Optimization of CO2 fixation in photosynthetic cells via thermodynamic buffering

Stable operation of photosynthesis is based on the establishment of local equilibria of metabolites in the Calvin cycle. This concerns especially equilibration of stromal contents of adenylates and pyridine nucleotides and buffering of CO2 concentration to prevent its depletion at the sites of Rubisco. Thermodynamic buffering that controls the homeostatic flux in the Calvin cycle is achieved by equilibrium enzymes such as glyceraldehyde phosphate dehydrogenase, transaldolase and transketolase. Their role is to prevent depletion of ribulose-1,5-bisphosphate, even at high [CO2], and to maintain conditions where the only control is exerted by the CO2 supply. Buffering of adenylates is achieved mainly by chloroplastic adenylate kinase, whereas NADPH level is maintained by mechanisms involving alternative sinks for electrons both within the chloroplast (cyclic phosphorylation, chlororespiration, etc.) and shuttling of reductants outside chloroplast (malate valve). This results in optimization of carbon fixation in chloroplasts, illustrating the principle that the energy of light is used to support stable non-equilibrium which drives all living processes in plants.