An acceptor analogue of β-1,4-galactosyltransferase: Substrate, inhibitor, or both?

Many glycosyltransferase inhibitors in the literature are structurally derived from the donor or acceptor substrate of the respective enzyme. A representative example is 2-naphthyl β-d-GlcNAc, a synthetic GlcNAc glycoside that has been reported as a galactosyltransferase inhibitor. This GlcNAc derivative is attractive as a chemical tool compound for biological and biochemical studies because of its reported potency as an inhibitor, and its short and straightforward synthesis from readily available starting materials. We report that in our hands, 2-naphthyl β-d-GlcNAc behaved, unexpectedly, as an acceptor substrate of the inverting β-1,4-galactosyltransferase (β-1,4-GalT) from bovine milk. This substrate activity has not previously been described. We found that 2-naphthyl β-d-GlcNAc can be an acceptor substrate both for recombinantly expressed β-1,4-GalT, and for a commercial batch of the same enzyme, and both in the presence and absence of bovine serum albumin (BSA). As expected for a full acceptor substrate, this substrate activity was time- and concentration-dependent. Additional experiments show that the observed inhibitor/substrate switch is facilitated by a phosphatase that is an essential component of our enzyme-coupled glycosyltransferase assay. These findings suggest that the behaviour of 2-naphthyl β-d-GlcNAc and related acceptor-based glycosyltransferase inhibitors is strongly dependent on the individual assay conditions. Our results therefore have important implications for the use of 2-naphthyl β-d-GlcNAc and related glycosides as tool compounds in glycobiology and glycobiochemistry.