• R Bhardwaj

      Articles written in Journal of Biosciences

    • Mechanism of reductive photoactivation of enzymes of C4 pathway

      V Maheshwari U Dwivedi R Bhardwaj Rashmi Mishra

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      Light, besides initiating primary photochemical processes, alters the redox state of soluble components in chloroplast. The present review attempts to cover the mechanism of reductive photoactivation of enzymes of photosynthetic carbon reduction cycle using key enzymes as examples. The reduced soluble components — ferredoxin, thioredoxin and NADPH, in turn, cause the reduction of disulphides to dithiols of chloroplastic enzymes. NADP-malate dehydrogenase is subject to activation by light through changes in NADPH/NADP. The key enzyme of C4 photosynthesis-PEP carboxylase, though cytosolic, has been shown to be activated by disulphide/sulphhydryl interconversion by reductants generated in light through chloroplast electron transport flow. PyruvatePi dikinase activity is controlled by the adenylate energy charge. It remains unclear how light controls the activation of cytosolic enzymes.

    • Cytochrome b6-f complex : The carburettor of exciton distribution in oxygenic photosynthesis

      U Dwlvedi R Bhardwaj

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      Efficient oxygenic photosynthesis not only requires synchronous turover and operation of photosystem I (PS I) and photosystem II (PS II) but also the preferential turnover of PS I for cyclic photophosphorylation to maintain required ATP and NADPH ratio during carbon dioxide reduction. Ohe initial higher rate of turnover of PS IIin viva is accounted by the fact that (i) PS I contains only about one-third of total chlorophylls, (ii) about 90% of light harvesting a/b protein (LAC) which accounts for about 50% of the total chlorophylls, remains associated with PS II as PS II-LHC II complexes (PS IIα and (iii) the ratio of PS II/PS I is always greater than unity, in the range of 1–2 : 1 under different environmental regimes. Ohe initial preferential feeding of PS II, due to its larger antenna, is bound to result in faster rate of turn over of PS II than PS I, leading to higher rate of reduction of an intersystem carrier than the rate of its oxidation by PS I. Ohe light dependent phosphorylation of a ‘mobile’ and small pool (−20%) of LHC II of PS IIα (possibly located at the edge of appressed regions of the membranes) increases the repulsive forces of LHC II resulting in its migration to non-appressed region associating itself with PS 1. Ohe phosphorylation itself is controlled by the redox state of an intermediate of electron transport.

      Several experimental approaches have provided evidence which suggest that (i) phosphorylation of LAC II involves interaction of cyt b5-f complex with LAC II kinase and the interaction of QA with cyt b5-f complex and (ii) different kinases may be involved in phosphorylation of LHC IIversus PS II polypeptides. Ohe major purpose of light dependent LAC II phosphorylation and its consequent migration close to PS I appears to balance the rate of cyclicversus non-cyclic photophosphorylation. Ohe mechanism by which cyt b5-f complex controls the activation of LAC II is not known. Ohe role of membrane bound ealmodulin, electron transfer through cyt b6-f complex in activation of LAC II kinase should be explored.

    • D1 protein of photosystem II: The light sensor in chloroplasts

      U Dwivedi R Bhardwaj

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      Light, controls the “blueprint” for chloroplast development, but at high intensities is toxic to the chloroplast. Excessive light intensities inhibit primarily photosystem II electron transport. This results in generation of toxic singlet oxygen due to impairment of electron transport on the acceptor side between pheophytin and QB -the secondary electron acceptor. High light stress also impairs electron transport on the donor side of photosystem II generating highly oxidizing species Z+ and P680+. A conformationsl change in the photosystem II reaction centre protein Dl affecting its QB-binding site is involved in turning the damaged protein into a substrate for proteolysis.

      The evidence indicates that the degradation of D1 is an enzymatic process and the protease that degrades D1 protein has been shown to be a serine protease Although there is evidence to indicate that the chlorophyll a-protein complex CP43 acts as a serine-type protease degrading Dl, the observed degradation of Dl protein in photosystem II reaction centre particlesin vitro argues against the involvement of CP43 in Dl degradation. Besides the degradation during high light stress of Dl, and to a lesser extent D2-the other reaction centre protein, CP43 and CP29 have also been shown to undergo degradation.

      In an oxygenic environment, Dl is cleaved from its N-and C-termini and the disassembly of the photosystem II complex involves simultaneous release of manganese and three extrinsic proteins involved in oxygen evolution. It is known that protein with PEST sequences are subject to degradation; D1 protein contains a PEST sequence adjacent to the site of cleavage on the outer side of thylakoid membrane between helices IV and V.

      The molecular processes of “triggering” of Dl for proteolytic degradation are not clearly understood. The changes in structural organization of photosystem II due to generation of oxy-radicals and other highly oxidizing species have also not been resolved. Whether CP43 or a component of the photosystem II reaction centre itself (Dl. D2 or cy1 b559 subunits), which may be responsible for degradation of Dl, is also subject to light modification to become an active protease, is also not known. The identity of proteases degrading Dl, LHCII and CP43 and C29 remains to be established

    • Alteration in the acceptor side of photosystem II of chloroplast by high light

      U Dwivedi R Bhardwaj M Sharma

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      The effect of high light on the acceptor side of photosystem II of chloroplasts and core particles of spinach was studied. BothVmax and apparentKm for DCIP were altered in photoinhibited photosystem II core particles. The double reciprocal plot analysis as a function of actinic light showed increased slope in chloroplasts photoinhibited in the presence of DCMU. Exposure of chloroplasts to high light in the presence of DCMU did not protect the chloroplast against high light induced decrease in Fm, level. Further the high light stress induced decrease inFm level was not restored by the addition of DCMU. These results suggest that the high light stress induced damage to chloroplast involves alteration in the binding site forQB on the DI protein on the acceptor side of photosystem II

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