Enzymatic deacetylation of oligomeric and polymeric chitin using AcCDA1 from Absidia coerulea

Chitin is a ubiquitous structural biopolymer with significant potential for various high-value applications. Chitin deacetylases (CDAs) can potentially deacetylate chitinous molecules, producing chitosans and partially acetylated chitooligosaccharides (COS). Here, we describe the expression and characterization of a CDA from the filamentous fungus Absidia coerulea, AcCDA1, active toward colloidal chitin, partially deacetylated chitin, chitosan with different degrees of acetylation, and COS. When incubated with COS with degrees of polymerization 3-6, the enzyme rapidly produced fully deacetylated COS, which is a relatively rare property among known CDAs. The mode of action of AcCDA1 toward penta-N-acetyl chitopentaose ((GlcNAc)₅) was studied by mass spectrometry, using a novel approach for labeling the reducing N-acetyl-D-glucosamine unit of the COS with chitooligosaccharide oxidase (FgChitO) from Fusarium graminearum. Kinetic studies with COS revealed high enzyme efficiency with k_cat/K_m values reaching 18.8 mM^-1 s^-1 for (GlcNAc)₅. The enzyme removed 12% of the acetyl groups in colloidal chitin whereas it drastically reduced the degree of acetylation of various chitosans. A comparative study with three other CDAs showed that AcCDA1 is highly efficient enzyme and revealed both similarities and differences. These findings show that various CDAs can achieve different deacetylation targets during the enzymatic processing of chitinous materials.