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alpha -Lactalbumin (LA) Stimulates Milk beta-1,4-Galactosyltransferase I (beta 4Gal-T1) to Transfer Glucose from UDP-glucose to N-Acetylglucosamine: CRYSTAL STRUCTURE OF beta 4Gal-T1·LA COMPLEX WITH UDP-Glc*

Ramakrishnan, Boopathy and Shah, Premal S. and Qasba, Pradman K. (2001) alpha -Lactalbumin (LA) Stimulates Milk beta-1,4-Galactosyltransferase I (beta 4Gal-T1) to Transfer Glucose from UDP-glucose to N-Acetylglucosamine: CRYSTAL STRUCTURE OF beta 4Gal-T1·LA COMPLEX WITH UDP-Glc*. Journal of Biological Chemistry, 276 (40). pp. 37665-37671. ISSN 0021-9258. https://resolver.caltech.edu/CaltechAUTHORS:RAMjbc01

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Abstract

beta-1,4-Galactosyltransferase 1 (Gal-T1) transfers galactose (Gal) from UDP-Gal to N-acetylglucosamine (GlcNAc), which constitutes its normal galactosyltransferase (Gal-T) activity. In the presence of alpha -lactalbumin (LA), it transfers Gal to Glc, which is its lactose synthase (LS) activity. It also transfers glucose (Glc) from UDP-Glc to GlcNAc, constituting the glucosyltransferase (Glc-T) activity, albeit at an efficiency of only 0.3-0.4% of Gal-T activity. In the present study, we show that LA increases this activity almost 30-fold. It also enhances the Glc-T activity toward various N-acyl substituted glucosamine acceptors. Steady state kinetic studies of Glc-T reaction show that the Km for the donor and acceptor substrates are high in the absence of LA. In the presence of LA, the Km for the acceptor substrate is reduced 30-fold, whereas for UDP-Glc it is reduced only 5-fold. In order to understand this property, we have determined the crystal structures of the Gal-T1·LA complex with UDP-Glc·Mn2+ and with N-butanoyl-glucosamine (N-butanoyl-GlcN), a preferred sugar acceptor in the Glc-T activity. The crystal structures reveal that although the binding of UDP-Glc is quite similar to UDP-Gal, there are few significant differences observed in the hydrogen bonding interactions between UDP-Glc and Gal-T1. Based on the present kinetic and crystal structural studies, a possible explanation for the role of LA in the Glc-T activity has been proposed.


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https://doi.org/10.1074/jbc.M102458200["eprint_fieldopt_related_url_type_DOI" not defined]UNSPECIFIED
Additional Information:Copyright © 2001 by The American Society for Biochemistry and Molecular Biology, Inc. Received for publication, March 19, 2001, and in revised form, August 2, 2001. Originally published In Press as doi:10.1074/jbc.M102458200 on August 2, 2001. We thank Dr. Zbgniew Dauter for help with synchrotron data collection. We thank Drs. Soma Kumar, Elizabeth Boeggeman, and P.V. Balaji for helpful discussions and comments during preparation of the manuscript. This work was supported in part by Contract NO1-CO-56000 from the NCI, National Institutes of Health.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. The atomic coordinates and the structure factors (code 1JN8, 1JNA, and 1JNC) have been deposited in the Protein Data Bank, Research Collaboratory for Structural Bioinformatics, Rutgers University, New Brunswick, NJ (http://www.rcsb.org/).
Issue or Number:40
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ID Code:9451
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Deposited On:03 Jan 2008
Last Modified:03 Oct 2019 00:00

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