Structural analysis and construction of thermostable antifungal chitinase

Abstract

Chitin is a biopolymer of N-acetyl-D-glucosamine with {beta}-1,4-bond and is the main component of arthropod exoskeletons and the cell walls of many fungi. Chitinase (EC 3.2.1.14) is an enzyme that hydrolyzes the {beta}-1,4-bond in chitin and degrades chitin into oligomers. It has been found in a wide range of organisms. Chitinase from Gazyumaru (Ficus microcarpa) latex exhibits antifungal activity by degrading chitin in the cell wall of fungi and is expected to be used in medical and agricultural fields. However, the enzymes thermostability is an important factor; chitinase is not thermostable enough to maintain its activity under the actual applicable conditions. We solved the crystal structure of chitinase to explore the target sites to improve its thermostability. Based on the crystal structure and sequence alignment among other chitinases, we rationally introduced proline residues, a disulfide bond, and salt bridge in the chitinase using protein engineering methods. As a result, we successfully constructed the thermostable mutant chitinases rationally with high antifungal and specific activities. The results provide a useful strategy to enhance the thermostability of this enzyme family. IMPORTANCEWe solved the crystal structure of the chitinase from Gazyumaru (Ficus microcarpa) latex exhibiting antifungal activity. Furthermore, we demonstrated that the thermostable mutant enzyme with a melting temperature (Tm) 6.9 {degrees}C higher than wild type (WT) and a half-life at 60{degrees}C that is 15 times longer than WT was constructed through 10 amino acid substitutions, including five proline residues substitutions, making disulfide bonding, and building a salt bridge network in the enzyme. These mutations do not affect its high antifungal activity and chitinase activity, and the principle for the construction of the thermostable chitinase was well explained by its crystal structure. Our results provide a useful strategy to enhance the thermostability of this enzyme family and to use the thermostable mutant as a seed for antifungal agents for practical use.