The lactose synthase acceptor site: a structural map derived from acceptor studies |
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Authors: | Lawrence J. Berliner Melanie E. Davis Kurt E. Ebner Thomas A. Beyer J. Ellis Bell |
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Affiliation: | (1) Department of Chemistry, Ohio State University, 43210 Columbus, OH, U.S.A.;(2) Department of Biochemistry, University of Kansas Medical Center, 66103 Kansas City, KS, U.S.A.;(3) Department of Biochemistry, Duke University Medical Center, 27710 Durham, NC, U.S.A.;(4) Department of Biochemistry, University of Rochester School of Medicine, 14642 Rochester, NY, U.S.A.;(5) Present address: Medical and Molecular Biology Section, Battelle Columbus Laboratories, 43204 Columbus, OH, U.S.A. |
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Abstract: | Summary A pictorial map of the lactose synthase (galactosyl transferase) acceptor binding site has been formulated from this and published studies on substrate analogs and inhibitors. The basic requirements are a pyranose, thiopyranose or inositol ring structure and equatorial substituents (if any) at C-2, C-3, C-4, and C-5. The aglycone (at C-1) may be either or -, but - is somewhat preferred. In the absence of -lactalbumin galactosyl transferase will accept long chain 2-N-acyl substituents on the glucosamine (GlcNH2) structure. An equatorial amino or N-acetyl substituent (e.g. mannosamine, N-acetylmannosamine) is also a suitable acceptor in the absence of -lactalbumin since both N-acetylglucosamine and N-acetylmannosamine have complementary binding loci for the N-acyl moiety. The aglycone moiety must be equatorial (-configuraation). However, upon -lactalbumin binding the aglycone specificity allows for axial (-configuration) as well as equatorial substituents. Furthermore, the 2-N-acyl substituent binding locus is blocked beyond a 2-N-hexanoyl group. It is suggested that -lactalbumin binds to a hydrophobic site some distance from the C-2 group. |
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