Structural analysis of group II chitinase (ChtII) catalysis completes the puzzle of chitin hydrolysis in insects.

Chitin is a linear homopolymer of -acetyl-β-d-glucosamines and a major structural component of insect cuticles. Chitin hydrolysis involves glycoside hydrolase family 18 (GH18) chitinases. In insects, chitin hydrolysis is essential for periodic shedding of the old cuticle ecdysis and proceeds via a pathway different from that in the well studied bacterial chitinolytic system. Group II chitinase (ChtII) is a widespread chitinolytic enzyme in insects and contains the greatest number of catalytic domains and chitin-binding domains among chitinases. In Lepidopterans, ChtII and two other chitinases, ChtI and Chi-h, are essential for chitin hydrolysis. Although ChtI and Chi-h have been well studied, the role of ChtII remains elusive. Here, we investigated the structure and enzymology of ChtII, a ChtII derived from the insect pest We present the crystal structures of two catalytically active domains of ChtII, ChtII-C1 and ChtII-C2, both in unliganded form and complexed with chitooligosaccharide substrates. We found that ChtII-C1 and ChtII-C2 both possess long, deep substrate-binding clefts with endochitinase activities. ChtII exhibited structural characteristics within the substrate-binding cleft similar to those in Chi-h and ChtI. However, ChtII lacked structural elements favoring substrate binding beyond the active sites, including an extra wall structure present in Chi-h. Nevertheless, the numerous domains in ChtII may compensate for this difference; a truncation containing one catalytic domain and three chitin-binding modules (ChtII-B4C1) displayed activity toward insoluble polymeric substrates that was higher than those of Chi-h and ChtI. Our observations provide the last piece of the puzzle of chitin hydrolysis in insects.