Determination of enzyme thermal parameters for rational enzyme engineering and environmental/evolutionary studies

Modeling of enzymatic activity of free β-glucosidase from palm weevil, Rhynchophorus palmarum Linn. (Coleoptera: Curculionidae) larvae: Effects of pH and temperature

Palm weevil, Rhynchophorus palmarum L., is an important pest of palm trees (Elaeis guineensis) around the tropical regions. Characterization of their digestive enzymes could be an important stage to develop appropriate pest control strategies. Study of these enzymes could also be of interest in different biotechnological applications. Among digestive enzymes, there is β-glucosidase which hydrolytically catalyzes the β-glycosidic linkage of glycosides. In the present work, the catalytic activity of β-glucosidase in the digestive juice of last larval instar of R. palmarum L. (Rpbgl) has been investigated using p-nitrophenyl-β-D-glucopyranoside (pNPG) as substrate. The "classical" physico-chemical properties for purified Rpbgl have been determined by the help of enzymatic activity modeling. Thus, the values of (325.4 ± 0.2) K, 5.28 ± 0.07 and (37.9 ± 0.6) kJ mol-1 were obtained for optimum temperature, optimum pH and activation energy, respectively. The pK values for enzyme-substrate complex are 4.25 ± 0.07 and 6.20 ± 0.07 for nucleophile and the proton donor, respectively. Enzyme kinetics study was also performed and the values of (127 ± 6) U mg-1 and (0.78 ± 0.08) mM were obtained for Vmax and Km, respectively. Using the Equilibrium model (EM), the thermal inactivation data were analyzed. ΔHeq, Teq, ΔGinact∗ and ΔGcat∗ were found to be (222 ± 4) kJ mol-1, (323.0 ± 0.1) K, (101.9 ± 0.2) kJ mol-1 and (53.37 ± 0.02) kJ mol-1, respectively. These results show that Rpbgl is less stable with a narrow temperature tolerance compared to other β-glucosidases.