Plant chitinases use two different hydrolytic mechanisms

Area of Science:

• Biochemistry
• Enzymology
• Glycosyl hydrolases

Background:

• Chitinases are enzymes that break down chitin, a major component of fungal and insect exoskeletons.
• Chitinases are classified into glycosyl hydrolase families 18 and 19, with distinct evolutionary origins and substrate specificities.
• Glycosyl hydrolases can operate via two main catalytic mechanisms: retention or inversion of the anomeric configuration.

Purpose of the Study:

• To investigate the stereochemical outcome of chitin hydrolysis catalyzed by plant chitinases from different families.
• To elucidate the catalytic mechanisms employed by cucumber chitinase (family 18) and bean chitinase (family 19).

Main Methods:

• High-performance liquid chromatography (HPLC) was used to analyze the products of chitin hydrolysis.
• Stereochemical analysis determined the anomeric configuration of the resulting chitooligosaccharides.

Main Results:

• Cucumber chitinase, belonging to family 18, demonstrated a retaining catalytic mechanism, consistent with bacterial chitinases and hen egg white lysozyme.
• Bean chitinase, a family 19 enzyme, exhibited an inverting catalytic mechanism during chitooligosaccharide hydrolysis.
• These findings highlight a divergence in the catalytic mechanisms between plant chitinase families.

Conclusions:

• Plant chitinases from families 18 and 19 utilize distinct catalytic mechanisms for chitin hydrolysis.
• The retaining mechanism is conserved in family 18 plant chitinases, similar to bacterial counterparts.
• The inverting mechanism observed in family 19 chitinases represents a different evolutionary and functional strategy.