Electron transport chain inhibition by Rotenone

An unskilled worker in a water garden was sent to sweep up a spill of white powder in the storage shed. Later he was found with labored breathing and convulsions. On further examination, the white powder was identified as rotenone. Respiratory distress is induced on rotenone exposure because it inhibits the complex that catalyzes which of the following?

A. Electron transfer from NADH to coenzyme Q

B. Oxidation of coenzyme Q

C. Reduction of cytochrome c

D. Electron transfer from cytochrome c to cytochrome a1/a3

E. Electron transfer from cytochrome a1/a3 to oxygen

The right answer is A) – Electron transfer from NADH to coenzyme Q

Components of Electron Transport Chain- Electrons flow through the respiratory chain through a redox span of 1.1V from NAD⁺/NADH to O₂/2H₂O, passing through four large protein complexes;

i) NADH-Q oxidoreductase (Complex I), where electrons are transferred from NADH to coenzyme Q (also called Ubiquinone);

ii) Succinate Q reductase (Complex II)-Some substrates with more positive redox potentials than NAD⁺/NADH (e.g., succinate) pass electrons to Q via succinate Q reductase (Complex II), rather than Complex I.

iii) Q-cytochrome c oxidoreductase (Complex III), which passes the electrons on to cytochrome c; and

iv) Cytochrome c oxidase (Complex IV), which completes the chain, passing the electrons to O₂ and causing it to be reduced to H₂O (Figure-1).

Mobile complexes

The four complexes are embedded in the inner mitochondrial membrane, but Q and cytochrome c are mobile. Q diffuses rapidly within the membrane, while cytochrome c is a soluble protein.

Figure-1- Components of the electron transport chain. The three complexes I, III and IV act as proton pumps, ATP is synthesized when protons flow back to the mitochondrial matrix through the ATP synthase complex.

Flavoproteins (are important components of Complexes I and II. The oxidized flavin nucleotide (FMN or FAD) can be reduced in reactions involving the transfer of two electrons (to form FMNH₂ or FADH₂), but they can also accept one electron to form the semiquinone. Iron-sulfur proteins (non-heme iron proteins, Fe-S) are found in Complexes I, II, and III. These may contain one, two, or four Fe atoms linked to inorganic sulfur atoms and/or via cysteine-SH groups to the protein. The Fe-S takes part in single electron transfer reactions in which one Fe atom undergoes oxidoreduction between Fe²⁺ and Fe³⁺.

Electron flow in ETC (figure-2)

NADH-Q oxidoreductase or Complex I is a large L-shaped multi-subunit protein that catalyzes electron transfer from NADH to Q, coupled with the transfer of four H⁺across the membrane: Electrons are transferred from NADH to FMN initially, then to a series of Fe-S centers, and finally to Q (Figure-2). In Complex II (succinate -Q reductase), FADH₂ is formed during the conversion of succinate to fumarate in the citric acid cycle and electrons are then passed via several Fe-S centers to Q.

Coenzyme Q or Ubiquinone (ubi). Ubi accepts electrons and transports them to Complex III. Ubi is capable of moving in the membrane between Complex I and III, picking up electrons at Complex I and dropping them off at Complex III. As is the case with Complex I, Complex III contains a series of proteins that transport electrons. The electrons are removed from Complex III by a small peripheral membrane protein known as cytochrome c. Cytochrome c transports the electrons from Complex III to Complex IV.

Reduced cytochrome c is oxidized by Complex IV (cytochrome c oxidase).

This transfer of four electrons from cytochrome c to O₂ involves two heme groups, a and a₃, and Cu. 

Complex IV finally gives its electrons to molecular oxygen (O₂), the final or terminal electron acceptor. The reduction of oxygen results in the formation of a water molecule from the oxygen.

Figure-2- Flow of electrons in the electron transport chain

Site-specific inhibitors -Specific inhibitors of electron transport, for example, rotenone and amobarbital block electron transfer in NADH-Q oxidoreductase and thereby prevent the utilization of NADH as a substrate. The central portion of the rotenone structure resembles the isoalloxazine ring of the FMN molecule, and when it binds to complex I, rotenone prevents the transfer of electrons from NADH to coenzyme Q. In contrast, electron flow resulting from the oxidation of succinate is unimpaired, because these electrons enter through QH2, beyond the block. Malonate is a competitive inhibitor of Complex II

BAL (British Anti Lewisite), Antimycin A interferes with electron flow from cytochrome b in Q-cytochrome c oxidoreductase.

Furthermore, electron flow in cytochrome c oxidase can be blocked by cyanide (CN-), azide (N3 -), and carbon monoxide (CO). Cyanide and azide react with the ferric form of heme a 3, whereas carbon monoxide inhibits the ferrous form. Inhibition of the electron-transport chain also inhibits ATP synthesis because the proton-motive force can no longer be generated.