CALCULATIONS ON THE SUBSTRATES OF CITRATE SYNTHASE - I - OXALOACETATE

Ab initio (MP2/6-31+G(d) and RHF/6-31+G(d)) and semi-empirical (AM1, P M3 and MNDO) molecular orbital calculations on oxaloacetate are presen ted. Minimum energy geometries are compared to the structures of oxalo acetate bound to citrate synthase, and also to that of citrate on the enzyme and in small molecule structures. It is found that the form bou nd to citrate synthase is of higher energy than the extended conformat ion that is the gas phase minimum. In particular, the enzyme binds a m ore compact conformation, with the alpha-ketocarboxylate moiety planar , Mechanistic implications of this binding mode are discussed. Compari son with crystallographic structures of citrate suggests that citrate synthase enforces a 'citrate-like' conformation on oxaloacetate at the active site which may assist the Claisen condensation reaction with a cetyl-CoA. Energy barriers to geometrical changes of oxaloacetate are calculated, and atomic charges derived for each conformation. AMI perf orms reasonably well for dianionic oxaloacetate. The results will be u seful in the development of molecular mechanical parameters for oxaloa cetate for use in simulations of citrate synthase/substrate complexes employing empirical potentials. (C) 1998 Elsevier Science B.V.