Kinetics of Acid Inactivation of Soybean Lipoxygenase and Its Effects on the Functional Properties of Soy Protein
Ali, Asbi Bin
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https://hdl.handle.net/2142/70105
Description
Title
Kinetics of Acid Inactivation of Soybean Lipoxygenase and Its Effects on the Functional Properties of Soy Protein
Author(s)
Ali, Asbi Bin
Issue Date
1988
Department of Study
Food Science
Discipline
Food Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Agriculture, Food Science and Technology
Abstract
Inactivation of soybean lipoxygenase by acid pH was investigated by grinding whole soybeans in water acidified with HCl. The enzyme was found to be completely and irreversibly inactivated at pH 3.0 and below. From a study of enzyme kinetics of lipoxygenase extracted from the acidified soy slurries, significant changes were observed with the values of the Michaelis constant and in the enzyme inhibition patterns when the enzyme was exposed to pH of about 4.6 and below. This suggests a change in the conformation of lipoxygenase below this pH limit. A separate study using a purified lipoxygenase-1 supported the evidence that the enzyme was stable when preincubated between pH 3.0 and 9.0 but lost all of its activity when exposed to pH 3.0 and below. The purified enzyme also obeyed Michaelis-Menten kinetics at pH 5.6 to 9.2. A Dixon plot suggested a histidine residue to be the active site in the enzyme for the hydroperoxidation reaction.
The effect of acid inactivation of soybean lipoxygenase on the functional properties of the soy protein were also investigated. Protein extractability and solubility was found to be significantly reduced when the soybeans were treated at pH 3.6 and below. Emulsifying activity was not affected by acidic pH. However, emulsion stability decreased while viscosity increased when soy proteins were treated at pH 3.0 and below. Water sorption isotherm studies suggested a decrease in water binding capacity for soy proteins treated at pH 3.0 and below. NMR measurements of hydrated soy proteins exhibited two states of water, namely polymer-plus-capillary and polymer-plus-capillary-plus-surface water. An electrophoretic analysis of dialyzed soy proteins revealed that the SDS-PAGE profiles for the proteins treated at pH 3.07 and below were similar to that of the heat-denatured protein sample suggesting that chemical denaturation had taken place. The protein quality as measured by a rapid microbiological assay for the soy protein treated at pH 3.0 was estimated to be at a PER of about 2.1.
This study shows the potential of producing soy proteins from whole soybeans treated at pH 3.0 which are free from lipoxygenase activity but which may have slightly reduced functional properties relative to those of the native soy proteins as a result of chemical denaturation.
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