Quantitative Evaluation of the Rate of 3-Phosphoglycerate Reduction in Chloroplasts

The 3-phosphoglycerate reduction has often been considered as a rate-limiting step in photosynthesis. To investigate this, a kinetic equation for 3-phosphoglycerate reduction is developed. The reaction catalyzed by 3-phosphoglycerate kinase was considered as close to the thermo-dynamic equilibrium and that of NADP-linked glyceral-dehyde 3-phosphate dehydrogenase as Michaelis-Menten type. Isolated intact spinach chloroplasts were used to obtain the data that are required to apply the equation for a description of photosynthesis in vivo. The apparent Km for 1,3-bisphosphoglycerate is evaluated to be 0.95 μM in isolated chloroplasts. The temperature dependence of the activity of NADP-linked glyceraldehyde 3-phosphate dehydrogenase is determined; the Q10 is 1.8 between 15° and 25°C. Numerous examples are presented where the developed kinetic equation is used to describe the reaction rate in the isolated chloroplast as well as in the leaf under steady-state and induction conditions. It is shown that within the physiological range, the rate of 3-phosphoglycerate reduction can be estimated by measuring the activity of NADP-linked glyceraldehyde 3-phosphate dehydrogenase,. the [ATP]/[ADP] ratio and the 3-phosphoglycerate concentration. It is shown that the assumption of a thermodynamic equilibrium at the 3-phosphoglycerate reduction cannot be reconciled with existing experimental data. The possible limiting role of NADP-linked glyceraldehyde 3-phosphate dehydrogenase in the Calvin cycle is discussed.