Crystal structure of lactose synthase reveals a large conformational change in its catalytic component, the beta1,4-galactosyltransferase-I.
The lactose synthase (LS) enzyme is a 1:1 complex of a catalytic component, beta1,4-galactosyltransferse (beta4Gal-T1) and a regulatory component, alpha-lactalbumin (LA), a mammary gland-specific protein. LA promotes the binding of glucose (Glc) to beta4Gal-T1, thereby altering its sugar acceptor specificity from N-acetylglucosamine (GlcNAc) to glucose, which enables LS to synthesize lactose, the major carbohydrate component of milk. The crystal structures of LS bound with various substrates were solved at 2 A resolution. These structures reveal that upon substrate binding to beta4Gal-T1, a large conformational change occurs in the region comprising residues 345 to 365. This repositions His347 in such a way that it can participate in the coordination of a metal ion, and creates a sugar and LA-binding site. At the sugar-acceptor binding site, a hydrophobic N-acetyl group-binding pocket is found, formed by residues Arg359, Phe360 and Ile363. In the Glc-bound structure, this hydrophobic pocket is absent. For the binding of Glc to LS, a reorientation of the Arg359 side-chain occurs, which blocks the hydrophobic pocket and maximizes the interactions with the Glc molecule. Thus, the role of LA is to hold Glc by hydrogen bonding with the O-1 hydroxyl group in the acceptor-binding site on beta4Gal-T1, while the N-acetyl group-binding pocket in beta4Gal-T1 adjusts to maximize the interactions with the Glc molecule. This study provides details of a structural basis for the partially ordered kinetic mechanism proposed for lactose synthase.
Ramakrishnan
B
B
Structural Glycobiology Section, Intramural Research Support Program-SAIC, Laboratory of Experimental and Computational Biology, CCR, NCI, Frederick, MD 21702, USA.
Qasba
P K
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D000117:Acetylglucosamine / Q000378:metabolism; D000818:Animals; D001665:Binding Sites; D002384:Catalysis; D020134:Catalytic Domain; D002417:Cattle; D018360:Crystallography, X-Ray; D005700:Galactosyltransferases / Q000737:chemistry* / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D006860:Hydrogen Bonding; D007700:Kinetics; D007768:Lactalbumin / Q000737:chemistry / Q000378:metabolism; D007788:Lactose Synthase / Q000737:chemistry* / Q000378:metabolism*; D008345:Manganese / Q000378:metabolism; D051379:Mice; D008958:Models, Molecular; D011485:Protein Binding; D011487:Protein Conformation; D013379:Substrate Specificity; D014531:Uridine Diphosphate Galactose / Q000378:metabolism
D014531:Uridine Diphosphate Galactose; D008345:Manganese; D007768:Lactalbumin; D005700:Galactosyltransferases; C119701:beta-1,4-galactosyltransferase I; D007788:Lactose Synthase; D005947:Glucose; D000117:Acetylglucosamine