Beyond the Active Site: The addition of a remote loop reveals a new complex biological function for chitinase enzymes

Abstract

Loops are small secondary structural elements that play a crucial role in the emergence of new 14 enzyme functions. However, our understanding of loop functions is mainly limited to the 15 catalytic loops. To understand the function of remote loops in enzymes, we studied Glycoside 16 hydrolase family 19 (GH19) chitinase - an essential enzyme family for pathogen degradation 17 in plants. By revealing the evolutionary history and loops appearance of GH19 chitinase, we 18 discovered that one loop which is remote from the catalytic site, is necessary to acquire the 19 new antifungal activity. We demonstrated that this remote loop directly accesses the fungal cell 20 wall, and surprisingly, it needs to adopt a defined structure supported by long-range 21 intramolecular interactions to perform its function. Our findings prove that Nature applies this 22 new strategy at the molecular level to achieve a complex biological function while maintaining 23 the original activity one in the catalytic pocket, suggesting an alternative way to design new 24 enzyme function.