1rxs
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(New page: 200px<br /><applet load="1rxs" size="450" color="white" frame="true" align="right" spinBox="true" caption="1rxs, resolution 2.80Å" /> '''E. coli uridine phos...)
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Revision as of 23:49, 20 November 2007
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E. coli uridine phosphorylase: 2'-deoxyuridine phosphate complex
Overview
Uridine phosphorylase (UP) is a key enzyme in the pyrimidine salvage, pathway that catalyses the reversible phosphorolysis of uridine to uracil, and ribose 1-phosphate. Inhibiting liver UP in humans raises blood uridine, levels and produces a protective effect ("uridine rescue") against the, toxicity of the chemotherapeutic agent 5-fluorouracil without reducing its, antitumour activity. We have investigated UP-substrate interactions by, determining the crystal structures of native Escherichia coli UP (two, forms), and complexes with 5-fluorouracil/ribose 1-phosphate, 2-deoxyuridine/phosphate and thymidine/phosphate. These hexameric, structures confirm the overall structural similarity of UP to E.coli, purine nucleoside phosphorylase (PNP) whereby, in the presence of, substrate, each displays a closed conformation resulting from a concerted, movement that closes the active site cleft. However, in contrast to PNP, where helix segmentation is the major conformational change between the, open and closed forms, in UP more extensive changes are observed. In, particular a swinging movement of a flap region consisting of residues, 224-234 seals the active site. This overall change in conformation results, in compression of the active site cleft. Gln166 and Arg168, part of an, inserted segment not seen in PNP, are key residues in the uracil binding, pocket and together with a tightly bound water molecule are seen to be, involved in the substrate specificity of UP. Enzyme activity shows a, twofold dependence on potassium ion concentration. The presence of a, potassium ion at the monomer/monomer interface induces some local, rearrangement, which results in dimer stabilisation. The conservation of, key residues and interactions with substrate in the phosphate and ribose, binding pockets suggest that ribooxocarbenium ion formation during, catalysis of UP may be similar to that proposed for E.coli PNP.
About this Structure
1RXS is a Single protein structure of sequence from Escherichia coli with PO4, K, DUR and V7O as ligands. Active as Uridine phosphorylase, with EC number 2.4.2.3 Full crystallographic information is available from OCA.
Reference
Crystal structures of Escherichia coli uridine phosphorylase in two native and three complexed forms reveal basis of substrate specificity, induced conformational changes and influence of potassium., Caradoc-Davies TT, Cutfield SM, Lamont IL, Cutfield JF, J Mol Biol. 2004 Mar 19;337(2):337-54. PMID:15003451
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