Modulation of the chloroplast ATP synthetase: Conformational States, Nucleotide Binding and Limited Accessibility to the Active Site

A kinetic scheme for photophosphorylation catalyzed by the chloroplast ATP synthetase is described by Equation 1. This sequence of reactions assumes that binding of ADP occurs prior to that of Pi (Selman, Selman-Reimer, 1981). Thus, the Pi-ATP exchange, one of the so-called partial reactions of photophosphorylation, might occur without significant dissociation of the E•ADP complex and without a concomitant ADP-ATP exchange reaction (k-1 ≪ k1) In this mechanism, formation and hydrolysis of ATP and the accompanying exchange reactions are reversible reactions that occur on the same catalytic site(s) on the energized membrane-bound enzyme (CF1). It has been about ten years since it was reported that the synthesis of ATP and its hydrolysis or the Pi-ATP exchange reaction show a dissimilar substrate specificity. Modified antibodies against CF1 were also shown to inhibit preferentially the Pi-ATP exchange. These and more recent data support the view that different enzyme conformations or altered catalytic sites may participate in the catalysis of ADP (photophosphorylation) and ATP (hydrolysis and exchange) utilizing reactions (Shavit, 1980).