Branch specificity of beta-D-galactosidase from Escherichia coli.

The "branch specificities" of the beta-D-galactosidases from Escherichia coli, jack bean, Aspergillus niger, and human liver were investigated with two branched oligosaccharide substrates, one which forms part of a complex-type biantennary N-linked glycan (compound 1) and a structure having blood group I activity (compound 2), respectively. Both substrates were available as radioactive compounds having a known distribution of 3H and 14C label in each of the terminal galactosyl groups, which allowed accurate estimation of the branch specificity of the enzymes from the ratio of 3H and 14C radioactivity in the galactose released by these hydrolases. It was found that the beta-D-galactosidase from E. coli preferentially released the galactosyl group at the 1----3 branch of compound 1 and that the 1----6 branch of compound 2. By contrast, the other beta-D-galactosidases investigated showed little or no branch specificity. These results suggest that the branch specificity of the beta-D-galactosidase from E. coli has to be explained from a specific recognition of certain parts of the aglycon of the substrates by this enzyme rather than from a better accessibility of the galactose at one particular branch.