Structure-function relationships in lactose synthase. Structural requirements of the UDP-galactose binding site

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Abstract
The structural requirements for the donor pyranosyl moiety of UDP-Gal in either galactosyl transfer or lactose biosynthesis were determined. The 4"-deoxy analog, UDP-4"-deoxyglucose, was synthesized and fully characterized as a donor substrate for galactosyltransferase [bovine milk, EC 2.4.1.22]. 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 .apprx. 1/20 that of UDP-Gal.

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