A New Family of Carbohydrate Esterases Is Represented by a GDSL Hydrolase/Acetylxylan Esterase from Geobacillus stearothermophilus

Abstract

Background: Acetylxylan esterases are enzymes that remove acetyl groups from the hemicellulolytic polymer xylan. Results: The axe2 gene product in Geobacillus stearothermophilus removes acetyl groups from acetylated xylan and xylosaccharides. Conclusion: Axe2 represents a new serine carbohydrate esterase family. Significance: The findings may provide new routes for the efficient utilization of biomass as a renewable energy source.

Understanding the structural and functional properties of carbohydrate esterases with a special focus on hemicellulose deacetylating acetyl xylan esterases

“…This type of assembly will accommodate the two active sites to act on the same substrate simultaneously (hemicellulose with several acetyl units) and increasing the rate of hydrolysis. Experiments conducted by Alalouf et al (2011) have also confirmed the catalytic function of AcXE2 on 2,3,4-tri-O-acetyl methyl-β-D-xylopyranoside, where AcXE2 was able to deacetylate at positions 3 and 4 simultaneously (Alalouf et al 2011). The octameric arrangement of AcXE2 is greatly stabilised by intermolecular interactions when compared to dimeric, tetrameric structures, thus enhancing the thermodynamic stability of the multimeric assembly (Lansky et al 2014) (Figure 7) (Table 3).…”

“…From the above interactions, the catalytic roles of Asp191 and His194 residues were confirmed to promote the nucleophilic attack on the deprotonated Ser15 side chains on the ester bonds of the substrate molecule, which constitutes for the first step of the hydrolysis. The structural roles of the catalytic residues were also supported by the site-directed mutagenesis experiments of the catalytic residues S15A, H194A and D191A, which were totally inactive (Alalouf et al 2011). The structural homology with the other members of SGNH hydrolase family proteins suggests that residues involved in oxyanion hole formation of Gly63 and Asn92 (Alalouf et al 2011).…”

“…The structural roles of the catalytic residues were also supported by the site-directed mutagenesis experiments of the catalytic residues S15A, H194A and D191A, which were totally inactive (Alalouf et al 2011). The structural homology with the other members of SGNH hydrolase family proteins suggests that residues involved in oxyanion hole formation of Gly63 and Asn92 (Alalouf et al 2011). The crystal structures of AcXE2 also confirms the arrangement of these residue to maintain exact distance and appropriate orientation for interacting and stabilising the catalytic anionic intermediate (Lansky et al 2014).…”

Phylogeny, classification and metagenomic bioprospecting of microbial acetyl xylan esterases

“…recognized CE families and a CE17 family has been proposed [4,21]. CEs act on soluble substrates and de-esterify saccharide residues esterified with short carboxylic acids such as acetic, propionic, butyric and phenolic acids (ferulic or p-coumaric acid) including peptidoglycan (CE4, 9, 11), glucuronic acid (CE14 and 15), chitin (CE4, 14), rhizobial Nod factors (CE4), xylan (CE1-7 and 16), pectin (CE8, 12, 13) and rhamnogalacturonan (CE12) [20,22].…”

“…Consequently, certain CAZymes have been grouped into subfamilies to serve as a platform through which CAZyme substrate specificities can be directly related to their structures [26], but no CE families have been sub-classified. So far, only the sub-classification of the CE16 family has been strongly supported [21,49].…”