Quantitative evaluation of the rate of 3-phosphoglycerate reduction in chloroplasts

Abstract

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 thermodynamic equilibrium and that of NADP-linked glyceraldehyde 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 K-m for 1,3-bisphosphoglycerate is evaluated to be 0.95 mu M in isolated chloroplasts. The temperature dependence of the activity of NADP-linked glyceraldehyde 3-phosphate dehydrogenase is determined; the Q(10) is 1.8 between 15 degrees and 25 degrees 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.