Question

1. The sequence of events in electron transport was elucidated in part by the use of inhibitors that block electron transfer at specific points along the chain. For example, adding rotenone (a plant toxin) or amytal (a barbiturate) blocks electron transport in Complex I; antimycin A (an antibiotic) blocks oxidation of Q in Complex III; and cyanide (CN^-) blocks electron transport in Complex IV by binding to the Fe²⁺in the Fe-Cu binuclear center.

1. What happens to oxygen consumption when these inhibitors are added to a suspension of respiring mitochondria?
2. What is the redox state (oxidized or reduced) of the electron carriers in the electron transport chain when each of the inhibitors is added separately to the mitochondrial suspension?

Answer 1

a. answer:

The consumption of oxygen will stop when inhibitors are added to a suspension of mitochondria because oxyen is reduced by the help of the electrons which are actually coming from respiratory substrates like glucose, with the aid of reducing powers like NADH and FADH2 ​​​​​​. These electrons will not reach to the complex V of the Electron Transport Chain when inhibitors are added and thus oxygen will not be reduced. It is this reduced oxygen which accepts two protons to give water. Thus, oxygen consumption will stop.

b. The redox state of the electron transporters depends upon the type of inhibitor used. The electron transporters which lie after the complex which is inhibited remain in oxidised state where as those which lie before that inhibited complex remain reduced.

For example,

If Actimycin inhibitor which inhibits complex III is added to one of the suspension then the complexes from which the electron pass before entering complex III ,i.e. complexes I and II will remain reduced including complex III itself. The complex or complexes which receive electron from this inhibited complex will remain in oxidised state. Here it is complex IV.

These inhibitors were used to find the order which these complexes receive electrons.