Biochemical and hydrolytic properties of multiple thermostable α-galactosidases from Streptomyces griseoloalbus: Obvious existence of a novel galactose-tolerant enzyme

The multiple α-galactosidases from Streptomyces griseoloalbus—α-Gal I, α-Gal II and α-Gal III were purified to homogeneity by a two-step chromatographic process. The molecular masses and pI of the three enzymes were 72, 57 and 35 kDa, and 4.41, 5.6 and 6.13, respectively. α-Gal I showed N-terminal sequence homology to S. coelicolor A3(2) family 27 α-galactosidase. The optimum pH and temperature of the three α-galactosidases were 5.0, 6.5 and 5.5 and 65, 50 and 55 °C, respectively. α-Gal I was stable up to 65 °C and α-Gal II and α-Gal III up to 55 °C for 2 h. Based on the hydrolytic properties α-Gal I could be classified as a member of GH27 family and α-Gal II and α-Gal III as members of GH36 family. Metal cations like Hg2+, Ag2+ and Cu2+ inhibited enzyme activity while Mg2+ enhanced the activity of α-Gal I. Interestingly α-Gal I showed unusual tolerance to even higher concentrations of galactose, unlike the other two α-galactosidases, which were competitively inhibited by galactose. Melibiose was a competitive inhibitor of all three enzymes. Histidine, tryptophan and carboxylic residues were essential for catalytic action of the three α-galactosidases.