Catalytic role of the calcium ion in GH97 inverting glycoside hydrolase.

The role of calcium ion in the active site of the inverting glycoside hydrolase family 97 enzyme, BtGH97a, was investigated through structural and kinetic studies. The calcium ion was likely directly involved in the catalytic reaction. The pH dependence of kcat/Km values in the presence or absence of calcium ion indicated that the calcium ion lowered the pKa of the base catalyst. The significant decreases in kcat/Km for hydrolysis of substrates with basic leaving groups in the absence of calcium ion confirmed that the calcium ion facilitated the leaving group departure.