The regulation of Rubisco activity

Abstract

The processes of photosynthesis and photorespiration are initiated by Rubisco, but the enzyme must be activated before it will catalyse either the carboxylation or oxygenation of ribulose bisphosphate. Rubisco is activated in vitro by CO ₂ and Mg ²⁺ . The dual roles of CO ₂ as both an activator and a substrate led to anomalously high K _m (CO ₂ ) values until the activation requirement was recognized. During activation, CO ₂ forms a carbamate at the ε-amino of a lysine residue on the large subunit of Rubisco. Under conditions thought to exist in the chloroplast during photosynthesis (10 μm CO ₂ , 5 mM Mg ²⁺ and pH 8.0), Rubisco is only partially active since the K _act (CO ₂ ) is in the range 25-30 μm CO ₂ . Thus the mechanism of activation as deduced from in vitro studies is incomplete. Higher activation levels can be obtained by preincubating Rubisco with phosphorylated metabolites, but these occupy ribulose bisphosphate binding sites and thus inhibit catalysis. Recently, a naturally occurring effector, which binds tightly to Rubisco and inhibits activity, has been found. This compound is synthesized in the dark and metabolized upon illumination, but its identity and physiological function are not yet known. In leaves, Rubisco is nearly fully activated at high light intensities. By analysing an Arabidopsis thaliana mutant deficient in the ability to activate Rubisco, we have determined that a soluble protein is required for the in vivo activation process. This enzyme, designated Rubisco activase, reduces the high K _act (CO ₂ ), observed with the isolated enzyme, to physiological levels in an illuminated reconstituted assay system containing washed thylakoid membranes, Rubisco and ribulose bisphosphate.