Electrogenic proton transport

Genetic Approaches to Electrogenic Proton Transport by a Yeast H+-ATPase... 

The plasma membrane H+-ATPase from Saccharomyces cerevisiae is a highly electrogenic proton pump that maintains cellular membrane potentials in excess of — 200 mV. This enzyme is used as a model system to probe the mechanism of electrogenic proton transport because it is readily amenable to genetic, biochemical, and biophysical analysis. The H+-ATPase plays an essential role in intracellular pH regulation and maintenance of electrochemi-... 

Mutations in the Saccharomyces cerevisiae H+-ATPase gene, PMA1, that confer cellular growth resistance to hygromycin B cause a generalized depolarization of cellular membrane potential. The normal hyperpolarized membrane potential in yeast is maintained by the H+-ATPase, and it is believed that the pmal mutations alter electrogenic proton transport by the enzyme. Electroneutral H+ transport by the mutant enzymes may involve the countertransport of K+, but other ions including H+ could participate. More direct evidence is needed to confirm the role of K+ as a counterion and to probe its putative transport mechanism. It will be important to determine whether H + and K+ use the same mechanistic pathway for transport. 

This rotational paradigm probably extends to the V-type ATPases, which are also multi-subunit pumps. These are electrogenic proton-transport multi-subunit ATPases present in a variety of membranes and are responsible for acid secretion into various intracellular compartments (lysosomes, Golgi, neurosecretory granules), into the lumen of the proximal tubule and the renal collecting duct, or across the ruffled border of the osteoclast. In contrast to the F,Fo ATPases, they are unable to synthesize ATP and, hence, operate only in the proton pumping direction. 

The free energy gained from the quinol oxidation inthe cytochrome-6c, complex allows further proton transfer from the cytoplasm to the periplasm. The 6c,-complex also mediates ET to the periplasmic side. There, soluble cytochromes accept the electrons and transport them back to the RC to reduce D+. The electron transfer is cycUc and therefore does not cause transmembrane potential. This potential is generated by the electrogenic proton translocation in the cytochrome-6c, complex. The electrochemical proton gradient is utilized by the ATP-synthase to form adenosine triphosphate from adenosine diphosphate and phosphate. 

Stdn A, Vasudevan G, Carter N et al. Equilibrative nudeoside transporter fiunily members from Leishmania donovani are electrogenic proton symporters. J Biol Chem 2003 278 35127-35134. 

Bakker EP, Mangerich WE. Inconveision of components of the bacterial proton motive force by electrogenic potassium transport. ] Bacieriol 1981 147 820-826. 

Besides the remarks made above, which might notably weaken one s confidence in the electrogenic proton pump theory, let us stress some further complications. First, it is impossible to perform direct proton permeability measurements by the use of tritium in radiotracer methods. Second, several processes might be correlated to the external pH change, like effects caused by CO2 transport, 0H efflux, HCO5 uptake, the permeability of H and ions via pores, and kinetic control of all ionic pumps. 

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