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Tryptophanyl residues of A niger glucoamylase G2 (EC 3.2.1.3) involved in substrate and inhibitor binding have been identified following N-bromosuccinimide (NBS) treatment in the presence and absence of protective ligands. Appropriate proteolytic cleavages of the glucoamylase derivatives enabled isolation of individual peptide fragments containing the 15 thytophan positions and the extent of tryptophan oxidation was measured employing normal and 2nd derivative UV-spectrophotometry.Trp-52,-141,-156,-228,-317, and-335 remained unoxidized while compflete oxidation to Trp-6,-28, and-466 and partial oxidation of Trp-170 and-178 was observed after NBS-treatment whether ligands were added or not. Trp-212,-417, and-437 were partially converted in uncomplexed glucoamylase while the presence of either the substrate maltose or the inhibitor acarbose prevented the oxidation of these residues. Trp-120, required for maintaining the active catalytic site, was protected by acarbose only, and its oxidation did not prevent ligand binding. The functional roles of Trp-212,-417,-437, and-120 are discussed.NBS-treatment of the acarbose-protected large form of glucoamylase, G1, destroyed its unique capacity to adsorb onto starch granules while catalytic activity was preserved towards soluble substrates. This effect could be correlated with the oxidation of Trp-590 and-615 located near the COOH-terminus.The reactivities, probably reflecting the degree of solvent exposure, were also assessed for individual tyrosyl residues in G2.The thermal stability of oxidized, catalytically active G1 and G2 was remarkably low as compared to the unmodified forms.

Influence of acarbose and maltose on the reactivity of individual tryptophanyl residues in glucoamylase from aspergillus niger