Physicochemical properties, kinetics and thermodynamic studies of polyphenol oxidase from sorghum (Sorghum bicolor (L.) Moench) for potential use in industry

Abstract

Advances Polyphenol Oxidase (PPO) from Sorghum bicolor (white and yellow varieties) grains were investigated for optimum processing condition. The partially purified enzyme was obtained from two varieties of Sorghum bicolor by step-wise separation through ion-exchange and size-exclusion chromatography. The final purification gave a yield of 7.33 % and 12.3 % for PPO from white and yellow sorghum respectively. The PPO has V_max and K_m of 2.66 U.mL⁻¹ and 19.72 mM for white sorghum, 1.33 U.mL⁻¹ and 12.92 mM for yellow sorghum. The optimal pH of PPO activity was found at pH 4 and pH 7 for white and pH 4 and pH 8 for yellow sorghum. The pKa 7.4 and 8.7 were obtained for PPO from white sorghum, and pK_a 5.4, 7.4 and 8.5 for yellow sorghum. The PPO residual activity were above 70 % at 5 hours of incubation within the neutral pH ranges for white sorghum, while those of yellow sorghum were below 40 %. The optimum temperature of 40 ºC and 30 ºC for white and yellow sorghum PPO respectively. The average value of enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG) obtained at 20 min of incubation and temperature 50 – 80 °C were respectively 49.03 kJ.mol⁻¹, - 129.52 J.mol⁻¹.K⁻¹, and 92.81 kJ.mol⁻¹ for white sorghum PPO, and 90.1 kJ.mol⁻¹, - 9.29 J.mol⁻¹.K⁻¹, and 93.37 kJ.mol⁻¹ for yellow sorghum PPO. Zn²⁺, Fe²⁺ and ascorbic acid inhibited PPO while Cu²⁺, Na⁺ and K⁺ activated the enzyme. The results suggest the processing parameters for controlling PPO in potential industrial application of white and yellow sorghum grains.