Active-pocket size differentiating insectile from bacterial chitinolytic β-N-acetyl-D-hexosaminidases

Chitinolytic β-N-acetyl-D-hexosaminidase is a branch of the GH20 (glycoside hydrolase family 20) β-N-acetyl-D-hexosaminidases that is only distributed in insects and micro-organisms, and is therefore a potential target for the action of insecticides. PUGNAc [O-(2-acetamido-2-deoxy-D-glucopyransylidene)-amino-N-phenylcarbamate] was initially identified as an inhibitor against GH20 β-N-acetyl-D-hexosaminidases. So far no crystal structure of PUGNAc in complex with any GH20 β-N-acetyl-D-hexosaminidase has been reported. We show in the present study that the sensitivities of chitinolytic β-N-acetyl-D-hexosaminidases towards PUGNAc can vary by 100-fold, with the order being OfHex1 (Ostrinia furnacalis β-N-acetyl-D-hexosaminidase)<SmCHB (Serratia marcescens chitobiase)<SpHex (Streptomyces plicatus β-N-acetyl-D-hexosaminidase). To explain this difference, the crystal structures of wild-type OfHex1 as well as mutant OfHex1(V327G) in complex with PUGNAc were determined at 2.0 Å (1 Å=0.1 nm) and 2.3 Å resolutions and aligned with the complex structures of SpHex and SmCHB. The results showed that the sensitivities of these enzymes to PUGNAc were determined by the active pocket size, with OfHex1 having the largest but narrowest entrance, whereas SpHex has the smallest entrance, suitable for holding the inhibitor, and SmCHB has the widest entrance. By widening the size of the active pocket entrance of OfHex1 through replacing the active site Val³²⁷ with a glycine residue, the sensitivity of OfHex1 to PUGNAc became similar to that of SmCHB. The structural differences among chitinolytic β-N-acetyl-D-hexosaminidases leading to different sensitivities to PUGNAc may be useful for developing species-specific pesticides and bactericides.