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-A 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 alpha-helices on each side. The catalytic residues Ser(90), His(187), and Asp(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.
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 A resolution exclusively by normal and anomalous scattering from I atoms, along with the structure of the iodinated complex at 1.80 A 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.
Acetyl xylan esterase from Penicillium purpurogenum, a single-chain 23 kDa member of a newly characterized family of esterases that cleaves side chain ester linkages in xylan, has been crystallized. The crystals diffract to better than 1 A resolution at the Cornell High Energy Synchrotron Source (CHESS) and are highly stable in the synchrotron radiation. The space group is P2(1)2(1)2(1) and cell dimensions are a = 34.9 A, b = 61.0 A, C = 72.5 A.
BACKGROUND Candida cylindracea cholesterol esterase (CE) reversibly hydrolyzes cholesteryl linoleate and oleate. CE belongs to the same alpha/beta hydrolase superfamily as triacylglycerol acyl hydrolases and cholinesterases. Other members of the family that have been studied by X-ray crystallography include Torpedo californica acetylcholinesterase, Geotrichum candidum lipase and Candida rugosa lipase. CE is homologous to C. rugosa lipase 1, a triacylglycerol acyl hydrolase, with which it shares 89% sequence identity. The present study explores the details of dimer formation of CE and the basis for its substrate specificity.
RESULTS:
The structures of uncomplexed and linoleate-bound CE determined at 1.9 A and 2.0 A resolution, respectively, reveal a dimeric association of monomers in which two active-site gorges face each other, shielding hydrophobic surfaces from the aqueous environment. The fatty-acid chain is buried in a deep hydrophobic pocket near the active site. The positioning of the cholesteryl moiety of the substrate is equivocal, but could be modeled in the hydrophobic core of the dimer interface.
CONCLUSIONS:
The monomer structure is the same in both the complexed and uncomplexed crystal forms. The dimers differ in the relative positions of the two monomers at the dimer interface. Of the 55 residues that are different in CE from those in C. rugosa lipase 1, 23 are located in the active site and at the dimer interface. The altered substrate specificity is a direct consequence of these substitutions.
Title: Monomeric and dimeric forms of cholesterol esterase from Candida cylindracea. Primary structure, identity in peptide patterns, and additional microheterogeneity Kaiser R, Erman M, Duax WL, Ghosh D, Jornvall H Ref: FEBS Letters, 337:123, 1994 : PubMed
Cholesterol esterase from Candida cylindracea was separated into two fractions, corresponding to a dimeric and a monomeric form. Fingerprint analysis after lysine cleavages shows identical patterns, suggesting lack of primary differences. Crystals obtained from the two proteins differ and suggest the possibility of an equilibrium between the two forms, influenced by the substrate cholesterol linoleate, which appears to stabilize the more active, dimeric form. All crystals have dimers as the asymmetric unit. The primary structure of the enzyme was determined at the peptide level and shows only one difference, Leu-350 instead of Ile, from a DNA-deduced amino acid sequence, and conservation of features typical for cholesterol esterases characterized.
Title: Crystallization and preliminary diffraction analysis of cholesterol esterase from Candida cylindracea Ghosh D, Erman M, Duax WL Ref: J Steroid Biochem Mol Biol, 38:663, 1991 : PubMed
Cholesterol esterase (EC 3.1.1.13) from the microorganism Candida cylindracea has been crystallized in two forms. Crystals, typically 0.30 x 0.15 x 0.10 mm in size, diffract rotating anode generated x-rays to beyond 3 A are suitable for data collection for an x-ray crystallographic investigation. A monoclinic crystal form in the space group P2(1) was found to have cell dimensions of a = 122.9 A, b = 101.0 A, c = 95.2 A and beta = 108.3 degrees. The asymmetric unit of the cell contains two dimers of 129 kDa each. A second crystal form, in the triclinic space group P1, has cell dimensions of a = 58.6 A, b = 88.7 A, c = 58.6 A, alpha = 93.3 degrees, beta = 113.8 degrees and gamma = 96.0 degrees, and has one dimer per asymmetric unit.
