Evidence for Cyclic Photophosphorylation during ¹⁴CO₂ Fixation in Intact Chloroplasts

Abstract

This study examines the effect of antimycin A and nitrite on 14CO2 fixation in intact chloroplasts isolated from spinach (Spiucia oeracea L.) leaves. Antimycin A (2 micromolar) strongly inhibited CO2 fixation but did not appear to inhibit or uncouple linear electron transport in intact chloroplasts. The addition of small quantities (40-100 micromolar) of nitrite A stoichiometric balance of ATP/NADPH in the chloroplast is essential for optimum photosynthesis. It is now widely accepted that such a stoichiometric balance of ATP/NADPH could not be achieved by linear electron transport from H20 to NADP+ alone (1 1). Assuming that the ATP/NADPH ratio generated via linear electron transport is 1.33, the extra ATP needed would presumably be derived from cyclic and/or pseudocyclic electron transport.High rates of 02 uptake have been measured in thylakoid preparations from spinach leaves in the presence of high concentrations of Fd (1; R. T. Furbank and M. R. Badger, unpublished), during photosynthesis in intact chloroplasts and cells (8,9), and in leaves of C3 plants (6,10). The values of 02 uptake measured in these studies are comparable to and, in several cases, exceeded the value (1 1.3% of total 02 evolution) required for the generation of all the extra ATP needed for CO2 fixation. This evidence suggests that pseudocyclic electron transport plays a significant (dominant) role in the ATP/NADPH balance during photosynthesis. Marsho et al. (18) found that, although there was a significant rate of 02 uptake in intact chloroplasts during the lag phase of photosynthesis, this diminished to a relatively insignificant level during steady-state photosynthesis. These low rates of 02 uptake were relatively insensitive to increases in light intensity which greatly stimulated photosynthesis (14). These and a number of other workers (3,4,20,23,32) have provided evidence for cyclic flow during photosynthesis in thylakoids, chloroplasts, and intact leaves. Quantitative measurements of Cyt turnover in intact chloroplasts (26) have also indicated substantial cyclic flow. But the corresponding rates of 02 evolution during these measurements were low. Thus, the relative contribution of cyclic versus pseudocyclic flow to the ATP/NADPH balance during photosynthesis remains unresolved. Furthermore, the reasons for the discrepancies between high (8, 9) and low (14, 18) rates of 02 uptake measured during photosynthesis in chloroplasts and cells remain to be resolved.In thylakoids, antimycin A is a reported potent inhibitor of Fddependent cyclic flow (2) but may function as an uncoupler at higher concentrations (7). In intact chloroplasts, antimycin A stimulated CO2 fixation under anaerobic conditions (19,22,25) and in aerobic conditions which were suboptimal (e.g. high Pi concentration) for CO2 fixation (28). But under aerobic conditions at optimal Pi, antimycin A strongly inhibited photosynthetic CO2 fixation in intact chloroplasts with concomitant decreases in energy-dependent processes such as Chl a fluorescence, 9-aminoacridin...

Regulation of Cyclic Photophosphorylation during Ferredoxin-Mediated Electron Transport

“…Antimycin A strongly inhibits photosynthesis driven by low intensity light, but inhibits only slightly in high intensity light (22). (Woo [24] reports a similar result, but Heber et al [7] found the opposite effect using high and low intensity red light. The reason for this discrepancy is unclear.…”

“…The results of these experiments indicate that two modes of cyclic electron transfer activity, which differ in their antimycin A sensitivity, can operate in the thylakoid membrane. Regulation of these activities can occur at the level of ferredoxin and is governed by the NADPH/NADP ratio.Evidence has accumulated for the existence of PSI cyclic photophosphorylation activity and its importance for CO2 fixation in chloroplasts (6,7,15,24,25). One of the remaining questions about PSI cyclic electron flow pertains to the mechanism by which the chloroplast regulates cyclic and noncyclic electron transport in order to maintain the necessary balance between ATP and NADPH for CO2 fixation activity.…”

Effect of Photon Fluence Rate on Oxygen Evolution and Uptake by Chlamydomonas reinhardtii Suspensions Grown in Ambient and CO2-Enriched Air

“…02 evolution (E.) clearly reflects photolysis of water in PSII (Hill reaction) (14). It only noncyclic and pseudocyclic but also cyclic transport of electrons may be coupled to ATP formation, although the contribution of the latter two to in vivo assimilation is not clear (9,11,29).…”

Oxygen Metabolism in Chloroplast

“…There were calculations of ATP synthesis coupled with O 2 reduction from the total ATP synthesis during simultaneous reductions of both NADP + and O 2 (Furbank & Badger, 1983;Ivanov et al, 1980;Robinson & Gibbs, 1982). Observed in some studies lower efficiency for ATP production of the pseudocyclic electron transport as compare with the efficiency of the non-cyclic electron transport in C 3 - (Woo, 1983) as well as in C 4 -plants (Ivanov & Edwards, 2000) can be probably explained now as a result of the use of the protons bound by PQH 2 not only for the ΔpH formation across the thylakoid membrane but also for the reduction of O 2…”