Browsing by Author "Lala, P"

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    Determination of a protein structure by iodination
    (1999) Ghosh, D; Erman, M; Sawicki, M; Lala, P; Weeks, DR; Li, NY; Pangborn, W; Thiel, DJ; Jörnvall, H; Gutierrez, R; Eyzaguirre, J
    Enzymatic and non-enzymatic iodination of the amino acid tyrosine is a well known phenomenon. The iodination technique has been widely used for labeling proteins, Using high-resolution X-ray crystallographic techniques, the chemical and three-dimensional structures of iodotyrosines formed by non-enzymatic incorporation of I atoms into tyrosine residues of a crystalline protein are described. Acetylxylan esterase (AXE II; 207 amino-acid residues) from Penicillium purpurogenum has substrate specificities towards acetate esters of D-xylopyranose residues in xylan and belongs to a new class of alpha/beta hydrolases. The crystals of the enzyme are highly ordered, tightly packed and diffract to better than sub-angstrom resolution at 85 K. The iodination technique has been utilized to prepare an isomorphous derivative of the AXE II crystal. The structure of the enzyme determined at 1.10 Angstrom resolution exclusively by normal and anomalous scattering from I atoms, along with the structure of the iodinated complex at 1.80 Angstrom resolution, demonstrate the formation of covalent bonds between I atoms and C atoms at ortho positions to the hydroxyl groups of two tyrosyl moieties, yielding iodotyrosines.
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    Multiple conformations of catalytic serine and histidine in acetylxylan esterase at 0.90 Å
    (2001) Ghosh, D; Sawicki, M; Lala, P; Erman, M; Pangborn, W; Eyzaguirre, J; Gutiérrez, R; Jörnvall, H; Thiel, DJ
    Acetylxylan esterase (AXEII; 207 amino acids) from Penicillium purpurogenum has substrate specificities toward acetate esters of D-xylopyranose residues in xylan and belongs to a new class of alpha/beta hydrolases. The crystal structure of AXEII has been determined by single isomorphous replacement and anomalous scattering, and refined at 0.90- and 1.10-Angstrom resolutions with data collected at 85 K and 295 K, respectively. The tertiary structure consists of a doubly wound alpha/beta sandwich, having a central six-stranded parallel beta -sheet flanked by two parallel ol-helices on each side. The catalytic residues Ser(90), His(187), and Ap(175) are located at the C-terminal end of the sheet, an exposed region of the molecule. The serine and histidine side chains in the 295 K structure show the frequently observed conformations in which Ser(90) is trans and the hydroxyl group is in the plane of the imidazole ring of His(187), However, the structure at 85 K displays an additional conformation in which Ser(90) side-chain hydroxyl is away from the plane of the imidazole ring of His(187). The His(187) side chain forms a hydrogen bond with a sulfate ion and adopts an altered conformation. The only other known hydrolase that has a similar tertiary structure is Fusarium solani cutinase, The exposed nature of the catalytic triad suggests that AXEII is a pure esterase, i.e. an alpha/beta hydrolase with specificity for nonlipidic polar substrates.