Different Cleavage Specificities of the Dual Catalytic Domains in Chitinase from the Hyperthermophilic Archaeon Thermococcus kodakaraensis KOD1

Abstract

The chitinase from the hyperthermophilic archaeon Thermococcus kodakaraensis KOD1, Tk-ChiA, has an interesting multidomain structure containing dual catalytic domains and triple chitin-binding domains. To determine the biochemical properties of each domain, we constructed deletion mutant genes corresponding to the individual catalytic domains and purified the recombinant proteins. A synergistic effect was observed when chitin was degraded in the presence of both catalytic domains, suggesting different cleavage specificity of these domains. Analyses of degradation products from N-acetyl-chitooligosaccharides and their chromogenic derivatives with thin layer chromatography indicated that the N-terminal catalytic domain mainly hydrolyzed the second glycosidic bond from the nonreducing end of the oligomers, whereas the C-terminal domain randomly hydrolyzed glycosidic bonds other than the first bond from the nonreducing end. Both catalytic domains formed diacetyl-chitobiose as a major end product and possessed transglycosylation activity. Further analysis of degradation products from colloidal chitin with high performance liquid chromatography showed that the N-terminal catalytic domain exclusively liberated diacetyl-chitobiose, whereas reactions with the C-terminal domain led to N-acetyl-chitooligosaccharides of various lengths. These results demonstrated that the N-terminal and C-terminal catalytic domains functioned as exo- and endochitinases, respectively. The biochemical results provide a physiological explanation for the presence of two catalytic domains with different specificity and suggest a cooperative function between the two on a single polypeptide in the degradation of chitin.