Modulation of surface charges of napin by enzymatic deamidation to improve its techno-functional properties

Napin is a major seed storage protein found in plants of the Brassica family and has an isoelectric point of 9-11, which may complicate its application in foods. Our aim was to reduce the isoelectric point of napin by enzymatic deamidation. Deamidation by protein glutaminase was optimized to reduce the isoelectric point of napin to 5. Intact protein LC-MS analysis showed that an average of 17 glutamines in napin were deamidated to glutamic acid, and the LC-MS data was found to correlate with ammonia release observed by an ammonia assay (R² = 0.973). Peptide-based LC-MS/MS analysis showed that the extent of deamidation was dependent on the location of glutamine residues in napin. Deamidation lowered the solubility to 70% at pH 5, which also facilitated formation of gels with 87.5 ± 2.2% water holding capacity after acidification with glucono-δ-lactone. Overall, these findings indicate that enzymatic deamidation to reduce the isoelectric point of napin enhanced its techno-functional properties and improving its potential as a food ingredient.