The role of residues R97 and Y331 in modulating the pH optimum of an insect β-glycosidase of family 1

Abstract

The activity of the digestive β-glycosidase from Spodoptera frugiperda (Sfβgly50, pH optimum 6.2) depends on E399 (pKa = 4.9; catalytic nucleophile) and E187 (pKa = 7.5; catalytic proton donor). Homology modelling of the Sfβgly50 active site confirms that R97 and Y331 form hydrogen bonds with E399. Site-directed mutagenesis showed that the substitution of R97 by methionine or lysine increased the E399 pK a by 0.6 or 0.8 units, respectively, shifting the pH optima of these mutants to 6.5. The substitution of Y331 by phenylalanine increased the pK a of E399 and E187 by 0.7 and 1.6 units, respectively, and displaced the pH optimum to 7.0. From the observed ΔpKa it was calculated that R97 and Y331 contribute 3.4 and 4.0 kJ·mol-1, respectively, to stabilization of the charged E399, thus enabling it to be the catalytic nucleophile. The substitution of E187 by D decreased the pK a of residue 187 by 0.5 units and shifted the pH optimum to 5.8, suggesting that an electrostatic repulsion between the deprotonated E399 and E187 may increase the pKa of E187, which then becomes the catalytic proton donor. In short the data showed that a network of noncovalent interactions among R97, Y331, E399 and E187 controls the Sfβgly50 pH optimum. As those residues are conserved among the family 1 β-glycosidases, it is proposed here that similar interactions modulate the pH optimum of all family 1 β-glycosidases.