PubMed · 4518402
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D Mauzerall. 1973. Why chlorophyll?. https://doi.org/10.1111/j.1749-6632.1973.tb43231.x
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Some properties of [4-vinyl] chlorophyllide a reductase are described. This enzyme converts divinyl chlorophyllide a to monovinyl chlorophyllide a. The latter is the immediate precursor of monovinyl chlorophyll a, the main chlorophyll in green plants. [4-Vinyl] chlorophyllide a reductase plays an important role in daylight during the conversion of divinyl protochlorophyllide a to monovinyl chlorophyll a. [4-Vinyl] chlorophyllide a reductase was detected in isolated plastid membranes. Its activity is strictly dependent on the availability of NADPH. Other reductants such as NADH and GSH were ineffective. The enzyme appears to be specific for divinyl chlorophyllide a, and it does not reduce divinyl protochlorophyllide a to monovinyl protochlorophyllide a. The conversion of divinyl protochlorophyllide a to monovinyl protochlorophyllide a has been demonstrated in barley and cucumber etiochloroplasts and appears to be catalyzed by a [4-vinyl] protochlorophyllide a reductase [Tripathy, B.C., & Rebeiz, C.A. (1988) Plant Physiol. 87, 89-94]. On the basis of reductant requirements and substrate specificity, it is possible that two different 4-vinyl reductases may be involved in the reduction of divinyl protochlorophyllide a and divinyl chlorophyllide a to their respective 4-ethyl analogues.
Non-radiative dissipation of absorbed excitation energy in chloroplast membranes is induced in the presence of the trans-thylakoid proton motive force; this dissipation is measured as high energy state quenching of chlorophyll fluorescence, qE. It has been suggested that this results from a low pH-induced structural alteration in the light harvesting complex of photosystem II, LHCII [(1991) FEBS Letters 292, 1-4]. The effect of the carboxyl-modifying agent, dicyclohexylcarbodiimide (DCCD), on energy dissipation in chloroplast membranes has been investigated. At concentrations below that required to inhibit electron transport, DCCD caused a decrease in the steady state delta pH, completely inhibited qE and also inhibited the low pH-dependent induction of qE. DCCD binding to polypeptides in the 22-28 kDa range correlated with inhibition of qE. It is suggested that DCCD reacts with amino acid residues in LHCII whose protonation is the primary event in the induction of energy dissipation. This LHCII domain may be identical to one forming a proton channel linking the site of PSII-dependent water oxidation to the thylakoid lumen [(1990) Eur. J. Biochem. 193, 731-736].
The mechanism of D1 protein degradation was investigated during photoinhibitory illumination of isolated photosystem II core preparations. The studies revealed that a proteolytic activity resides within the photosystem II core complex. A relationship between the inhibition of D1 protein degradation and the binding of the highly specific serine protease inhibitor diisopropyl fluorophosphate to isolated complexes of photosystem II was observed, evidence that this protease is of the serine type. Using radiolabeled inhibitor, it was shown that the binding site, representing the active serine of the catalytic site, is located on a 43-kDa polypeptide, probably the chlorophyll a protein CP43. The protease is apparently active in darkness, with the initiation of breakdown being dependent on high light-induced substrate activation. The proteolysis, which has an optimum at pH 7.5, gives rise to primary degradation fragments of 23 and 16 kDa. In addition, D1 protein fragments of 14, 13, and 10 kDa were identified. Experiments with phosphate-labeled D1 protein and sequence-specific antisera showed that the 23- and 16-kDa fragments originate from the N- and C-termini, respectively, suggesting a primary cleavage of the D1 protein at the outer thylakoid surface in the region between transmembrane helices D and E.