Kinetic Studies of Polyphenol Oxidase from White Yam (Dioscorea rotundata Poir) Tuber

Abstract

Enzymatic browning is engendered by conversion of monophenols (cresolase activity) and ortho-diphenols (catecholase activity) to reactive ortho-quinones in the presence of molecular oxygen by polyphenol oxidases (PPOs). The present study ascertained the inhibition kinetics of PPO extracted from Dioscorea rotundata tuber in the presence of non-toxic sulfhydryl amino acid (L-cysteine) and appraised other connecting kinetic and thermodynamic properties of D. rotundata PPO (DR-PPO). DR-PPO was extracted and purified by (NH 4) 2 SO 4 precipitation, ultrafiltration and dialysis. Thermodynamic parameters and DR-PPO activity were measured by standard methods. Kinetic analyses showed that 4.0 mM L-cysteine displayed non-competitive inhibition prototype with DR-PPO experimental substrate. The affinity of L-cysteine for DR-PPO inhibitor binding site was ≈ 25 folds greater than that of catechol for the enzyme active site. Arrhenius plot gave an approximate first-order reaction with activation energy (Ea)=60.07 kJ mole-1. DR-PPO activity upon incubation at T=50°C for 5 min resulted to 28.11 ± 0.05% decay in relative enzyme activity. Likewise, within experimental pH values (pH 7.5-10.5 units), decay in relative DR-PPO activity ranged between 19.39 ± 0.01%-65.11 ± 0.05%. Furthermore, pH/DR-PPO activity profile displayed three peak values, which suggests the presence of DR-PPO isoforms. The present study propose the use of L-cysteine as an alternative inhibitor for DR-PPO activity alongside with thermal inactivation (T>70°C for 15 min) and pH adjustment (pH<7.5 units for 15 min) to alleviative and serve as control measures against enzymatic browning reactions in D. rotundata tuber.

Thermal inactivation kinetics parameters of browning enzymes in starfruit (Averrhoa carambola L.) juice

“…This indicates that when the heating temperature increased, PPO was inactivated at a slower rate than that of POD, hence it is more stable than POD. The E a value of the PPO for the current study was closed to that reported by Zhang & Shao (2015) for loquat fruit, but lower than E a for PPO in peaches (Toralles et al ., 2005), pineapple (Lee et al ., 2009), strawberry (Chisari et al ., 2007) and white yam (Chikezie, 2015).…”

The Development and Validation of Critical and Creative Thinking Skills Test in Enzyme for Undergraduate Chemistry Students

“…Enzymes like proteins are made up of many amino acids through peptide bonds. Amino acids are composed of carboxyl groups (-COOH) and amine groups (-NH 2 ) which are bound to the same C atom (also called alpha carbon) (Chikezie, 2014;Demir etal., 2012). The carboxyl group provides acidic properties and the amine group provides basic properties.…”