Studies on Thermodynamics and Kinetics of Thermo- Inactivation of Some Quality-Related Enzymes in White Yam (Dioscorea rotundata)

Abstract

The effect of heat treatment on the activities of three quality related enzymes:-peroxidase (POD), polyphenol oxidase (PPO), and lipoxygenase (LOX), from edible white yam (Dioscorea rotundata) was studied over a temperature range of 50 to 80°C using mathematical analysis of the kinetic and thermodynamic parameters for the thermoinactivation of the enzymes. Denaturation of these enzymes, measured by loss in activity, could be described by a simple first-order reaction that was resolved into biphasic inactivation curves. This indicates the existence of two isoforms of different thermal stabilities with k-values between 0.032 and 0.525. D-values decreased with increasing temperature, indicating faster inactivation of the enzymes at higher temperatures. Results suggested that peroxidase is relatively more thermostable than polyphenol oxidase and lipoxygenase with a Z-value of 4.11, Ea of 2510kJ mol -1 for the first phase of the biphasic inactivation reaction. The Gibbs free energy (ΔG), values range from -552.95 to 279.01kJ/mol for the three enzymes. The results indicate that the oxidation reactions were: (1) not spontaneous (ΔG > 0) for peroxidase at low temperatures, (2) spontaneous (ΔG < 0) for lipoxygenase (3) slightly endothermic (ΔH > 0) for lipoxygenase and (4) reversible (ΔS < 0) for all the three enzymes at all temperature. The high z-value obtained for peroxidase in the first phase of inactivation indicates that a high amount of energy was required to initiate its denaturation, and supports why it is used as a marker for inactivation of quality-related enzymes. Materials and Methods Preparation of homogenatesFresh uninfected yam tubers were peeled, washed and diced into Citation: Eze SOO, Chilaka FC, Nwanguma BC (2010) Studies on Thermodynamics and Kinetics of Thermo-Inactivation of Some Quality-Related Enzymes in White Yam (Dioscorea rotundata). J Thermodyn Catal 1:104.

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“…The fact that ΔH values decreased with increase in temperature reveals that less energy is required to denature the enzyme at high temperature (Bhatti et al, 2006). A similar trend in the values of ΔH was observed in peroxidase from white yam (Eze et al, 2010) though with different magnitude, α-amylase from digestive tract of tropical house cricket (Kouadio et al, 2013), α-amylase from sorghum bicolar (Kumar, 2008). As a rule of thumb, the higher the Ea (D) , the higher the ΔH (D) .…”

“…The negative values for change in entropy could have some implications which include; compactation of the protein, ordering of water molecules in the vicinity of the hydrophobic residues and possible aggregation during denaturation (Gummadi, 2003;Marin et al, 2003). A similar trend was observed in peroxidase, lipoxygenase and polyphenol oxidase from white yam (Eze et al, 2010) and xylanase from Aspergillus niger (Pal and Khanum, 2011) but the magnitude was higher in enzymes from white yam, showing that they undergo more aggregation and compactation during inactivation. The differences observed here could be as a result of change in the conformation of the enzymes studied (Gouda et al, 2003).…”

“…This is an indication that the two enzymes may have similar amino acid sequence. However, Eze et al (2010) obtained a negative ΔG (D) values from peroxidase of white yam showing that the enzyme is less thermostable when compared to the one studied herein. Also the difference could be attributed to the fact that two enzymes might have different primary structure.…”

“…4) (Tables I and II) for α-1 and α-2 respectively. Ea (D) of 2510KJmol -1 k -1 was obtained from peroxidase from white yam (Eze et al, 2010). Ea (D) of 245.89KJmol -1 K -1 and 182.92KJmol -1 K -1 were obtained from the digestive tract of tropical house cricket α-amylase (Kouadio et al, 2013).…”

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“…Discrete molecules or nanoparticles of finishes can be brought individually to designated sites on textile materials in a specific orientation and trajectory through thermodynamics, [35] electrostatic or other technical approaches.…”