Transglutaminase (TGase, EC 184.108.40.206) has the systematic name of protein-glutamine -glutamyltransferase. It catalyzes the acyl transfer reaction between -carboxyamide groups of glutamine residues in proteins, peptides, and various primary amines. When the -amino group of lysine acts as acyl acceptor, it results in polymerization and inter- or intramolecular cross-linking of proteins via formation of -( -glutamyl) lysine linkages.
This occurs through exchange of the -amino group of the lysine residue for ammonia at the carboxyamide group of a glutamine residue in the protein molecule(s). Formation of covalent cross-links between proteins is the basis for TGase to modify the physical properties of protein foods. The addition of microbial TGase to surimi significantly increases its gel strength, particularly when the surimi has lower natural setting abilities (presumably due to lower endogenous TGase activity).
Thus far, the primary applications of TGase in seafood processing have been for cold restructuring, cold gelation of pastes, or gel-strength enhancement through myosin cross-linking. In the absence of primary amines, water may act as the acyl acceptor, resulting in deamination of -carboxyamide groups of glutamine to form glutamic acid (Ashie and Lanier 2000).
PROTEOLYSIS DURING CHEESE FERMENTATION
Chymosin (rennin) is an enzyme present in the calf stomach. In cheese making, lactic acid bacteria (starter) gradually lower the milk pH to the 4.7 that is optimal for coagulation by chymosin. Most lactic acid starters have limited proteolytic activities. However, other added lactic acid bacteria have much stronger proteolytic activities.
These proteases and peptidases break down the milk caseins to smaller protein molecules and, together with the milk fat, provide the structure of various cheeses. Other enzymes such as decarboxylases, deaminases, and transaminase are responsible for the degradations of amino acids into secondary amines, indole, -keto acids, and other compounds that give the typical flavor of cheeses.