| Summary: | The effects of microbial transglutaminase (MTGase) cross-linking on the physicochemical characteristics of individual caseins were investigated. MTGase was used to modify three major individual caseins, namely, κ-casein (κ-CN), α<sub>S</sub>-casein (α<sub>S</sub>-CN) and β-casein (β-CN). The SDS-PAGE analysis revealed that MTGase-induced cross-linking occurred during the reaction and that some components with high molecular weights (>130 kDa) were formed from the individual proteins κ-CN, α<sub>S</sub>-CN and β-CN. Scanning electron microscopy (SEM) and particle size analysis respectively demonstrated that the κ-CN, α<sub>S</sub>-CN and β-CN particle diameters and protein microstructures were larger and polymerized after MTGase cross-linking. The polymerized κ-CN (~749.9 nm) was smaller than that of β-CN (~7909.3 nm) and α<sub>S</sub>-CN (~7909.3 nm). The enzyme kinetics results showed <i>K</i><sub>M</sub> values of 3.04 × 10<sup>−6</sup>, 2.37 × 10<sup>−4</sup> and 8.90 × 10<sup>−3</sup> M for κ-CN, α<sub>S</sub>-CN and β-CN, respectively, and, furthermore, <i>k</i><sub>cat</sub> values of 5.17 × 10<sup>−4</sup>, 1.92 × 10<sup>−3</sup> and 4.76 × 10<sup>−2</sup> 1/s, for κ-CN, α<sub>S</sub>-CN and β-CN, respectively. Our results revealed that the cross-linking of β-CN catalyzed by MTGase was faster than that of α<sub>S</sub>-CN or κ-CN. Overall, the polymers that formed in the individual caseins in the presence of MTGase presented a higher molecular weight and larger particles.
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