Regulatory electron transport pathways in cyclic photophosphorylation: Reduction of C-550 and cytochrome b6 by ferredoxin in the dark

Abstract

Recent work [l-4] has strengthened and extended in several important respects the concept that ferredoxin is the physiological catalyst of cyclic photophosphorylation in chloroplasts and that this type of phosphorylation is a source of ATP needed for COZ assimilation, protein synthesis and other energydependent reactions [S-IO]. As reviewed in [2,1 A 1, noncyclic photophosphorylation alone cannot fully meet the ATP requirements of CO, assimilation because of the marked excess of ATP over NADPH that is needed: Cq plants require 5 ATPand Cs plants require 3 ATP per 2 NADPH used in the assimilation of 1 COZ to the level of sugar phosphate (cf. [ 121). The recent findings pertaining to cyclic photophosphorylation in chloroplasts may be summarized as follows: (i) Cyclic photophosphorylation by chloroplasts proceeds optimally in the presence of air and is catalyzed by low concentrations of ferredoxin (10 PM), the same as those required for NADP’ reduction [ 1,2]; (ii) Pseudocyclic photophosphorylation, which depends on electron transport from photoreduced ferredoxin to oxygen [7], is of minor importance as a source of ATP in chloroplasts [ 1,4]; (iii) In the presence of NADP+, the ferredoxin-depen-