The steady state kinetics of photophosphorylation

The steady state kinetics of phenazine methosulfate-catalyzed, photosystem I-driven photophosphorylation with spinach thylakoid membranes has been measured. The Km for ADP is 85 +/- 20 microM and the Km for phosphate is 0.58 +/- 0.11 mM. Adenosine-5'-O-(thiodiphosphate) (beta-ThioADP) is a competitive inhibitor with respect to ADP at high phosphate concentrations (Kis = 105 +/- 14 microM), but is a noncompetitive inhibitor with respect to phosphate (at all ADP concentrations) and ADP at low phosphate concentrations. Thiophosphate is a competitive inhibitor with respect to phosphate (Kis = 95 +/- 9 microM), but an uncompetitive inhibitor with respect to ADP. ADP also inhibits the rate of phosphorylation at high ADP to phosphate ratios. An ordered Bi Uni mechanism with ADP preceding the binding of phosphate is proposed to explain the kinetic data.