Question

When the antibiotic valinomycin is added to actively respiring mitochondria, several things happen; the yield of ATP decreases, the rate of O2 consumption increases, heat is released, and the pH gradient across the inner mitochondrial membrane increases. Does valinomycin act as an uncoupler or an inhibitor of oxidative phosphorylation? Explain

Answer 1

Ans. Note the following points-

#1. The rate of O₂ consumption increases: O₂ acts as the final electron acceptor. It is reduced to H₂O at complex IV of ETC.

Of oxidative phosphorylation is inhibited due to inhibition of any of the ETC complexes, the rate of O₂ consumption decreases drastically or may even completely inhibited.

Since rate of O₂ consumption does not decreases, valinomycin does NOT act as an inhibitor of oxidative phosphorylation (OxP).

#2. ATP yield decreases: If OxP is inhibited, the process of ATP synthesis is STOPPED because proton gradient is no longer established by inhibited one or more ETC complexes in an inhibited OxP.

Since ATP synthesis is NOT completely ceased, valinomycin does not act as the inhibitor of OxP.

#3. Heat is released: Translocation of protons flow intermembrane space (IMS) to mitochondrial matrix (MM) without involving ATP synthase is called proton leak, and also uncoupling of OxP. Proton leak causes conversion of electrochemical potential energy of proton gradient into thermal energy, thus exhibits thermogenic effect, i.e. increase heat production.

Such proton flow across the inner mitochondria membrane (IMM) from IMS to MM decreases the magnitude of proton gradient. The protons leaking across IMM can’t be used to synthesize ATP.

# The lower magnitude of proton gradient has lesser chemiosmotic potential energy in it, thus proton translocation from IMS to MM via ATP synthase produces relatively lesser amount of ATP.

# However, since there is no inhibition of any ETC complex, the rate of O₂ consumption, and in turn, the rate of cellular respiration (the rate at which O₂ is consumed) remains unaffected. The rate of O₂ consumption may also increase because the cell reduces O₂ at relatively higher rate in order to unsuccessfully restore the rate of ATP production.

# Conclusion: Valinomycin acts as an uncoupler but NOT as an OxP inhibitor.