Why an increase in activity of an enzyme in the Calvin Benson Cycle does not always lead to an increased photosynthetic CO2 uptake rate? – A theoretical analysis

Overexpressing Calvin Benson cycle (CBC) enzyme shown to limit the flow of CO2 through the cycle is a major approach to improve photosynthesis. Though control coefficients of CBC enzymes vary under different environmental and developmental conditions, it is usually implicitly assumed that enzymes in the CBC have a monotonic impact on the CBC fluxes. Here, with a dynamic systems model of the photosynthetic carbon metabolism, we show that, for glycerate-3-phosphate kinase (PGAK), fructose-1,6-bisphosphatase (FBPase), fructose-1,6-bisphosphate aldolase (FBA) and transketolase (TKa), individually increasing activity of these CBC enzymes theoretically leads to an initial increase then decrease in the fluxes through the CBC. Also, the inhibition constants of ADP for PGAK and of F6P for FBPase influence the CBC flux in a biphasic manner. These predicted enzymes showing a biphasic manner are always located in different sub-cycles of the CBC, which consume the shared substrates in the early steps in the CBC and produce intermediates used as substrates for enzymes in the later reactions. We show that the excessive increase in activities of enzymes in one sub-cycle consuming the shared metabolite could cause low concentrations of metabolites in the other sub-cycles, which results in low reaction rates of the later reactions and hence lowers overall CBC flux. This study provides a model to explain the underlying reasons that overexpression of enzymes in the CBC sometimes can negatively impact photosynthesis. We find that balanced activities of enzymes in the sub-cycles of the CBC are required to gain a higher efficiency of the CBC.