Abstact

BACKGROUND Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) catalyzes the addition of CO2 to ribulose 1,5-bisphosphate in all photosynthetic organisms. During catalysis, the bisphosphate is depleted by reactions other than carboxylation and some of the products are potent inhibitors of rubisco. We have used one of these, xylulose 1,5-bisphosphate as an analogue of the natural substrate and co-crystallized it with the enzyme. RESULTS We have solved the crystal structure of Synechococcus rubisco with bound xylulose 1,5-bisphosphate to 2.3 A and compared it with the previously solved 2'-carboxylarabinitol 1,5-bisphosphate (2CABP) enzyme quaternary complex. Unlike 2CABP, xylulose 1,5-bisphosphate forms a binary complex with no activating CO2 or essential metal present. Five flexible elements that restrict access to the active site in the 2CABP complex also close off the active site in the xylulose 1,5-bisphosphate complex, stabilized by interactions with the hydrated form of the analogue. CONCLUSIONS Xylulose 1,5-bisphosphate induces closure of critical loops of the protein without essential cofactors resident at the active site. In the case of rubisco in one species, catalysis is completely inhibited.