A unique octameric structure of Axe2, an intracellular acetyl-xylooligosaccharide esterase from Geobacillus stearothermophilus

作者：Lansky Shifra; Alalouf Onit; Solomon Hodaya Vered; Alhassid Anat; Govada Lata; Chayen Naomi E; Belrhali Hassan; Shoham Yuval^*; Shoham Gil

来源：Acta Crystallographica Section D-Biological Crystallography, 2014, 70(2): 261-278.

DOI：10.1107/S139900471302840X

Geobacillus stearothermophilus T6 is a thermophilic, Gram-positive soil bacterium that possesses an extensive and highly regulated hemicellulolytic system, allowing the bacterium to efficiently degrade high-molecular-weight polysaccharides such as xylan, arabinan and galactan. As part of the xylan-degradation system, the bacterium uses a number of side-chain-cleaving enzymes, one of which is Axe2, a 219-amino-acid intracellular serine acetylxylan esterase that removes acetyl side groups from xylooligosaccharides. Bioinformatic analyses suggest that Axe2 belongs to the lipase GDSL family and represents a new family of carbohydrate esterases. In the current study, the detailed three-dimensional structure of Axe2 is reported, as determined by X-ray crystallography. The structure of the selenomethionine derivative Axe2-Se was initially determined by single-wavelength anomalous diffraction techniques at 1.70 angstrom resolution and was used for the structure determination of wild-type Axe2 (Axe2-WT) and the catalytic mutant Axe2-S15A at 1.85 and 1.90 angstrom resolution, respectively. These structures demonstrate that the three-dimensional structure of the Axe2 monomer generally corresponds to the SGNH hydrolase fold, consisting of five central parallel beta-sheets flanked by two layers of helices (eight alpha-helices and five 3(10)-helices). The catalytic triad residues, Ser15, His194 and Asp191, are lined up along a substrate channel situated on the concave surface of the monomer. Interestingly, the Axe2 monomers are assembled as a 'doughnut-shaped' homo-octamer, presenting a unique quaternary structure built of two staggered tetrameric rings. The eight active sites are organized in four closely situated pairs, which face the relatively wide internal cavity. The biological relevance of this octameric structure is supported by independent results obtained from gel-filtration, TEM and SAXS experiments. These data and their comparison to the structural data of related hydrolases are used for a more general discussion focusing on the structure-function relationships of enzymes of this category.

• 出版日期2014-2