Steady-state kinetics of the photosystem I reaction in chloroplasts of Dunaliella which contain variable concentrations of plastocyanin.

The endogenous plastocyanin (PC) concentrations of Dunaliella cultures were varied from 0.3 to 3.1 molecules per pigment 700 (P700) by decreasing the Cu(+) supply of the nutrient. With these cultures the amount of PC which is sufficient for maximum photosynthesis in intact cells was determined to be about 1 to 1.5 PC/P700. Chloroplasts were also prepared from these cells and were employed in enzyme kinetic measurements of the PSI reaction from ascorbate reduced diaminodurene (DAD) to methylviologen/O2. The k m value for DAD in this reaction was 106 μM. A decrease of the endogenous PC concentration caused no change of the k m value but affected the V max in the DAD-dependent reaction. A similar interference of the PC concentration on the maximum reaction rate could also be observed when the light intensity was varied.