Non-cyclic electron flow

Fig. 3. Five possible fates of the primary oxidant, P700, and the primary reductant, P430", formed in the photochemical reaction in pholosystem I in D144 particles. (A) Charge recombination (B) Cyclic electron flow (C) Non-cyclic electron flow (D) Photoaccumulation of P700 and (E) Photoaccumulation of P430. Each case is illustrated with experimental results with PS-I D144 particles. From Ke (1973) The primary electron acceptor of photosystem I. Biochim Biophys Acta 301 1 -33. See references in this review for the original data sources.

The work done in Ziegler s laboratory has shown that irradiation of green leaves and isolated chloroplasts causes a rapid and reversible increase (up to 4,5-fold) in the activity of the NADP-dependent GAld3P dehydrogenase. The in vivo light-induced activation of the enzyme is closely connected with the operation of the non-cyclic electron flow of photosynthesis, as shown by experiments with inhibitors. NADPH causes a specific activation (up to about 10-fold) of the inactive dark-enzyme , and ATP is also a similarly eflFective activating agent in vitro. Anderson has achieved the conversion of one electrophoretic-... 

Vennesland s group has concluded that the inactivation in vivo after the cells have been treated with ammonia involves the formation of a firmly bound complex of reduced enzyme and cyanide. They have speculated that the product of ammonium assimilation which inhibits nitrate reductase is cyanide. In this respect the COj requirement for inactivation by ammonia in the light could be relevant. On the other hand, COg could potentiate the stimulating effect of ammonia on the photosynthetic non-cyclic electron flow.< If this interpretation would be correct, the activation of the ADP-dependent pyruvate kinase reaction by ammonia discovered by Bassham in Chlorella might be better explained by an increase in the cellular ADP level induced by the uncoupling effect of ammonia on photophosphorylation than by a direct activation of the kinase by the ammonium cation, as it has been postulated. 

Thus, one can expect that, due to their different composition in acyl lipids, the two monolayers will be assigned different roles in the thylakoid membrane function. One of the approaches to test this hypothesis has been to use the acyl lipid depletion technique in which special precautions have to be taken to remove all hydrolysis products (free fatty acids and lysoderivatives) which could alter membrane function [6]. Recent evidence concerning the role of phospholipids in sustaining the uncoupled non-cyclic electron flow activity is that several distinct populations of PG and PC have to be considered [3,6]. A first one, which is easily accessible to phospholipase A2, supports only about 15% of the activity. A second phospholipid population which is less accessible to phospholipase A2 sustains the remaining 85% of the activity. Finally, a third population of phospholipids does not seem to be involved in the uncoupled non-cyclic electron flow activity. These results and several other reports using mainly reconstitution procedures [4,5] point to the fact that acyl lipids may sustain the photosynthetic membrane function. 

Fig. 2 Dependency of uncoupled non-cyclic electron flow activity on various PG populations in the outer and inner thylakoid membrane leaflets. PG depletion refers to either depletion per se (steps A and D) or to delocalization of inner PG to the outer leaflet (steps B and C). See explanations in the text.

These results suggest that as far as the uncoupled non-cyclic electron flow activity (H20/NADP ) is dependent on PG, one can distinguish between two PG populations the first one, weakly involved, located in the outer and the second one, strongly involved, located in the inner monolayer of the thylakoid membrane. In addition, the inner PG population is likely to contain several subpopulations of PG molecules, some of them being discrete but very efficient in sustaining electron flow activity, in agreement with earlier suggestions [3,5,6]. 

Fig. 2. Kinetics of the hydrolysis of PG and PC in phospholipase A2-treated thylakoid membranes (A) and of the uncoupled non-cyclic electron flow activity H20/NADP (B). activity following BSA addition. 

Siegenthaler PA, Rawyler A, Smutny J. The phospholipid population which sustains the uncoupled non-cyclic electron flow activity is localized in the inner monolayer of the thylakoid membrane. Biochim Biophys Acta 1989 975 104-111. 

A high NADPH/NADP ratio generated by photoreduction of NADP with NADH (or H2) is a prerequisite for photosynthetic electron transport to nitro-genase. The NADPH/NADP ratio may balance cyclic and non-cyclic electron flow around PS I, adjusting the optimum ATP/e-ratio for nitrogenase (or other biosynthetic activities) to operate. A fixed spatial arrangement of FNR between ferredoxin and the b /f-complex (Fig.4 see Carrillo,... 

Table 2 summarizes the effects of inhibitors of photosynthesis and PS-I electron transport mediators on the accumulation of inorganic carbon and evolution. The accumulation of inorganic carbon was insensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea(DCMU), an inhibitor of PS-II and non-cyclic electron transport. This indicates that the HCD3 transport does not require non-cyclic electron flow from PS-II, being consistent with the action spectra for HCO3 transport. The HCO transport was accelerated by the mediator of PS-I cyclic electron transport and cyclic photophosphorylation, phenazine... 

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