Cytochrome function in the cyclic electron transport pathway of chloroplasts

Abstract

Flash excitation of isolated intact chloroplasts promoted absorbance transients corresponding to the electrochromic effect ( P-518) and the α-bands of cytochrome b 6 and cytochrome f. Under conditions supporting coupled cyclic electron flow, the oxidation of cytochrome b 6 and the reduction of cytochrome f had relaxation half-times of 15 and 17 ms, respectively. Optimal poising of cyclic electron flow, achieved by addition of 0.1 μM 3-(3,4-dichlorophenyl)-1,1-dimethylurea, increased phosphorylation of endogenous ADP and prolonged these relaxation times. The presence of NH 4Cl, or monensin plus NaCl, decreased the half-times for cytochrome relaxation to approximately 2 ms. Uncouplers also revealed the presence of a slow rise component in the electrochromic absorption shift, with formation half-time of about 2 ms. The inhibitors of cyclic phosphorylation antimycin and 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone abolished the slow rise in the electrochromic shift and prolonged the uncoupled relaxation times of cytochromes b 6 and f by factors of ten or more. These observations indicate that cytochrome b 6, plastoquinone and cytochrome f participate in a coupled electron transport process responsible for cyclic phosphorylation in intact chloroplasts. Estimations of cyclic phosphorylation rates from 40 to 120 μmol ATP/mg chlorophyll per h suggest that this process can provide a substantial fraction of the ATP needed for CO 2 fixation.