Light-scattering study on subunit association-dissociation equilibria of bovine liver glutamate dehydrogenase

The subunit dissociation of bovine liver glutamate dehydrogenase (L-glutamate: NAD(P)+ oxidoreductase (deaminating), EC 1.4.1.3) induced by guanidine hydrochloride ( GdnHCl ) in 0.2 M phosphate buffer (pH 7.3) was investigated by light-scattering molecular-weight measurements. With increasing GdnHCl concentration, two-step transition was observed in the molecular weight change. The dissociation behavior was well described by assuming the dissociation-association equilibria expressed as HK1 in equilibrium 2T K2 in equilibrium 6M where H, T, and M represent the hexameric, trimeric and monomeric forms of the enzyme, respectively. GdnHCl concentration dependence of the two equilibrium constants was interpreted in terms of the binding of GdnHCl on the protein. According to this treatment, the numbers of amino acid residues present at the trimer-trimer contact area within hexamer, N3, and at the monomer-monomer contact area within trimer, N1, were estimated to be as follows; N3 = 21 +/- 2 and N1 = 27 +/- 5. These values seem to be reasonable considering the physical model proposed for this enzyme.