Regulation of the Calvin cycle for CO2 fixation as an example for general control mechanisms in metabolic cycles

The theory of a metabolic cycle with the main portion of its intermediates remaining inside the cycle during one turnover has been developed. On this basis, the regulation of the Calvin cycle is analyzed. It is demonstrated t hat not only the reactions of non-equilibrium enzymes, as the carboxylation of ribulose 1,5-bisphosphate, but reactions that operate close to a thermo dynamic equilibrium, especially the reduction of 3-phosphoglycerate and the transketolase reaction can significantly influence the total turnover peri od in the Calvin cycle. The role of compensating mechanisms in the maintena nce of the photosynthesis rate upon changes of environmental conditions and of enzyme contents is analyzed for the Calvin cycle. It is shown that the change of the total quantity of the metabolites is one of the main self-reg ulated mechanisms in the Calvin cycle. A change of the ATP/ADP ratio can be used by the cell to maintain the CO2 assimilation rate, when the total qua ntity of the metabolites is changed. The developed analysis permits to expl ain some experimental data obtained with transgenic plants with restricted efflux of carbon from the chloroplasts. (C) 1999 Elsevier Science Ireland L td. All rights reserved.