ADP KINASE AND ATPASE IN CHLOROPLASTS

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Abstract
Treatment of chloroplasts with trypsin activates a light-requiring ATPase whose properties are strikingly similar to those of the light-requiring ADP kinase of chloroplasts. The observations here presented suggest that there exists, in chloroplasts, a reducible enzyme which, in its reduced state, catalyzes the reversible reaction: P i -2 + ADP -3 + H + ⇌ ATP -4 + H 2 O. By reduction and protonation of the catalytic site of this enzyme, light-driven electron flow in the chloroplast drives the reaction to the right. Hydrolysis of ATP proceeds only when the enzyme is reduced and when the proton concentration within the chloroplast is kept at low levels, viz., in the absence of light, in the presence of uncoupling agents which decrease the concentration of internal H + , or in the presence of electron acceptors which by oxidizing the internal electron acceptors also decrease the proton potential. Activation of the enzyme requires light; it remains active only in the presence of ATP. Hydrolysis of all the ATP results in inactivation of the ATPase. The membrane-bound protein CF 2 limits the reversibility of the reaction by excluding ATP and H 2 O from the enzyme site. It also facilitates the ability of the chloroplasts to accumulate and to maintain high internal concentrations of such ions as ADP, P i , PMS + , and imidazole.