Microbial transglutaminase-induced structural and rheological changes of cationic and anionic myofibrillar proteins

This study investigated the effects of microbial transglutaminase (TG) on structural changes in porcine myofibrillar protein (MP) under varying pH (2.0–6.0) and two ionic strength conditions (0.1 M versus 0.6 M NaCl). Lowering the pH below the isoelectric point (pI) of myosin induced protein unfolding as revealed by surface hydrophobicity and differential scanning calorimetry. Although the MP solubility at the low ionic strength (0.1 M NaCl) was maximal at pH 3.0, both SDS-PAGE profiles and dynamic rheology indicated TG could not cross-link MP under this condition. Based on the carboxyl group content, the TG-catalyzed deamidation was dominant at a pH lower than the pI of myosin (pH 5.0) while cross-linking occurred at higher pH. Moreover, deamidation had no effect on rheological properties of MP. The results indicate that the TG reaction was governed by the pH of substrate protein, and the reaction intensity was related to the solubility of protein.