Question

When the antibiotic nigericin is added to actively respiring mitochondria, several things happen: the yield of ATP decreases, the rate of oxygen consumption increases, heat is released, and the pH gradient across the inner membrane is lost.

Explain all four of these experimental observations in terms of nigericin’s mode of action. In other words specifically explain what nigericin is doing and why each of these side effects are occurring. What is common word for nigericin?

Answer 1

The common word for Nigericin is IONOPHORE/ pottasium ionophore. Ionophores are  "ion carrier" that catalyze transport of ion across hydrophobic membranes such as liquid polymeric membranes or lipid bilayers (lipophilic) . Here Nigericin ( derived from Streptomyces hygroscopicus) acts as an H+, K+, Pb2+ ionophore. Mostly  it is an antiporter of H+ and K+. To carry out cell's function membrane potential is very much essential. But Nigericin transports K+ ions from inner membrane space(IMS) of mitochondria to matrix and H+ ions from matrix to IMS (Antiporter).

According to Mitchell's Chemiosmotic theory electron transport chain and (ETC) oxidative phosphorylation(OP) are coupled reactions. The H+s pumped (exergoinic reaction) in ETC from matrix to IMS creates electrochemical gradient (proton motive force) and across the membrane and activates the flow of H+s from IMS to matrix through ATPase that helps to convert ADP and P to form ATP.

i) Why ATP decreases? - Changes the intra- cellular K+ and H + concentrations, that change the electrochemical gradient across the inner membrane of the mitochondria.

For ATP synthesis IM potential of mitochondria is very much needed. H+ pumping through ATPase is much needed process. If nigericin carries K+ outside and H+ inside that will affect the membrane potential so that ATP synthesis doesnot continue and decreases.

ii) Why oxygen consumption increases ?

As there is decrease in ATP synthesis leads to ADP accumulation, and this leads to increase the consumption of oxygen. ETC continues and so the oxygen consumption increases that is uncoupling of ETC and OP.

iii) Why heat is released?

Due to the decrease in ATP production because of ionophore,  the respiratory rate increased and more heat is released. Proton leakage leads to the release and oxidation of free fatty acids in the case of the mitochondria of brown adipocytes leads to heat production.

iv) Why pH gradient loss?

As the ionophore Nigericin induces proton leakage that changes the pH gradient accross the oinner mitochondrial membrane.