Purification and characterization of two α-galactosidases associated with catabolism of guar gum and other α-galactosides by Bacteroides ovatus

Abstract

When Bacteroides ovatus is grown on guar gum, a galactomannan, it produces α-galactosidase I which is different from α-galactosidase II which it produces when grown on galactose, melibiose, raffinose, or stachyose. We have purified both of these enzymes to apparent homogeneity. Both enzymes appear to be trimers and have similar pH optima (5.9 to 6.4 for α-galactosidase I, 6.3 to 6.5 for α-galactosidase II). However, α-galactosidase I has a pI of 5.6 and a monomeric molecular weight of 85,000, whereas α-galactosidase II has a pI of 6.9 and a monomeric molecular weight of 80,500. α-Galactosidase I has a lower affinity for melibiose, raffinose, and stachyose (K(m) values of 20.8, 98.1, and 8.5 mM, respectively) than does α-galactosidase II (K(m) values of 2.3, 5.9, and 0.3 mM, respectively). Neither enzyme was able to remove galactose residues from intact guar gum, but both were capable of removing galactose residues from guar gum which had been degraded into large fragments by mannanase. The increase in specific activity of α-galactosidase which was associated with growth on guar gum was due to an increase in the specific activity of enzyme I. Low, constitutive levels of enzyme II also were produced. By contrast, enzyme II was the only α-galactosidase that was detectable in bacteria which had been grown on galactose, melibiose, raffinose, or stachyose.