Initial events of light-dependent ATP synthesis in spinach subchloroplast particles

Enzymatic activities in thylakoid membranes, which form medium [32P]NDP and [32P]ATP from 32Pi. Polynucleotide phosphorylase and adenylate kinase

Soluble chloroplast coupling factor 1 (CF1) and the ATP synthase complex, under uncoupled conditions, can form bound ATP from tightly bound ADP and medium Pi. This partial reaction is a powerful probe of the mechanism of ATP synthesis. During our study of the synthesis of bound ATP by CF1 other enzyme activities, which generate [32P]nucleotides from 32Pi, were characterized and controlled. Two enzymes present at significant levels in the preparations are polynucleotide phosphorylase and adenylate kinase. Polynucleotide phosphorylase (PNPase) was found both in thylakoid and CF1 preparations and catalyzed the formation of [beta-32P]ADP via its Pi----ADP exchange activity. The formation of [beta-32P]ADP during net photophosphorylation is attributable to adenylate kinase action on the [32P]ATP formed since hexokinase and glucose effectively block its production. In addition, PNPase also degraded RNA present in thylakoid preparations yielding all four [32P]nucleoside diphosphates. PNPase was also shown to catalyze a Pi----ATP exchange that is dependent on RNA primers and other cofactors.

Source of rapidly labeled ATP tightly to non-catalytic sites on the chloroplast ATP synthetase

Bound [32P]ATP is found on deenergized, washed chloroplast thylakoids which were illuminated in the presence of ADP and [32P]Pi. Tight binding of [32P]ATP occurred both during and after energization. Different classes of bound [32P]ATP were distinguished on the basis of their rates of formation, susceptibility to hexokinase and displacement by unlabeled ATP. 1. The rates of formation and discharge of the rapidly labeled tightly bound ATP class were much lower than that of ATP formation. The level of this bound ATP saturates at lower concentrations of substrates than does the rate of phosphorylation. Unlabeled ATP, present in the reaction medium, displaces the rapidly labeled tightly bound ATP without affecting the rate of phosphorylation. 2. We therefore conclude that the rapidly labeled bound ATP class does not fulfill the requirements expected for a catalytic intermediate and that the nucleotide tight binding site(s) on the ATP synthetase differ from the catalytic site(s) for ATP formation. 3. Since the rapidly labeled tightly bound [32P]ATP is not abolished by high concentrations of hexokinase, but is nevertheless displaced by exogenous ATP, we propose that tight binding of ATP to non-catalytic sites occurs via a free species of newly synthesized ATP which diffuses slowly to the medium from a space accessible to ATP but not to hexokinase.

Relation between the initial kinetics of ATP synthesis and of conformational changes in the chloroplast ATPase studied by external field pulses

ATP formation and the energy-dependent release of tightly bound [14C]-adenine nucleotides from the chloroplast coupling factor CF1 has been studied as a function of the time of energization of the membrane in the range of 500 mus up to 60 ms. The high time resolution was achieved because the energization was generated artificially by external electric field pulses. Applying external electric field pulses to a chloroplast suspension induces an electric potential difference across the thylakoid membrane. The following results were obtained: (1) The amount of ATP generated increases linearly with the time of energization. The steady-state rate of ATP formation is reached in less than 500 mus. (2) A fraction of the adenine nucleotides tightly bound to CF1 is released on energization with a half-rise-time of about 2 ms. The size of the fraction, i.e., the amplitude of the fast phase of the release, increases with the magnitude of the induced transmembrane electric potential difference. A further slow release is superimposed. (3) The initial rate of the release of adenine nucleotides is practically identical with the rate of ATP formation. It is concluded that the release of tightly bound nucleotides monitors an initial conformational change by which the ATPase turns from an inactive into an activated state. For the explanation of the results a reaction scheme is proposed which takes into account a preceding activation of the ATPase.

AMP photophosphorylation and binding by chloroplasts

Stoichiometric amounts of chloroplast thylakoids photophosphorylate free AMP to tightly bound ADP. Free ADP is a poor competitor for this AMP photoreaction, which saturates below 16 micronAMP. The inhibitor, diadenosine pentaphosphate, abolishes AMP photophosphorylation, and inhibits dark ADP binding. Taken together, these data imply that this photoreaction involves the high affinity nucleotide binding site(s) of chloroplast coupling factor CF1, and that little mixing with free nucleotides occurs.