1nah
UDP-GALACTOSE 4-EPIMERASE FROM ESCHERICHIA COLI, REDUCED
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OverviewOverview
UDP-galactose 4-epimerase catalyzes the conversion of UDP-galactose to, UDP-glucose through a mechanism involving the transient reduction of NAD+., Here we describe the X-ray structures for epimerase complexed with, NADH/UDP, and NAD+/UDP, refined to 1.8 and 2.0 angstrom, respectively. The, alpha-carbon positions for the two forms of the enzyme are superimposed, with a root-mean-square deviation of 0.36 A. Overall, the models for the, reduced and oxidized proteins are very similar except for the positions of, several side chains including Phe 178 and Phe 218. The most striking, difference between the oxidized and reduced enzymes is the conformation of, the nicotinamide ring of the dinucleotide. In the reduced protein, the, nicotinamide ring adopts the anti conformation while in the oxidized, enzyme the syn conformation is observed. There are also significant, structural differences in UDP binding between the oxidized and reduced, forms of the protein which most likely explain the observation that, uridine nucleotides bind more tightly to epimerase/NADH than to, epimerase/NAD+. Both van der Waals and electrostatic interactions between, epimerase and NAD+ are extensive with 35 contacts below 3.2 angstrom as, would be expected for enzyme that binds the dinucleotide irreversibly., This is in sharp contrast to the patterns typically observed for the, NAD+-dependent dehydrogenases which bind nucleotides in a reversible, fashion. While it has been postulated that the active site of epimerase, must contain a base, the only potential candidates within approximately 5, A of both the NAD+ and the UDP are Asp 31, Asp 58, and ASP 295. These, amino acid residues, however, are intimately involved in nucleotide, binding and most likely do not play a role in the actual catalytic, mechanism. Thus it may be speculated that an amino acid residue, other, than glutamate, aspartate, or histidine, may be functioning as the active, site base.
About this StructureAbout this Structure
1NAH is a Single protein structure of sequence from Escherichia coli with NA, NAD, UDP, EDO and PEG as ligands. Active as UDP-glucose 4-epimerase, with EC number 5.1.3.2 Full crystallographic information is available from OCA.
ReferenceReference
Crystal structures of the oxidized and reduced forms of UDP-galactose 4-epimerase isolated from Escherichia coli., Thoden JB, Frey PA, Holden HM, Biochemistry. 1996 Feb 27;35(8):2557-66. PMID:8611559
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