Abstract
Carbohydrate esterases family 4 (CE4 enzymes) includes chitin and peptidoglycan deacetylases, acetylxylan esterases, and poly-N-acetylglucosamine deacetylases that act on structural polysaccharides, altering their physicochemical properties, and participating in diverse biological functions. Chitin and peptidoglycan deacetylases are not only involved in cell wall morphogenesis and remodeling in fungi and bacteria, but they are also used by pathogenic microorganisms to evade host defense mechanisms. Likewise, biofilm formation in bacteria requires partial deacetylation of extracellular polysaccharides mediated by poly-N-acetylglucosamine deacetylases. Such biological functions make these enzymes attractive targets for drug design against pathogenic fungi and bacteria. On the other side, acetylxylan esterases deacetylate plant cell wall complex xylans to make them accessible to hydrolases, making them attractive biocatalysts for biomass utilization. CE4 family members are metal-dependent hydrolases. They are highly specific for their particular substrates, and show diverse modes of action, exhibiting either processive, multiple attack, or patterned deacetylation mechanisms. However, the determinants of substrate specificity remain poorly understood. Here, we review the current knowledge on the structure, activity, and specificity of CE4 enzymes, focusing on chitin deacetylases and related enzymes active on N-acetylglucosamine-containing oligo and polysaccharides.
Highlights
Carbohydrate esterases family 4 (CE4 enzymes) includes chitin and peptidoglycan deacetylases, acetylxylan esterases, and poly-N-acetylglucosamine deacetylases that act on structural polysaccharides, altering their physicochemical properties, and participating in diverse biological functions
The deacetylation pattern exhibited by chitin deacetylases and related CE4 enzymes active on COS is diverse, reflecting different substrate specificities and pattern recognition on their linear substrates
The diversity of deacetylation patterns exhibited by chitin deacetylases and related CE4 enzymes active on COS (Table 1) can be attributed to the differential accessibility of the linear chitin oligosaccharide to the separate subsites along the substrate binding cleft of their structures
Summary
Carbohydrate esterases (CEs) are enzymes that catalyze the de-O- or de-N-acetylation of glycans and substituted saccharides. Some active CDAs were identified and purified from very diverse organisms, such as archaea, marine bacteria, fungi, and insects, which in many cases, are not even capable of producing chitosans [10]. These enzymes, like their sources, are very diverse in their characteristics and optimal working conditions. It has been shown that the addition of a lytic polysaccharide monooxygenase (LPMO), which oxidatively cleaves the chitin chains on the surface of the fibrils, greatly increased the activity of a CDA on β-chitin [12]. CDAs are either intracellular, as in Rhizobium species where they are involved in Nod factor biosynthesis, or extracellular, involved in the catabolism of chitin, as in marine bacteria [8,10,13]
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