Influence of Microbial Transglutaminase on Physicochemical and Cross-Linking Characteristics of Individual Caseins

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_S-casein (α_S-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_S-CN and β-CN. Scanning electron microscopy (SEM) and particle size analysis respectively demonstrated that the κ-CN, α_S-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_S-CN (~7909.3 nm). The enzyme kinetics results showed K_M values of 3.04 × 10^-6, 2.37 × 10^-4 and 8.90 × 10^-3 M for κ-CN, α_S-CN and β-CN, respectively, and, furthermore, k_cat values of 5.17 × 10^-4, 1.92 × 10^-3 and 4.76 × 10^-2 1/s, for κ-CN, α_S-CN and β-CN, respectively. Our results revealed that the cross-linking of β-CN catalyzed by MTGase was faster than that of α_S-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.