Food Biochemistry and Food Processing (2 edition)

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14 Seafood Enzymes: Biochemical Properties and Their Impact on Quality 273

(dry basis), respectively. With pre-swelling in acid for 24 hours,

collagen extracted with bigeye snapper pepsin for 48 hours had

the yield of 19.79%, which was greater than that of collagen

extracted using porcine pepsin at the same level (13.03%) (Nali-

nanon et al. 2007).

TRANSGLUTAMINASE

Transglutaminase (TGase; EC 2.3.2.13) is a transferase whose

systematic name is protein-glutamineγ-glutamyltransferase.

The enzyme catalyzes the acyl transfer reaction in which the

γ-carboxyamine groups of glutamine residues in proteins, pep-

tides, and various primary amines, act as acyl donors and primary

amino groups includingε-amino groups of lysine residues, either

as peptide-proteins bound or free lysine, act as the acyl accep-

tor (Folk 1980, Worratao and Youngsawatdigul 2005). When

acceptors areε-amino groups of lysine residues, the forma-

tion ofε-(γ-glutamyl)lysine linkages occur both intra-and in-

termolecularly (Folk 1980). This occurs through the exchange

of theε-amino groups of the lysine residues for the ammo-

nia at theγ-carboxyamide group of a glutamine residue in

a protein molecule (Fig. 14.4). Formation of covalent cross-

links between proteins is the basis of the ability of TGase to

modify the physical characteristics of protein foods (Ashie and

Lanier 2000). These bonds are stable and resistant to proteolysis

(Worratao and Youngsawatdigul 2005). Amino acid sequence

and the charge of amino acids surrounding the susceptible glu-

tamine residue and local secondary structures affected the cross-

linking capability (Folk 1980).

TGase has been widely used to improve functional proper-

ties of various food proteins (Benjakul et al. 2004a). TGase

is an endogenous fish enzyme responsible for cross-linking of

proteins and change textural properties of fish sol during pro-

cessing, especially surimi production (Ashie and Lanier 2000).

This is known to reinforce the thermal myosin gelation that oc-

curs during surimi production by the formation of intra- and

intermolecular cross-links between myosin heavy chains known

as setting. This setting phenomenon, called suwari, is one of the

important processes for manufacturing surimi-based products

(Shahidi and Kamil 2001).

Transglutaminase

Protein solution Protein gel

=H 2 O

=ε-(γ-glutamyl) lysine crosslink

Figure 14.4.Cross-linking of proteins by transglutaminase.