Kinetic and thermodynamic properties of wall bound invertase in heat tolerant and susceptible cultivars of wheat

Abstract

We have identified two types of invertases, one bound ionically and the other covalently to the particulate fraction in grains of heat tolerant C 306 and heat susceptible WH 542 cultivars of wheat (Triticum aestivum L.). The cell walls contained a high level of invertase activity, of which 79.2–72.8% was extractable by 2 M NaCl and 14.9–21.1% by 0.5% EDTA in C 306 and WH 542, respectively. The NaCl-released invertase constituted the predominant fraction. Using 5–100 mM sucrose and pH range of 4.0–7.0, the apparent Michaelis constant (K _m, enzyme substrate affinity measure) of enzyme ranged from 5.73 to 16.06 mM for C 306 and from 6.08 to 19.86 mM for WH 542. The V _max (maximum catalytic rate) values at these pH were higher in C 306 (0.63–11.04 μg sucrose hydrolysed min⁻¹) than WH 542 (0.51–8.73 μg sucrose hydrolysed min⁻¹). By employing photo-oxidation and by studying the effect of pH on K _m and V _max, the involvement of histidine and α-carboxyl groups at the active site of the enzyme was indicated. The two cultivars also showed differential response in terms of thermodynamic properties of the enzyme i.e. energy of activation (E _a), enthalpy change (ΔH) and entropy change (ΔS). NaCl-released invertase showed differential response to metal ions in two cultivars suggesting their distinctive nature. Mn²⁺, Cu²⁺, Hg²⁺, Mg²⁺, Zn²⁺ and Cd²⁺ were strong inhibitors in WH 542 as compared to C 306 while K⁺, Ca²⁺ were stimulators in both the cultivars. Overall the results suggest that genetic differences exist in wall bound invertase properties of wheat grains as evident in its altered kinetic behaviour.