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Query: UMLS:C0847097 (acidity)
 15,165 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Part of the chloroplast photoprotection response to excess light absorption involves formation of zeaxanthin (and antheraxanthin) via the violaxanthin deepoxidase enzyme, the activity of which requires lumen acidity near or below pH 6.0. Clearly, the violaxanthin de-epoxidase activity is strongly regulated because at equivalent energization levels (including the parameters of H(+) accumulation and ATP formation rates), there can be either low or high violaxanthin de-epoxidase enzyme activity. This work shows that the factor or factors responsible for regulating the violaxanthin deepoxidase correlate directly with those which regulate the expression of membrane-localized or delocalized proton gradient (Delta[Formula: see text] (H+)) energy coupling. The most clearly identified factor regulating switching between localized and delocalized energy coupling modes is Ca(2+) binding to the lumen side of the thylakoid membrane; in particular, Ca(2+) binding to the 8 kDA subunit III of the CF(o) H(+) channel. The activity of violaxanthin deepoxidase in pea (Pisum sativa) and spinach (Spinacea oleracea) thylakoids is shown here to be strongly correlated with conditions known from previous work to displace Ca(2+) from the CF(o) H(+) channel and thus to modulate the extent of lumenal acidification while maintaining a fairly constant rate of ATP formation.
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PMID:Influence of Ca(2+) on the thylakoid lumen violaxanthin de-epoxidase activity through Ca(2+) gating of H(+) flux at the CF(o) H(+) channel. 1622 81

The presence of an acidic lumen and the xanthophylls, zeaxanthin and antheraxanthin, are minimal requirements for induction of non-radiative dissipation of energy in the pigment bed of Photosystem II. We recently reported that ascorbate, which is required for formation for these xanthophylls, also can mediate the needed lumen acidity through the Mehler-peroxidase reaction [Neubauer and Yamamoto (1992) Plant Physiol 99: 1354-1361]. It is demonstrated that in non-CO2-fixing intact chloroplasts and thylakoids of Lactuca sativa, L. c.v. Romaine, the ascorbate available to support de-epoxidase activity is influenced by membrane barriers and the ascorbate-consuming Mehler-peroxidase reaction. In intact chloroplasts, this results in biphasic kinetic behavior for light-induced de-epoxidation. The initial relatively high activity is due to ascorbate preloaded into the thylakoid before light-induction and the terminal low activity due to limiting ascorbate from the effects of chloroplast membranes barriers and a light-dependent process. A five-fold difference between the initial and final activities was observed for light-induced de-epoxidation in chloroplasts pre-incubated with 120 mM ascorbate for 40 min. The light-dependent activity is ascribed to the competitive use of ascorbic acid by ascorbate peroxidase in the Mehler-peroxidase reaction. Thus, stimulating ascorbic peroxidase with H2O2 transiently inhibited de-epoxidase activity and concomitantly increased photochemical quenching. Also, the effects inhibiting ascorbate peroxidase with KCN, and the KM values for ascorbate peroxidase and violaxanthin de-epoxidase of 0.36 and 3.1 mM, respectively, support this conclusion. These results indicate that regulation of xanthophyll-dependent non-radiative energy dissipation in the pigment bed of Photosystem II is modulated not only by lumen acidification but also by ascorbate availability.
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PMID:Membrane barriers and Mehler-peroxidase reaction limit the ascorbate available for violaxanthin de-epoxidase activity in intact chloroplasts. 2431 Oct 66