Histidine as an essential residue in the active site of the copper enzyme galactose oxidase.

The pH dependence of the oxidation of β-methyl-d-galactopyranoside by galactose oxidase at 1.33 mm O₂ has been determined. The k_cat exhibits a bell-shaped dependence on the ionization of at least two groups in the enzyme-substrate complex, pK_b' = 6.3 and pK_a' = 7.1, respectively. The pH-independent value for k_cat at 1.33 mm O₂ (nonsaturating) and saturating glycoside is 1435 s^?; the pH optimum is 6.7. Galactose oxidase is inactivated rapidly by iodoacetamide. Although the reaction is much slower, iodoacetate also inactivates the enzyme. The inactivation by iodoacetamide obeys saturation kinetics; at pH 7.0 k₃ = 2.19 min^?1 and K_i = 5.1 mM; k₃ but not K_i exhibits a bell-shaped pH dependence, with pK_a values of 6.3 and 7.6, respectively. Labeling with [¹⁴C]iodoacetamide establishes that one carboxamidomethyl group is incorporated per enzyme molecule. This incorporation parallels the loss of enzymatic activity. Only N-3-carboxymethylhistidine is detected in chromatograms following hydrolysis of the labeled protein. The protein-bound copper is not lost as a consequence of alkylation. Apogalactose oxidase does not react with iodoacetamide. The alkylation is inhibited by the oxidation of an active center tryptophan residue (s) by N-bromosuccinimide. The fraction of residual enzyme activity remaining after tryptophan oxidation corresponds to the extent of labeling by [¹⁴C]iodoacetamide. Although alkylation causes little change in the spin Hamiltonian parameters of the Cu(II) atom, it nearly abolishes both the optical activity and optical absorbance of the metal. The native tryptophan fluorescence of the enzyme, which is a sensitive probe of its active site, is also markedly affected. Since binding of a substrate, β-methyl-d-galactopyranoside, reduces fluorescence as it does in the active enzyme and binding of CN^? at the Cu(II) site as detected by electron spin resonance appears unaffected by the alkylation, the effect of alkylation is on catalysis, per se. Both a catalytic and a subtle conformational role for the active site histidine are inferred from the results.