Mode of Action and Specificity of a Chitinase from Unicellular Microalgae, Euglena Gracilis

Euglena gracilis is a unicellular microalga showing characteristics of both plants and animals, and extensively used as a model organism in the research works of biochemistry and molecular biology. Biotechnological applications of E. gracilis have been conducted for production of numerous important compounds. However, chitin-mediated defense system intensively studied in higher plants remains to be investigated in this microalga. Recently, Taira et al. (Biosci Biotechnol Biochem 82:1090–1100, 2018) isolated a unique chitinase gene, comprising two catalytic domains almost homologous to each other (Cat1 and Cat2) and two chitin-binding domains (CBD1 and CBD2), from E. gracilis. We herein examined the mode of action and the specificity of the recombinant Cat2 by size exclusion chromatography and NMR spectroscopy. Both Cat1 and Cat2 appeared to act toward chitin substrate with non-processive/endo-splitting mode, recognizing two contiguous N-acetylglucosamine units at subsites − 2 and − 1. This is the first report on a chitinase having two endo-splitting catalytic domains. A cooperative action of two different endo-splitting domains may be advantageous for defensive action of the E. gracilis chitinase.