CATALYSIS BY ENTROPIC GUIDANCE FROM ENZYMES

Certain features underlying enzymatic catalysis, such as energetic sta bilization from binding interactions or proximity and orientation of c hemical groups, are evident in the equilibrium-averaged structure of a n enzymatic complex determined by crystallography or NMR. Transient fe atures are not apparent from an average structure. Here, we report on a catalytically relevant property of an enzymatic complex revealed by thermal fluctuations from a molecular dynamics study. The conformation al fluctuations of the cofactor NADH are altered by binding the enzyme lactate dehydrogenase (LDH) compared to those of free NADH; thermal m otions give rise to structures similar to that of the putative transit ion state. The alteration is stereospecific, in agreement with measure d changes in vibrational spectra, and leads to an understanding of the correlation, established some time ago by crystallography and NMR, be tween the nicotinamide glycosidic bond torsion angle (anti/syn) and th e stereospecificity of hydride transfer. These results suggest that on e catalytic role of the enzyme is to funnel the population of NADH con formers to the transition state and reduce the entropic barrier to act ivation. The specific motions in an enzyme complex that might function to enhance transition state formation are described.