1986 Volume 34 Issue 9 Pages 3812-3817
In order to differentiate tryptophan residues in the active site of a major glucoamylase from a Rhizopus sp. (Gluc1), N-bromosuccinimide (NBS) oxidation of Gluc1 in the presence of maltitol and tris(hydroxymethyl)aminomethane (Tris) was studied.1. When Gluc1 was oxidized in the presence of maltitol, the hydrolytic activities of Gluc1 towards both soluble starch and p-nitrophenyl α-D-glucopyranoside (PNPG) were scarcely protected from NBS oxidation. On the other hand, when Gluc1 was oxidized in the presence of Tris, the activity towards PNPG increased due to NBS oxidation of about 2 mol of tryptophan residues, whereas the activity towards maltose or soluble starch decreased.2. The fluorescence quenching and the ultraviolet (UV) difference spectrum of Gluc1 induced by the addition of maltitol decreased with NBS oxidation. The decreases were small in the case of Gluc1 oxidized in the presence of maltitol, but were large and nearly parallel with the decrease in the activity towards maltose or soluble starch when Gluc1 was oxidized in the presence of Tris.3. It seems that at least two tryptophan residues exist in the catalytic locus of the enzyme active site, and one of them is related to the binding with a glucose moiety of substrates such as maltose but is scarcely related to the binding with the p-nitrophenyl moiety of PNPG. Another tryptophan residue seemed to be involved mainly in the catalytic action and to contribute little to maltose binding.
