Activation of Rubisco from non-green algae

Abstract

The CO2 fixing enzyme d-ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) requires both CO2 and Mg2+ for its activation. A molecule of CO2, distinct from the substrate CO2, binds to the e -lysyl amino group of a conserved lysine residue at the active site, forming a carbamate that completes an anionic binding site for the Mg2+. This metal-coordinated carbamate appears to be the general base that carries out several proton abstractions that are essential to the catalytic sequence, a function without precedent elsewhere in biology. Formation of the activated enzyme-CO2-Mg2+ (ECM) complex reaches an equilibrium governed by the CO2 and Mg2+ concentrations: The product of the dissociation constants for CO2 and Mg2+, KcKMg, therefore dictates the extent of activation of Rubisco at particular CO2 and Mg2+ concentrations. It has recently been found that certain non-green algae have Rubiscos with more efficient kinetic properties than those of higher-plant Rubiscos. Consequently, attention has focused on these Rubiscos as potential candidates for transfection into higher plants. Nothing, however, is known about the activation properties of non-green-algal Rubiscos and whether they would be suitable to the conditions prevailing in higher-plant chloroplasts. The carbamylation equilibrium of Rubisco was measured under varying CO2 and Mg2+ concentrations using a method that exploits the difference in binding affinity of the reaction-intermediate analog, 2¿-carboxy-d-arabinitol-1,5-bisphosphate, to carbamylated and uncarbamylated active sites. A comparison of data for non-green-algal and higher-plant Rubiscos will be presented.