Electron Transport and Photophosphorylation in Chloroplasts as a Function of the Electron Acceptor

Abstract

Electron acceptors may be divided into three distinct classes on the basis of the nature of the electron transport and phosphorylation processes which accompany their reduction by illuminated chloroplasts. Class I acceptors are reduced slowly in the absence of phosphorylation. The addition of ADP and Pi usually increases the rate 2- to 3-fold and up to 1.3 molecules of ATP are formed for each pair of electrons transferred to the acceptor. Uncouplers such as methylamine increase the rate of reduction still further. Most of the electron acceptors heretofore employed in studies of noncyclic photophosphorylation (ferredoxin, ferricyanide, viologens, and flavins) belong to this class. All are ionic or otherwise polar substances with negligible lipid solubilities. Class II acceptors are reduced rapidly whether or not phosphorylation occurs and the addition of uncouplers often does not increase the rapid rate of reduction. Little or no ATP is formed. Substances such as phenolindophenols belonging to this class have dual functions, serving both as electron acceptors and as uncouplers. They are weak acids, lipid-soluble in their nonionized, protonated forms. Their uncoupling function is enhanced as the pH is lowered and the amount of lipid-soluble acid increases. Class III acceptors are also reduced rapidly whether or not phosphorylation occurs and uncouplers again cause little or no increase in the rate of reduction. However, when ADP and Pi are added in the absence of uncouplers a high rate of phosphorylation is observed. The phosphorylation associated with Class III acceptors is usually more rapid than is the phosphorylation associated with Class I acceptors, especially at suboptimal pH. Nevertheless, over a wide range of conditions, the efficiency of phosphorylation in terms of electrons transported (P:e2) is rather precisely half of the efficiency observed when Class I acceptors are reduced. All nonionic, lipid-soluble acceptors we have investigated belong to Class III. Thus the lipid-soluble p-benzoquinone is a Class III acceptor but the lipid-insoluble p-benzoquinone sulfonate is a Class I acceptor. From these observations we conclude that electrons are transported to Class I acceptors through two sites of phosphorylation whereas the transport of electrons to Class III acceptors utilizes only one of the sites. Presumably lipid-soluble acceptors have access to and accept electrons from carriers which normally transfer electrons between the two sites of phosphorylation.