The structure and action of chitinases.

Chitin is second only to cellulose in biomass and it is an important component of many cell wall structures. Several families of enzymes, of distinctly different structure, have evolved to hydrolyze this important polysaccaride. Glycohydrolase family 18 enzymes, chitinases, are characterized by an eight-fold alpha/beta barrel structure; it has representatives among bacteria, fungi, and higher plants. In general these chitinases act through a retaining mechanism in which beta linked polymer is cleaved to release a beta anomer product. Family 19 chitinases are found primarily in plants but some are found in bacteria. Members of this family are related to one another by amino acid sequence, but are unrelated to family 18 proteins. They have a bilobal structure with a high alpha-helical content. Despite any significant sequence homology with lysozymes, structural analysis reveals that family 19 chitinases, together with family 46 chitosanases, are similar to several lysozymes including those from T4-phage and from goose. The structures reveal that the different enzyme groups arose from a common ancestor glycohydrolase antecedent to the procaryotic/eucaryotic divergence. In general, the family 19 enzymes operate through an inverting mechanism.