Lecture 34 Summary

Update for Fall 2009 in progress... Topics to be covered: Respiratory control, uncouplers and inhibitors of electron transport. Introduction to photosynthesis and a comparison of photosynthetic phosphorylation in chloroplasts and oxidative phosphorylation of mitochondria and bacteria.

The integral membrane complexes of the mitochondrial electron transport chain (ETC).

Regulation of the citric acid cycle

Most of the regulation of the citric acid cycle can be accounted for by three main influences: substrate availability, product inhibition, and competitive feedback inhibition. The rate of citrate production is effectively under the control of oxaloacetate levels. NADH, which is a product of several different steps of the citric acid cycle competes with the substrate NAD+. This product inhibition slows the rate of the citric acid cycle when the electron transport chain is unable to reoxidize NADH sufficiently. ATP, a broadly acting allosteric effector, inhibits the same citric acid cycle components subject to product inhibition by NADH, namely isocitrate dehydrogenase (IDH) and α-ketoglutarate dehydrogenase complex, as well as PDH complex. Both α-keto acid dehydrogenase complexes are also inhibited by high levels of their cognate thioester products (acetyl CoA and succinyl CoA). ADP is apparently an allosteric activator of IDH.

Regulation of oxidative phoshorylation

The regulation of oxidative phoshorylation is based upon the demand for ATP. Furthermore, the electron transport chain and phosphorylation of ADP are closely coupled, at least in the typical physiological circumstance. The rate of oxygen consumption by isolated mitochondria slows as the supply of ADP is exhausted.

Uncouplers and inhibitors of electron transport

2,4-Dinitrophenol, a lipid-soluble substance, can carry protons across the inner mitochondrial membrane. Thus, it can uncouple the electron transport chain from ATP synthesis by dissipating the proton-motive force.

Uncoupling can be a physiological process - albeit a regulated one under special circumstances requiring thermogenesis. UCP proteins function as physiological uncouplers in brown adipose tissue.

Various inhibitors have been discovered, the use of which in research have illuminated the sequence of carriers and complexes shown in the scheme above. They have been used to show, for example, that there must be three distinct entry points for electrons to enter into the ETC and contribute to the generation of the proton gradient.

Inhibitors of electron transport through Complex I include rotenone and amytal. Rotenone is a plant-derived substance used by Amazonian tribes to poison fish. It has also been employed as an insecticide. Amytal is a type of barbituate.

Electron transport through Complex III is inhibited by the antibiotic compound antimycin A.

Learning objectives

• Describe the regulation of the citric acid cycle: the enzymes involved, regulators and modes (allosteric, product inhibition, etc.)
• Describe how the rate of oxidative phosphorylation would be regulated by demand for ATP and availability of ADP, and how this rate impacts the rates of electron transport and the citric acid cycle.

Page update in progress, 12-18-09

References

1. Jeremy M. Berg, John L. Tymoczko, Lubert Stryer. (2007). Biochemistry (6th edition) (WH Freeman)
2. Michel H, et al. (1998) Cytochrome c oxidase: Structure and spectroscopy. Ann Rev Biophys Biomol Struct 27: 329-356.