Free energy changes and metabolic regulation in steady-state photosynthetic carbon reduction

Abstract

The standard physiological free energy changes of reactions of glycolysis, the reductive pentose phosphate cycle (photosynthetic carbon reduction cycle) and the oxidative pentose phosphate cycle have been calculated from available data. The concentrations of metabolites of the photosynthetic carbon reduction cycle, measured during steady-state photosynthesis in Chlorella pyrenoidosa in the presence of radioactive tracers, and the concentrations of some intermediates of the oxidative pentose phosphate cycle measured during a subsequent dark period, have been employed to calculate the free energy changes of each reaction of the reductive cycle and of some of the reactions of the oxidative cycle during steady-state light and dark conditions. With respect to the magnitude of the negative free energy change, at steady state, such reactions have been found to be of two types. Those with high negative free energy changes (−6 to −11 kcal) are in each case reactions from which there exists independent evidence of a role in metabolic regulation. Those with small negative free energy changes (o to −2 kcal) are not regulated reactions and are highly reversible. Thus most of the negative free energy change occurring under steady-state conditions in this metabolic system is dissipated for purposes of control. By the criterion of negative free energy change, ribulosediphosphate carboxylase, and fructose- and heptosediphosphatases are regulated enzymes. The activities of these enzymes are known to be high in the light and low in the dark. Phosphoribulokinase, which mediates the one reaction with an intermediate negative free energy change (−3.82 kcal) also may be a regulated enzyme with greater activity in the light than in the dark. In the oxidative cycle, the reaction mediated by glucose-6-phosphate dehydrogenase has a very high negative free energy change and appears to be active in the dark and inactive in the light. One function of these controls is thought to be the exclusive operation of the reductive cycle in the light and the oxidative cycle in the dark.