Proton oxidative phosphorylation

ATP results from the movement of approximately three protons from the cytosol into the matrix through Fg. Altogether this means that approximately four protons are transported into the matrix per ATP synthesized. Thus, approximately one-fourth of the energy derived from the respiratory chain (electron transport and oxidative phosphorylation) is expended as the electrochemical energy devoted to mitochondrial ATP-ADP transport. 

The thylakoid membrane is asymmetrically organized, or sided, like the mitochondrial membrane. It also shares the property of being a barrier to the passive diffusion of H ions. Photosynthetic electron transport thus establishes an electrochemical gradient, or proton-motive force, across the thylakoid membrane with the interior, or lumen, side accumulating H ions relative to the stroma of the chloroplast. Like oxidative phosphorylation, the mechanism of photophosphorylation is chemiosmotic. 

Many inhibitors of substrate oxidations, substrate transport, electron transport, and ATP synthesis are known including many well-known toxins (see Sherratt, 1981 Harold, 1986 Nicholls and Ferguson, 1992). These are not discussed here except to mention specific uncouplers of oxidative phosphorylation. Classic uncouplers such as 2,4-dinitrophenol have protonated and unprotonated forms, both of which are lipid soluble and cross the inner mitochondrial membrane discharging the proton gradient. This prevents ATP synthesis and stimulates respiration. 

Vance, J.E., eds.), pp. 116-142, Benjamin/Cummings Publishing Co., Menlo Park, California. Senior, A.E. (1988). ATP synthesis by oxidative phosphorylation. Physiological Rev. 68, 177-230. Senior, A.E. (1990). The proton-translocating ATPase of Esherichia colt. Ann. Rev. Biophys. Chem. 19,7- 1. 

Uncouplers of oxidative phosphorylation Compounds that uncouple oxidative phosphorylatiou from electron transport in the inner mitochondrial membrane. Most are weak lipophilic acids that can run down the proton gradient across this membrane. 

Figure 12-8. Principles of the chemiosmotic theory of oxidative phosphorylation. The main proton circuit is created by the coupling of oxidation in the respiratory chain to proton translocation from the inside to the outside of the membrane, driven by the respiratory chain complexes I, III, and IV, each of which acts as a protonpump. Q, ubiquinone C, cytochrome c F Fq, protein subunits which utilize energy from the proton gradient to promote phosphorylation. Uncoupling agents such as dinitrophenol allow leakage of H" across the membrane, thus collapsing the electrochemical proton gradient. Oligomycin specifically blocks conduction of H" through Fq.

This can be used in several ways. H+-ATPase plays a key role in oxidative phosphorylation (ATP synthesis) as it transfers protons back into the matrix space with simultaneous synthesis of ATP (with temporary enzyme phosphorylation, cf. page 451). 

Fig. 6.9 The catalysts for denitrification. Nitrate is reduced by a molybdenum enzyme while nitrite and oxides of nitrogen are reduced today mainly by copper enzymes. However, there are alternatives, probably earlier iron enzymes. The electron transfer bct complex is common to that in oxidative phosphorylation and similar to the bf complex of photosynthesis, while cytochrome c2 is to be compared with cytochrome c of oxidative phosphorylation. These four processes are linked in energy capture via proton (H+) gradients see Figure 6.8(a) and (b) and the lower parts of Fig. 6.9 which show separately the active site of the all iron NO-reductase, and the active site of cytochrome oxidase (02 reductase).

The chemi-osmotic theory of oxidative phosphorylation has been reviewed,74 a model for mitochondrial oxidative phosphorylation in which a membrane potential or proton gradient might transmit energy from an oxidation step to ATP synthesis has been proposed,76 and adenine nucleotide transport in mitochondria has been reviewed.76... 

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