Specific reaction of Cys-149 with 3,3,3-trifluorobromacetone allows one to probe symmetry relation between the active center regions of tetrameric glyceraldehyde-3-phosphate dehydrogenase by 19F nuclear magnetic resonance (nmr) techniques. Nmr titration studies in the pH range of greatest enzymic activity reveal the existence of species with an (alphaalpha')2 structure; this symmetry is not induced by the coenzyme. Addition of NADH to the ketone-labeled protein causes the enzymic reduction of the ligand in a stereospecific manner and is used to demonstrate the functionality of residues other than Cys-149 that are essential for catalysis. The interpretation of chemical shift characteristics found for the trifluoroacetonyl group together with the kinetics of its reduction allows the derivation of a dynamic model for the enzymic structure which may contribute to understanding of the half-of-the-sites phenomenon.