Abstract
We have found that the hyperthermophilic archaeon Pyrococcus kodakaraensis KOD1 produces an extracellular chitinase. The gene encoding the chitinase (chiA) was cloned and sequenced. The chiA gene was found to be composed of 3,645 nucleotides, encoding a protein (1,215 amino acids) with a molecular mass of 134,259 Da, which is the largest among known chitinases. Sequence analysis indicates that ChiA is divided into two distinct regions with respective active sites. The N-terminal and C-terminal regions show sequence similarity with chitinase A1 from Bacillus circulans WL-12 and chitinase from Streptomyces erythraeus (ATCC 11635), respectively. Furthermore, ChiA possesses unique chitin binding domains (CBDs) (CBD1, CBD2, and CBD3) which show sequence similarity with cellulose binding domains of various cellulases. CBD1 was classified into the group of family V type cellulose binding domains. In contrast, CBD2 and CBD3 were classified into that of the family II type. chiA was expressed in Escherichia coli cells, and the recombinant protein was purified to homogeneity. The optimal temperature and pH for chitinase activity were found to be 85 degrees C and 5.0, respectively. Results of thin-layer chromatography analysis and activity measurements with fluorescent substrates suggest that the enzyme is an endo-type enzyme which produces a chitobiose as a major end product. Various deletion mutants were constructed, and analyses of their enzyme characteristics revealed that both the N-terminal and C-terminal halves are independently functional as chitinases and that CBDs play an important role in insoluble chitin binding and hydrolysis. Deletion mutants which contain the C-terminal half showed higher thermostability than did N-terminal-half mutants and wild-type ChiA.