# Different responses of fish/microbial transglutaminase to salt and ultrasound: Implications for myosin cross-linking

## TL;DR

This study compares how fish and microbial enzymes respond to salt and sound waves when used to link muscle proteins in food processing.

## Contribution

The study reveals distinct effects of salt and ultrasound on fish and microbial transglutaminase activity and structure during myosin cross-linking.

## Key findings

- Microbial transglutaminase activity increased with salt concentration, while fish transglutaminase activity decreased.
- High-intensity ultrasound combined with salt enhanced microbial transglutaminase activity and structural changes.
- Ultrasound pretreatment improved myosin cross-linking efficiency but had conflicting effects on water holding capacity depending on the enzyme.

## Abstract

This study investigated the differential responses of fish transglutaminase (FTGase) and microbial transglutaminase (MTGase) to NaCl concentrations and high intensity ultrasound (HIU) during myosin cross-linking. Increasing NaCl concentrations enhanced MTGase activity by 34.5% at 0.8 mol/L compared to the control without NaCl. This enhancement was accompanied by structural unfolding, as evidenced by increased UV absorption intensity, which indicated greater exposure of aromatic residues. In contrast, FTGase activity progressively declined with increasing NaCl concentrations, with minimal structural changes observed. Given its Ca2+-independent property, MTGase was used to explore the synergistic effect of HIU and NaCl. The combination of 400 W HIU and 0.3 mol/L NaCl induced the most pronounced structural changes in MTGase, which exposed some buried reactive sulfhydryl groups and elevated activity by 17.0%. Furthermore, HIU pretreatment of both enzymes enhanced their catalytic efficiency for myosin cross-linking, as evidenced by increased formation of ε-(γ-Glu)-Lys isopeptide bonds. Notably, while HIU-pretreated FTGase improved the water holding capacity (WHC) of myosin samples, HIU-pretreated MTGase likely induced excessive cross-linking, which paradoxically reduced WHC. Conversely, applying HIU directly to preformed enzyme-myosin complexes disrupted established cross-links. These findings provide a basis for optimizing transglutaminase applications in food processing using HIU.