Modification of the kinetic parameters of enzymes by protein engineering requires extensive knowledge of the structural details of the enzyme and its complexes with different reaction intermediate analogues. Such structural studies are described here for Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase, which catalyzes the initial reactions of two important but competing physiological events in green plants; carbon dioxide fixation and photorespiration. Observed functional changes in mutants of Rubisco are correlated with structural details as well as with the defined conformational changes that occur during catalysis. A possible functional role of the small subunit of Rubisco is described based on a comparison of the active-site geometry of a bacterial L2 molecule with that of higher plant L8S8 molecules. The ultimate aim of these studies is to engineer Rubisco mutants that are more efficient than the wild-type enzyme by decreasing the oxygenase/carboxylase ratio.