Lactose synthase was found to show anomeric preference for beta-D-glucose. This information was utilized in the design of methyl, ethyl, propyl, butyl, and pentyl N-acetyl-beta-D-glucosaminides, which were subsequently demonstrated to be substrates for galactosyltransferase with apparent Km values in the low millimolar range. alpha-Lactalbumin competitively inhibits the transferase activity against these N-acetylglucosamine derivatives. This pattern of inhibition has also been observed when the dimer, trimer, and tetramer of N-acetylglucosamine and ovomucoid served as the galactose acceptor. The data suggest that the binding of alpha-lactalbumin and the N-acetylglucosamine derivatives is mutually exclusive. This assertion is further supported by the inability of methyl and butyl N-acetyl-beta-D-glucosaminides to facilitate retention of galactosyltransferase on a column of alpha-lactalbumin immobilized onto Sepharose. Free N-acetylglucosamine, on the other hand, does cause retention of the transferase under the same conditions. Thus, alpha-lactalbumin must bind to a region on galactosyltransferase in close proximity to the monosaccharide binding site and exert its substrate-specifying action through competitive and mutually exclusive binding with the N-acetylglucosamine analogues accompanied by an increased affinity for glucose. In short, our substrate analogue studies have revealed that the association-dissociation modulation of galactosyltransferase activity is effected through a topographical blockade of glycoprotein binding by alpha-lactalbumin.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.