Marine Aspergillus terreus produces a chitinase exhibiting a dual mode of enzymatic action

Marine Aspergillus terreus has been explored as an important chitinase-producing fungal strain for-N-Acetyl-D-Glucosamine (GlcNAc) production from chitin substrates. Here, a purified extracellular 45 kDa chitinase of marine Aspergillus terreus (accession number JQ248076) was characterized in terms of substrate specificity. Conventionally, endochitinase cleaves the chitin substrate randomly to produce GlcNAc and its different multimers. So, it requires at least tetramer to characterize the endochitinases; whereas, exochitinases cleaves the chitin substrate from its reducing end and produce either GlcNAc or chitobiose (GlcNAc dimer). In present chitinase characterization, the HPLC followed by HRMS analyses revealed differential product formation from the chitin substrates of varied chain length. With swollen chitin polymer, the enzyme produced GlcNAc as a sole product; whereas with chitohexaose substrate, a mixture of GlcNAc and its oligomers were obtained. Although, mass spectrometry-based proteomics analysis identified the isolated chitinase as an endochitinase 1 precursor (Accession XP_001217186). However, the enzyme kinetic study exhibited higher catalytic efficiency for exochitinase specific dimeric chromogenic substrate in comparison to endochitinase specific tetrameric fluorogenic substrate, which indicated predominantly exochitinase behavior of the enzyme. Further, the in-silico study predicted the differential cleavage pattern of the enzyme, which could be due to different mode of substrate binding and processive mechanism through the tunnel shaped binding cleft of the enzyme. The dual mode of catalytic activity of the present chitinase was further confirmed by a molecular docking study with different lengths of substrates. With the unique dual mode of action, the chitinase of marine Aspergillus terreus offers a great promise towards its utility in the production of GlcNAc.