After the epoch-making report on X-ray crystal structure of a lysozyme-N-acetylglucosamine trisaccharide complex in 1967, catalytic mechanisms of glycosyl hydrolases have been discussed with reference to the lysozyme mechanism. From the recent findings of chitinolytic enzymes, however, the enzymes were found to have catalytic and substrate binding mechanisms different from those of lysozyme. Based on the X-ray crystal structures of chitinases and their complexes with substrate analogues, the catalytic mechanisms were discussed considering the relative locations of catalytic residues to the bound substrate analogues. Resembling the lysozyme catalytic center, family 19 chitinases, family 46 chitosanases, and family 23 lysozymes have two carboxyl groups at the catalytic center, which are separated ( > 10 Å) on either side of the catalytic cleft. The catalytic reaction of the enzymes takes place through a single displacement mechanism. In family 18 chitinases, one can identify only one catalytic carboxylate as a proton donor, but not the second catalytic carboxylate whose function and location are similar to those of Asp52 in lysozyme. The catalytic reaction of family 18 chitinases is most likely to take place through a substrate-assisted mechanism. Hen egg white lysozyme has the binding cleft represented by (-4)(-3)(-2)(-1)(+1)(+2). The binding cleft of family 19 chitinases, family 46 chitosanases, and family 23 lysozymes, however, is represented by (-3)(-2)(-1)(+1)(+2)(+3). Molecular dynamics calculation suggests that family 18 chitinases have the binding cleft, (-4)(-3)(-2)(-1)(+1)(+2). The functional diversity of the chitinolytic enzymes might be related to different physiological functions of the enzymes. The enzymes are now being applied to plant protection from fungal pathogens and insect pests. Structure of the targeted chitinous component was determined by a combination of enzyme digestion and solid state CP/MAS NMR spectroscopy, and have been taken into consideration for efficient application of the enzymes. Recent understanding of the catalytic and substrate binding mechanisms would be helpful as well for arrangement of a powerful strategy in such an application.
Laboratory of Biophysical Chemistry, Faculty of Agriculture, Kinki University, 3327-204 Nakamachi,Nara 631-8505, Japan.