Modelling the reaction mechanism of ribulose-1,5-bisphosphate carboxylase/oxygenase and consequences for kinetic parameters

Although ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) was discovered nearly 60 years ago, the associated chemical mechanism of the reaction is still incompletely understood. The catalytic cycle consists of four major steps: ribulose-1,5-bisphosphate binding, enolization, CO₂ or O₂ addition and hydration, and cleavage of the intermediate. The use of individual rate constants for these elemental steps yields mathematical expressions for usual kinetic constants (k_cat, K_m), CO₂ versus O₂ specificity (S_c/o) as well as other chemical parameters such as the ¹²C/¹³C isotope effect. That said, most of them are not simple and thus the interpretation of experimental and observed values of k_cat, K_m and S_c/o may be more complicated than expected. That is, Rubisco effective catalysis depends on several kinetic parameters that are influenced by both the biological origin and the cellular medium (which, in turn, can vary with environmental conditions). In this brief review, we present the basic model of Rubisco kinetics and describe how subtle biochemical changes (which may have occurred along Evolution) can easily modify Rubisco catalysis. The chemical mechanism of Rubisco is at the heart of photosynthetic metabolism since it dictates the rate of carboxylation. Here, advantage is taken of chemical formalism to give general equations describing carboxylation velocity, specificity and isotope effects. The limits of these and uncertainties on intrinsic chemical events are discussed so as to appreciate possibilities of optimization.