Existence of Two Types of Electrostatic Interaction in Reactions Catalyzed by Ferredoxin-NADP^+ Oxidoreductase

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Effects of salt and pH on diaphorase and NADP^+ photoreduction activities were studied with broken spinach chloroplasts and purified ferredoxin-NADP^+ oxidoreductase. Two types of electrostatic interactions, which regulate the reaction rate, were observed. One is the long-range electrostatic interaction which determines the local concentrations of reactants in the surface-mediated processes due to the change in the surface potential. In addition to the salt-induced change in the reaction rate, the pH optimum shift by salt was also remarkable: the optimum pH in the diaphorase activity of chloroplasts shifted to the more acidic pH region with an increase of salt concentration, while that of the membrane-free enzyme was not affected by salts. A more specific, short-range electrostatic interaction in reactions between NADP(H) and ferredoxin-NADP^+ oxidoreductase was observed. This interaction became clearer when fixed charges on the membrane surface were masked by an addition of salts. Complete dissociation of the 2'-phosphate group of NADP(H) was necessary for its association with the enzyme. The electrostatic attraction between the negatively-charged part of INTADPH and the positively-charged part of the enzyme (probably lysyl and arginyl residues) may play a role in the short-range interaction.
日本植物生理学会の論文