INSIGHTS INTO THE MECHANISM OF RUBISCO. Thomas C. Taylor, Inger Andersson. Swedish University of Agricultural Science, Uppsala, Sweden

So far little has been known about the precise mechanism by which Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase) is able to catalyse the enolisation, carboxylation and hydrolysis of its substrate. We have collected data from several complexes of Rubisco in both the non-active and the active forms with the substrate; the inhibitors 2-carboxyarabitinol bisphosphate (2-CABP), 4-CABP, and xylulose bisphosphate (XuBP); with the product, 3-phosphoglycerate, and also the unliganded active enzyme. With this information it has been possible to identify residues in the active site capable of assisting the chemistry, to identify active site loop movements necessary for correct catalysis and to propose a catalytic mechanism for the carboxylation reaction.

Rubisco is activated by the post-translational carbamylation of a specific lysine residue. The carbamylate, along with an aspartate and a glutamate coordinates a magnesium ion. The magnesium ion and the carbamate appear to play a key role in the reaction. Activation does not seem to affect the conformation of the enzyme. In the unliganded form the large subunit N-terminus and C-terminus along with an active site loop (Loop 6) are disordered and the active site is open to bulk solvent. When liganded with 2-CABP, a reaction intermediate analogue, these disordered regions are locked over the active site thus shielding it from the solvent and protecting the ligand. This is important during the slow reaction cycle to prevent misprotonation of the reaction intermediates.