Purification andCharacterization of a Newly IdentifiedGH18 Chitinase BhGH18 from <i>Bacillus haynesii</i> and Its Ultrasound-Assisted Hydrolysis of Chitin

Abstract A previously uncharacterized GH18 Chitinase from Bacillus haynesii (BhGH18) was heterologously expressed and characterized. BhGH18 displayed maximal activity at pH 7.5 and 40 °C, and activity was enhanced by Mg2+/Zn2+ but inhibited by Cu2+/Fe2+/Ni2+. After further purification, the enzyme exhibited markedly improved kinetics toward pNP-(GlcNAc)2 (Km = 0.103 mM, Vmax = 0.217 μmol/min). Importantly, on the natural substrate colloidal chitin, BhGH18 gave an apparent Km of 3.51 mg/mL and Vmax of 1.69 μmol/min (for colloidal chitin). The enzyme displayed an endohydrolytic preference, converting ultrasonically pretreated chitin predominantly into chitobiose (DP2). Ultrasonication at 500 W increased the hydrolysis yield nearly 6-fold (23.86% conversion). Molecular docking identified Trp157, Glu197, Tyr198 and Arg333 as key substrate-binding residues. This work demonstrates an integrated ultrasound-enzyme strategy using BhGH18 for sustainable production of DP2-enriched chitin oligosaccharides.