Using a spectrokinetic approach the absorption spectra of the short-lived transient products in the enzymatic reaction of the glutamate decarboxylase with natural substrate are determined for the first time. The quantum-chemical calculations of the electronic structure and spectra of various ionic species of numerous vitamin Bb derivatives allowed a hypothesis on the nature of the intermediate products detected to be suggested. A model of the enzyme functioning taking into account the charge equilibria and some electronic-conformational relations is proposed.The impressive current advances in enzymology are dependent, to a considerable degree, on the active attention to this branch of science on the part of investigators working in a variety of other fields. In fact, solution of the most topical problems in biomedical sciences cannot, at present, be achieved without a thorough understanding of the mechanisms of enzyme action.A large and very important group of enzymes are those requiring pyridoxal-5'-phosphate as cofactor and effecting a variety of transformations of amino acids in all types of organisms-the so-called PLP-enzymes [l]. A large number of studies have been devoted to the investigation of enzymes belonging to this group, with studies aiming at elucidation of the mechanism of enzymic transamination reactions as a predominant topic.As opposed to our previous communication [ 2 ] , where a tentative model was formulated for the mechanism of enzymatic a-amino acid decarboxylation, the present paper discusses the interaction of glutamate decarboxylase from Escherichia coli with its natural substrate, taking into consideration the charge equilibria of coenzyme and substrate. The electronic structures of some nonenzymatic (model) coenzyme-substrate complexes are discussed. A more correct assignment to possible intermediates is suggested for the absorption bands recorded by means of the stop-flow procedure. (In the earlier paper [2] the spectroscopic resolution rested on the assumption that all spectral changes at 420 nm and beyond this wavelength were due only to the decrease in amount of the "internal" Schiff base; this resulted in somewhat inexact evaluation of the long-wave band in the difference spectrum.) In addition, some general aspects of the catalytic action of pyridoxal enzymes will be discussed.It should be stated that in the postulations regarding the model of enzymatic decarboxylation, the electronic structure and total energy, in the ground state, of