Ribulose Bisphosphate Carboxylase-Oxygenase: Its Role in Photosynthesis

Synthesis of triose phosphate by the chloroplast requires three substrates: light, CO_2 and orthophosphate (P_i). In the response of the rate of carbon assimilation to the concentration of CO_2, the kinetic properties of RuBP carboxylase-oxygenase (Rubisco) constitute the main limitation at low CO_2 concentrations, while at higher concentrations of CO_2 the limitation is shifted towards the reactions leading to the regeneration of the substrate, RuBP, driven by electron transport. In these circumstances, light or P_i, or both, can become limiting. The characteristics of Rubisco that can affect photosynthesis fall under three main headings: (1) amount and kinetic constants; (2) activation state; and (3) regulation of catalysis (including the role of effectors, such as P_i and glycerate 3-phosphate (PGA)). These characteristics are analysed, and the role of changes in activity of the enzyme is discussed in the context of limitation and regulation of the photosynthetic process. Other factors considered are the regeneration of RuBP and its relation to electron transport, P_i supply, and photorespiration. The influence that expected increases in atmospheric CO_2 concentration, and/or genetic improvements in the characteristics of the enzyme, may have on the present balance between the partial processes of photosynthesis, is discussed.