Structure-function relationships in lactose synthase. Structural requirements of the uridine 5'-diphosphate galactose binding site.

The structural requirements for the donor pyranosyl moiety of UDP-Gal in either galactosyl transfer or "lactose" biosynthesis have been determined. The 4"-deoxy analogue, UDP-4"-deoxyglucose, was synthesized and fully characterized as a donor substrate for galactosyltransferase. The relative rate of deoxyglucosyl transfer to glucose or GlcNAc acceptors was 5.5 +/- 0.6% of that of UDP-Gal as the substrate, with Km values in the same range as that for UDP-Gal or UDP-Glc. Several conclusions may be drawn as to the detailed structural requirements of the UDP-Gal binding site: an axial 4"-hydroxyl group on the pyranosyl moiety is necessary for precise substrate alignment as is also an equatorial 6"-CH2OH moiety. Where one or the other moiety was lacking (UDP-dGlc or UDP-Arab), the maximal rate of glycosyl transfer was ca. 1/20th that of UDP-Gal.