Studies on chitosan: 6. Relationship between N-acetyl group distribution pattern and chitinase digestibility of partially N-acetylated chitosans

Abstract

The digestibility of partially N-acetylated chitosans by microbial chitinases was investigated in view of the distribution pattern of N-acetyl groups along the polysaccharide chain. Partially N-acetylated chitosans are classified into two groups; moderately N-deacetylated chitosans (MDC) with 10–30% acetyl content obtained by heterogeneous N-deacetylation of chitin and partially N-acetylated chitosans (PAC-H) with 20–70% acetyl content prepared by homogeneous N-acetylation of highly N-deacetylated chitosans (HDC). MDC have some blocks of N- acetyl- d- glucosamine (GlcNAc) sequences but PAC-H are random-type copolymers of GlcNAc and d-glucosamine. The apparent K m values of Streptomyces griseus chitinase were 0.14 g l −1 for 30% N-acetylated MDC and 0.16 g l −1 for 30% N-acetylated PAC-H. The K m values decreased with increased N-acetylation but the values for both MDC and PAC-H with similar acetyl content were almost the same. The chitinase from S. griseus could not distinguish the difference between block and random distributions of GlcNAc. The chitinases from Bacillus sp. and Bacillus sp. PI-7S also hydrolysed MDC and PAC-H in the same manner. From these results we conclude that sequences of GlcNAc are not necessary for recognition by these chitinases in contrast to lysozyme.