron transport involves following 4 enzyme complexes (multiprotein complexes are also identified by Roman numerals I to IV):

(i) NADH-Q reductase or NADH dehydrogenase complex (Complex I)

(ii) Succinate Q-reductase complex (Complex II)

(iii) QH2-cytochrome c reductase complex (Complex III)

(iv) Cytochrome c oxidase complex (Complex IV)

The enzyme complexes consists of following different prosthetic groups:

Table: Four enzyme complexes of mitochondrial electron transport chain

	Enzyme Complex	Mass (KDa)	Number of subunits	Prosthetic groups
Complex I	NADH-Q oxidoreductase (NADH dehydrogenase)	880	42(14)	FMN, FeS
Complex II	Succinate Q-reductase complex (Succinate dehydrogenase)	140	4	FAD, FeS
Complex III	Q-cytochrome c oxido reductase complex	250	10	Cyt bH (Heme bH), Cyt bL (Heme bL), FeS, Cyt cL (Heme cL)
Complex IV	Cytochrome c oxidase complex	160	10(3-4)	Cyt a (Heme a) Cyt a3 (Heme a3), CuA, CuB

Process of mitochondrial electron transport:

NADH-Q reductase or NADH dehydrogenase complex (Complex I)

Complex I is also termed as NADH-Q reductase and the enzyme in complex I is NADH dehydrogenase.  It is a large protein, and also contains 45 amino acid chains.  NADH generates electrons in the mitochondrial matrix during the TCA cycle.  Oxidation of these electrons thus occurs by complex I termed as NADH-ubiquinone oxido-reductase or NADH-Q reductase.

In addition to several proteins, this complex also contains a tightly bound molecule of flavin mononucleotide (FMN) and also several nonheme iron- sulfur (Fe-S) complexes.  Both electrons and protons pass from NADH2 to FMN.  As a result the latter is reduced.  However, FMNH2 breaks to release protons (H⁺) and electrons.  As a result protons pass out through the inner mitochondrial membrane to outer chamber.

NADH+ H⁺ +FMN  →  FMNH₂+NAD⁺

FMNH₂   → FMN+2H+2e^–

Electrons now move to the iron sulfur complex (Fe-S) and from there, to quinone.  The common quinone is coenzyme Q, also termed as ubiquinone (UQ).  It is highly soluble in lipids and thus diffuses freely in the plane of the membrane.  Thus ubiquinone (UQ) forms a pool of mobile electron acceptors that conveys electrons between complex I and complex III

2e^–+2Fe³⁺S →  2Fe²⁺S

2Fe²⁺S+Q → 2Fe³⁺S+Q^2-

Charged quinone picks the proton from mitochondrial matrix and thus passes it into the outer chamber with the help of cytochrome b.

Succinate Q-reductase complex (Complex II)

Complex II is also termed as Succinate ubiquinone oxido-reductase or- Succinate Q-reductase complex. It contains flavin adenine dinucleotide (FAD).  Like Complex I, succinic dehydrogenase also transfers electrons from succinate to a molecule of ubiquinone from the membrane pool.

FADH₂ produced during the reduction of succinate passes its electrons and protons to coenzyme Q through iron sulphur complex (Fe-S).

FADH₂ +2Fe³⁺S →  2Fe²⁺S+2H⁺+FAD

2Fe²⁺S+Q+2H⁺  →   2Fe³⁺S+QH₂

QH2-cytochrome c reductase complex (Complex III)

Complex-III is also termed as cytochrome c reductase. The fully reduced form of ubiquinone is ubiquinol and it is oxidised by cytochrome c reductase.

QH2-cytochrome c reductase complex has 3 components- cytochrome b, iron sulphur complex (Fe-S) and also cytochrome c₁

Cytochrome proteins also have a prosthetic group of heme.  The heme molecule is similar to the heme in hemoglobin, but it carries electrons, not oxygen.  As a result, the iron ion at its core is reduced and oxidised as it passes the electrons, fluctuating between different oxidation states: Fe++(reduced) and Fe +++(oxidised).  The heme molecules in the cytochromes have slightly different characteristics due to the effects of the different proteins binding them, giving slightly different characteristics to each complex.

Complex III in turn reduces a molecule of cytochrome c.  It is a peripheral protein located on the side of membrane facing the intermembrane space.  Like ubiquinone, cytochrome c is also a mobile carrier and conveys electrons between Complex III and Complex IV, the terminal complex in the chain.

  QH₂+2Fe³⁺ cyt b  →  Q+2H⁺+2Fe²⁺ cyt b

2Fe²⁺ cyt b +2Fe³⁺S →   2Fe³⁺ cyt b+2Fe²⁺S

2Fe²⁺S+Q+2H⁺  →    2Fe³⁺S+ QH₂

QH₂+2Fe³⁺ cyt c₁ →   Q+2H⁺+ 2Fe²⁺ cyt c₁

Cytochrome c₁ hands over its electron to cytochrome c . Like Coenzyme Q, cytochrome c is also mobile carrier of electrons.

2Fe²⁺ cyt c₁+2Fe³⁺ cyt c →   2Fe³⁺ cyt c₁+2Fe²⁺ cyt c

Cytochrome c oxidase complex(Complex IV):

The fourth complex also termed as Cytochrome c oxidase complex contains cytochrome proteins c, a and cytochrome a₃. This complex also contains two heme groups (one in each of the two cytochromes, a and a3 . It also consist of three copper ions (a pair of Cu_A and one Cu_B in cytochrome a₃), which helps in the transfer of electrons to oxygen.

2Fe²⁺ cyt c +2Fe³⁺ cyt a   →    2Fe³⁺ cyt c +2Fe²⁺ cyt a

2Fe²⁺ cyt a +2Fe³⁺ cyt a₃Cu²⁺  →  2Fe³⁺ cyt a +2Fe² cyt a₃Cu²⁺

Electrons first pass from cytochrome c to cytochrome a and then to cytochrome a₃ and finally to molecular oxygen.

2Fe² cyt a₃Cu²⁺  → 2Fe³ cyt a₃Cu¹⁺

Oxygen is the ultimate acceptor of electrons.  It becomes reactive and thus combines with protons to form metabolic water.

2Fe³ cyt a₃Cu¹⁺+O →  2Fe³⁺ cyt a₃Cu²⁺+O

2H⁺+O   →   2H₂O

Energy released during passage of electrons from one carrier to the next is made available to specific transmembrane complexes which pump protons from the matrix side of the inner mitochondrial membrane to the outer chamber. As a result there is an increase in proton concentration in the outer chamber or outer surface of the inner mitochondrial membrane. Thus the difference in the proton concentration on the outer and inner sides of the inner mitochondrial membrane is termed as proton gradient.