title: Metal ion and substrate binding to bovine galactosyltransferase.
abstract: Bovine milk galactosyltransferase was examined by ESR and NMR proton relaxation measurements to determine the stoichiometry and nature of manganese and UDP-Gal substrate binding. The ESR and NMR data clearly showed the binding of two (Mn(II) per mol of enzyme in the ternary complex (enzyme-manganese-UDP-Gal). The affinity of the enzyme for manganese is much higher in the presence of UDP-Gal than in its absence. A deenhancement was observed in both water and UDP-Gal proton relaxation rates upon ternary complex formation [enzyme-Mn(II)-UDP-Gal] relative to the metal-substrate [Mn(II)-UDP-Gal] binary complex, yet the temperature dependence of the water proton relaxation rate was consistent with fast exchange. A simple model was proposed which accounted for the pronounced deenhancement, involving a slow conformational interconversion of an initially formed, rapidly exchanging conformer of the enzyme-Mn(II)-UDP-Gal complex to a second form which contributes negligibly to the relaxation.
mesh_terms: D000818:Animals; D001665:Binding Sites; D002417:Cattle; D004578:Electron Spin Resonance Spectroscopy; D005260:Female; D005700:Galactosyltransferases / Q000378:metabolism*; D009682:Magnetic Resonance Spectroscopy; D008345:Manganese; D008433:Mathematics; D008892:Milk / Q000201:enzymology*; D011485:Protein Binding; D014531:Uridine Diphosphate Galactose; D014539:Uridine Diphosphate Sugars
chemical_list: D014539:Uridine Diphosphate Sugars; D014531:Uridine Diphosphate Galactose; D008345:Manganese; D005700:Galactosyltransferases