Biochemical And Hydrolytic Properties Of Multiple Thermostable α-galactosidases From Streptomyces Griseoloalbus: Obvious Existence Of A Novel Galactose-tolerant Enzyme

摘要

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 pl 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 ℃, respectively. α-Gal Ⅰ was stable up to 65 ℃ and α-Gal Ⅱ and α-Gal Ⅲ up to 55 ℃ for 2 h. Based on the hydrolytic properties a-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 Hg~(2+), Ag~(2+) and Cu~(2+) inhibited enzyme activity while Mg~(2+) enhanced the activity of a-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.