Optimization of the Enzymatic Deamidation of Soy Protein by Protein-Glutaminase and Its Effect on the Functional Properties of the Protein

Abstract

The effects of enzymatic deamidation by protein-glutaminase (PG) on the functional properties of soy protein isolate (SPI) were studied. Conditions for the deamidation were evaluated by means of response surface methodology (RSM). Optimal conditions based on achieving a high degree of deamidation (DD) with a concurrently low degree of hydrolysis (DH) were 44 °C, enzyme:substrate ratio (E/S) of 40 U/g protein and pH 7.0. Under optimal conditions, both DD and DH increased over time. SDS-PAGE results indicated that lower molecular mass subunits were produced with increasing DD. Far-UV circular dichroism spectra revealed that the α-helix structure decreased with higher DD, while the β-sheet structure increased until 15 min of deamidation (32.9% DD), but then decreased at higher DD. The solubility of deamidated SPI was enhanced under both acidic and neutral conditions. SPI with higher DD showed better emulsifying properties and greater foaming capacity than SPI, while foaming stability was decreased. It is possible to modify and potentially improve the functional properties of SPI by enzymatic deamidation using PG.

Oat protein solubility and emulsion properties improved by enzymatic deamidation

“…In other words, the PG deamidated oat protein did not need extra heating for solubilizing. The significant increase of oat protein solubility was in agreement with the effects of PG deamidation on maize a-zein (Yong et al, 2004), wheat gluten (Yong et al, 2006) and soy protein isolate (Suppavorasatit et al, 2011). The combined effects of the increase in the protein structural flexibility and the increase in the protein net charge might be assumed as the mechanism for the improvement in oat protein water solubility by PG deamidation.…”

“…1 A). The increase of DD caused by increasing PG dosage was also shown in PG reactions with soy protein isolate and maize a-zein (Yong et al, 2004;Suppavorasatit et al, 2011).…”

“…Different superscript letters indicate significant differences at P < 0.05 (Tukey's test). (Suppavorasatit et al, 2011). These findings revealed that the accessibility of PG to the glutamine amino acids in the globular structures (oat protein and soy protein) were not as good as in the more elongated wheat gluten (Gu et al, 2001).…”

“…Suppavorasatit et al (2011) found commercial PG significantly hydrolyzed the soy protein isolate into smaller polypeptides. One possible reason could be that the PG (Amano 500) used in the work of Suppavorasatit et al (2011) was different from the PG (Amano 50) in the present work and might contain residual protease activity present as contaminant. In addition, the conversion of glutamine to charged glutamic acid might cause intramolecular electrostatic repulsive force within soy globulin and result in fragmentation of the protein molecules.…”

Gene Cloning, Purification, and Characterization of a Novel Peptidoglutaminase-Asparaginase from Aspergillus sojae

“…Although the deamidation of proteins can be performed either enzymatically or nonenzymatically (i.e., chemically), enzymatic protein modification has several advantages over chemical treatment, such as milder reaction conditions, increased reaction rate, and higher specificity. Specifically, transglutaminase, protease, peptidoglutaminase, and protein-glutaminase enzymes have been shown to catalyze protein deamidation (5,6,26,34).Soy sauce is a traditional Japanese seasoning that is manufactured by fermenting steamed soybeans and roasted wheat with koji molds (Aspergillus sojae or Aspergillus oryzae) along with water and salt. A. sojae is preferentially used to make soy sauce due to its high proteolytic activity.…”