1mas: Difference between revisions

Protozoan parasites rely on the host for purines since they lack a de novo, synthetic pathway. Crithidia fasciculata salvages exogenous inosine, primarily through hydrolysis of the N-ribosidic bond using several, nucleoside hydrolases. The most abundant nucleoside hydrolase is, relatively nonspecific but prefers inosine and uridine as substrates. Here, we report the three-dimensional structure of the inosine-uridine, nucleoside hydrolase (IU-NH) from C. fasciculata determined by X-ray, crystallography at a nominal resolution of 2.5 A. The enzyme has an open, (alpha, beta) structure which differs from the classical dinucleotide, binding fold. IU-nucleoside hydrolase is composed of a mixed, eight-stranded beta sheet surrounded by six alpha helices and a small, C-terminal lobe composed of four alpha helices. Two short antiparallel, beta strands are involved in intermolecular contacts. The catalytic pocket, is located at the C-terminal end of beta strands beta 1 and beta 4. Four, aspartate residues are located at the bottom of the cavity in a geometry, which suggests interaction with the ribose moiety of the nucleoside. These, groups could provide the catalytically important interactions to the, ribosyl hydroxyls and the stabilizing anion for the oxycarbonium-like, transition state. Histidine 241, located on the side of the active site, cavity, is the proposed proton donor which facilitates purine base, departure [Gopaul, D. N., Meyer, S. L., Degano, M., Sacchettini, J. C., &, Schramm, V. L. (1996) Biochemistry 35, 5963-5970]. The substrate binding, site is unlike that from purine nucleoside phosphorylase, phosphoribosyltransferases, or uracil DNA glycosylase and thus represents, a novel architecture for general acid-base catalysis. This detailed, knowledge of the architecture of the active site, together with the, previous transition state analysis [Horenstein, B. A., Parkin, D. W., Estupinan, B., & Schramm, V. L. (1991) Biochemistry 30, 10788-10795], allows analysis of the interactions leading to catalysis and an, explanation for the tight-binding inhibitors of the enzyme [Schramm, V., L., Horenstein, B. A., & Kline, P. C. (1994) J. Biol. Chem. 269, 18259-18262].

1MAS is a [http://en.wikipedia.org/wiki/Single_protein Single protein] structure of sequence from [http://en.wikipedia.org/wiki/Crithidia_fasciculata Crithidia fasciculata] with K as [http://en.wikipedia.org/wiki/ligand ligand]. Active as [http://en.wikipedia.org/wiki/Purine_nucleosidase Purine nucleosidase], with EC number [http://www.brenda-enzymes.info/php/result_flat.php4?ecno=3.2.2.1 3.2.2.1] Structure known Active Site: ACT. Full crystallographic information is available from [http://ispc.weizmann.ac.il/oca-bin/ocashort?id=1MAS OCA].  

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