Effect of glycoside hydrolase 18 chitinase on alternately β-1,4-linked glucose and N-acetylglucosamine derived from the sulfur-oxidizing bacterium Thiothrix nivea.

Abstract

The filamentous sulfur-oxidizing bacterium Thiothrix nivea produces glucosaminoglucan (GG), which is β-1,4-linked copolymer of glucose and glucosamine. GG is a major component of the sheath which covers the bacterial filament, and the bacterium uses CO₂ and organic acids as carbon sources in the presence of H₂S. Similar to chitin, N-acetylated GG (NGG) can be degraded by lysozyme. However, it has not yet been confirmed whether NGG can be degraded by chitinase. Glycoside hydrolase 18 (GH18) chitinase is present in mammals and is produced by various microorganisms. In this study, the degradation of NGG by GH18 chitinase was investigated. When GH18 chitinase was added to the NGG suspension, a decrease in turbidity and an increase in reducing power were observed, indicating that it can degrade NGG. HPLC analysis and Nuclear magnetic resonance spectroscopy revealed that the enzyme degrades NGG into a specific disaccharide β-Glc-(1→4)-GlcNAc. Accordingly, it was found that GH18 chitinase hydrolyzes the GlcNAc→Glc linkage but not the Glc→GlcNAc linkage. The NGG degradation rate was much lower than that of chitin. Hence, NGG was recognized as a material with better biological durability than that of chitin. NGG-chitin composites with different NGG contents were prepared, and as the NGG content increased, the susceptibility of the hydrogel to chitinase decreased. Importantly, this study proposes a novel method for controlling the stability of chitin hydrogels using GH18 chitinase and enhances current understanding of the enzyme.