Structure and function of a broad-specificity chitin deacetylase from Aspergillus nidulans FGSC A4

Abstract

Enzymatic conversion of chitin, a β-1,4 linked polymer of N-acetylglucosamine, is of major interest in areas varying from the biorefining of chitin-rich waste streams to understanding how medically relevant fungi remodel their chitin-containing cell walls. Although numerous chitinolytic enzymes have been studied in detail, relatively little is known about enzymes capable of deacetylating chitin. We describe the structural and functional characterization of a 237 residue deacetylase (AnCDA) from Aspergillus nidulans FGSC A4. AnCDA acts on chito-oligomers, crystalline chitin, chitosan, and acetylxylan, but not on peptidoglycan. The K m and k cat of AnCDA for the first deacetylation of penta-N-acetyl-chitopentaose are 72 μM and 1.4 s-1, respectively. Combining mass spectrometry and analyses of acetate release, it was shown that AnCDA catalyses mono-deacetylation of (GlcNAc)2 and full deacetylation of (GlcNAc)3-6 in a non-processive manner. Deacetylation of the reducing end sugar was much slower than deacetylation of the other sugars in chito-oligomers. These enzymatic characteristics are discussed in the light of the crystal structure of AnCDA, providing insight into how the chitin deacetylase may interact with its substrates. Interestingly, AnCDA activity on crystalline chitin was enhanced by a lytic polysaccharide monooxygenase that increases substrate accessibility by oxidative cleavage of the chitin chains.