Structural Insights into the Dual-Substrate Recognition and Catalytic Mechanisms of a Bifunctional Acetyl Ester-Xyloside Hydrolase from Caldicellulosiruptor lactoaceticus

Enzymes are usually characterized by their evolutionarily conserved catalytic domains; however, this work presents the incidental gain-of-function of an enzyme in a loop region by natural evolution of its amino acids. A bifunctional acetyl ester-xyloside hydrolase (CLH10) was heterologously expressed, purified, and characterized. The primary sequence of CLH10 contains the fragments of the conserved sequence of esterase and glycosidase, which distribute in a mixed type. The crystal structure revealed that the primary sequence folded into two independent structural regions to undertake both acetyl esterase and β-1,4-xylanase hydrolase functions. CLH10 is capable of cleaving both the β-1,4-xylosidic bond-linked main chain and the ester bond-linked acetylated side chain of xylan, which renders it valuable because it can degrade acetylated xylan within one enzyme. Significantly, the β-1,4-xylanase activity of CLH10 appears to have been fortuitously obtained because of the variable Asp10 and Glu139 located in its loop region, which suggested that the exposed loop region might act as a potential hot-spot for the design and generation of promising enzyme function in both directed evolution and rational protein design.