Abstract
In search of alpha-galactosidases with improved kinetic properties for removal of the immunodominant alpha 1,3-linked galactose residues of blood group B antigens, we recently identified a novel prokaryotic family of alpha-galactosidases (CAZy GH110) with highly restricted substrate specificity and neutral pH optimum (Liu, Q. P., Sulzenbacher, G., Yuan, H., Bennett, E. P., Pietz, G., Saunders, K., Spence, J., Nudelman, E., Levery, S. B., White, T., Neveu, J. M., Lane, W. S., Bourne, Y., Olsson, M. L., Henrissat, B., and Clausen, H. (2007) Nat. Biotechnol. 25, 454-464). One member of this family from Bacteroides fragilis had exquisite substrate specificity for the branched blood group B structure Gal alpha 1-3(Fuc alpha 1-2) Gal, whereas linear oligosaccharides terminated by alpha 1,3-linked galactose such as the immunodominant xenotransplantation epitope Gal alpha 1-3Gal beta 1-4GlcNAc did not serve as substrates. Here we demonstrate the existence of two distinct subfamilies of GH110 in B. fragilis and thetaiotaomicron strains. Members of one subfamily have exclusive specificity for the branched blood group B structures, whereas members of a newly identified subfamily represent linkage specific alpha 1,3-galactosidases that act equally well on both branched blood group B and linear alpha 1,3Gal structures. We determined by one-dimensional H-1 NMR spectroscopy that GH110 enzymes function with an inverting mechanism, which is in striking contrast to all other known alpha-galactosidases that use a retaining mechanism. The novel GH110 subfamily offers enzymes with highly improved performance in enzymatic removal of the immunodominant alpha 3Gal xenotransplantation epitope.