| Abstract: | The method of centrifugation of chloroplast thylakoids through silicone fluid, previously used to estimate the uptake of solutes by thylakoids, is shown to be an excellent method for measuring binding of nucleotides to thylakoids. This binding, which is probably an exchange (Harris, D. A. and Slater, E. C. (1975) Biochim. Biophys. Acta 387, 335-348), is enhanced by light and is sensitive to uncoupling. Half-maximal binding of adenosine 5'-triphosphate (ATP) or adenosine 5'-diphosphate (ADP) at 10 mjM was reached within less than 0.1 s. With illumination times sufficient to elicit maximal binding, saturation of the site(s) is approached at 20 muM nucleotide and dissociation constants of 5 muM and 7 muM were calculated for ADP and ATP, respectively. At saturation, the binding corresponds to 1 mol/mol of coupling factor 1 or less. Although the light-dependent binding of ADP does not require Mg2+, that of ATP is markedly enhanced by Mg2+. A 10-fold molar excess of guanosine di- or triphosphate or adenyl-5'-yl imidodiphosphate had little effect on the binding. Adenosine 5'-phosphosulfate, a competitive inhibitor of phosphorylation with respect to ADP, decreases the binding. Thylakoids, previously illuminated in the absence of added nucleotides, retain the capacity to bind ADP or ATP in the dark long after the H+ electrochemical gradient has decayed. The conformation of coupling factor 1 in darkened thylakoids following illumination in the absence of added nucleotides may thus differ from that in thylakoids either illuminated in the presence of nucleotides or kept in the dark. Approximately 20% of the ADP bound to coupling factor 1 in thylakoids is converted to ATP by a 2-s illumination. Bound inorganic phosphate, derived either from ATP or from inorganic phosphate itself, serves as the phosphoryl donor. Bound ADP may, therefore, be of catalytic significance in the mechanism of phosphorylation. |
