The stoichiometric relation of phosphorylation to electron transport in isolated chloroplasts

Abstract

Electron transport in chloroplasts can proceed by two pathways. One pathway is dependent on the presence of ADP and orthophosphate and is responsible for photophosphorylation. This pathway is sensitive to the photophosphorylation inhibitor phlorizin, to antimycin A and to p-chloromercuribenzoate (PCMB). The other pathway is independent of phosphorylation, insensitive to phlorizin, relatively insensitive to antimycin A and insensitive to PCMB. Apparently amines increase the electron transport rate by uncoupling the antimycin-sensitive phosphorylating pathway while carbonylcyanide phenylhydrazones uncouple by increasing the activity of the antimycin-resistant non-phosphorylating pathway. During photophosphorylation the electron transport rate normally exceeds the corresponding rate measured in the absence of phosphate and, under many conditions, the amount of ATP formed is proportional to the additional electron transport which occurs in the presence of phosphate. Indeed there is often a rather precise mole for mole correspondence between ATP formation and the accompanying increase in ferricyanide reduction. This is true when phosphorylation conditions are optimal and it is also true when the phosphorylation rate is varied over a wide range by limiting concentrations of phosphate, by the addition of antimycin A, by the addition of PCMB, or by the addition of carbonylcyanide phenylhydrazones. It is not true unless the rate of electron transport is controlled at the level of the phosphorylation reaction. Thus the correspondence between phosphorylation and extra electron transport is no longer observed when electron transport is limited by low light intensities or by the addition of phenylureas. Under these conditions phosphorylation may greatly exceed the increase in electron transport associated with phosphorylation. Moreover the correspondence is not always found when the chloroplasts have been partially uncoupled by amines such as Tris or methylamine. Our observations suggest that, at high light intensities and in the absence of amine uncouplers, the phosphorylating electron transport is superimposed on a constant non-phosphorylating electron transport. If this is so the phosphorylating process must yield two molecules of ATP for every pair of electron it transfers.