Isopeptide bond formation

Figure 21. General scheme for isopeptide bond formation between amino and carboxyl side groups of proteins and additional amino acids. X and Z are amino and carboxyl protecting groups, respectively Y is a carboxyl-activating group (114).

Figure 3. Structure of peptide and isopeptide bonds resulting from covalent attachment of amino acids to proteins by chemical methods. In isopeptide bond formation Rt = -CH2- or -CH2CH2- of aspartic or glutamic acid and R2 = -(CH2)n- of lysine.

The transglutaminases are calcium-dependent enzymes that catalyse the cross-linking of proteins by promoting the formation of isopeptide bonds between the /-carboxyl group of a glutamine in one polypeptide chain and the e-amino group of a lysine in the second (Greenberg et al., 1991). These... 

Heating of foods rich in proteins may lead to formation of crosslinking isopeptide bonds between the S-NH2 group of lysine and the p- and y-carboxyl groups of aspartic and glutamic acid residues or their amides. 

A peptide is any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another. The amide bonds in peptides are called peptide bonds. The word peptide is usually applied to compounds whose amide bonds (sometimes called eupeptide bonds) are formed between C-1 of one amino acid and N-2 of another, but it includes compounds with residues linked by other amide bonds (sometimes called isopeptide bonds). Peptides with fewer than about 10-20 residues may also be called oligopeptides those with more residues are called polypeptides. Polypeptides of specific sequence of more than about 50 residues are usually known as proteins, but authors differ greatly on where they start to apply this term. 

Fig. 8. Formation of isopeptide bond catalyzed by Factor XHIa. The chemical reaction was catalyzed by Factor XHIa, yielding insoluble fibrin crosslinked by Ne-(7 glutamyl) lysine bonds. Factor XIII is activated to Factor XHIa by thrombin in the presence of calcium ions and fibrin.

Each a chain contains potential glutamine acceptor sites at 221, 237, 328, and 366, and donor sites at lysine 508, 539, 556, 580, and 601 (Greenberg et al., 2003 Matsuka et al., 1996). Since the aC domains associate even in the absence of crosslinking, these interactions probably bring acceptor and donor sites in proximity, facilitating the formation of the isopeptide bonds. These bonds create a covalendy connected network of aC domains, although litde is known of its structure. In addition, there are lesser amounts of 7 trimers, tetramers, and 07 complexes. Factor XIII polymorphisms can have effects on the structure and properties of the fibrin clot (Ariens et al., 2002). 

Ubiquitin is highly conserved in eukaryotes yeast and human ubiquitin differ at only 3 of 76 residues. The carboxyl-terminal glycine residue of ubiquitin (Ub) becomes covalently attached to the e-amino groups of several lysine residues on a protein destined to be degraded. The energy for the formation of these isopeptide bonds (iso because e- rather than a-amino groups are targeted) comes from ATP hydrolysis. 

It would be useful to look at the possibility of using these enzymes for formation of cross-links in proteins leading to textured products. Asquith et al. (65) have recently reviewed the occurrence of isopeptide bonds in proteins. 

A calcium-dependent transglutaminase existing in the inner root sheath and the medullary cells [186-188] catalyzed the formation of isopeptide bonds. Evidence obtained by Rogers et al. [189] suggests that proteins containing the gJu-lys bond, or, more correctly. 

Modifications ensuing from the formation of novel covalent bonds in food proteins include (1) intra-and intermolecular formation of isopeptide bonds (also as a consequence of the use of transglutaminase... 

The clot is stabilized by the formation of covalent bonds introduced by a transglutaminase. Factor Xllla (Lorand et al, 1980 Henschen McDonagh, 1986). The active enzyme. Factor Xllla, is generated from its precursor. Factor XIII, by the action of thrombin (Fi Me 1). The acti dty of Factor Xllla is dependent on both calcium ion concentration and fibrin (ogen). As many as six isopeptide bonds are formed between the side chains of lysine (donor) and glutamine (acceptor) residues. The Y chains of fibrinogen are ligated first, followed by the carboxyl terminal a. chains. The formation of these covalent bonds renders the clot mechanically... 

Transglutaminase. Transglutaminase is a calcium-dependent enzyme that catalyses the formation of E-(y-glutamyl)lysine isopeptide bonds between a polypeptide-bound glutamine and a lysine of the protein. 

Heating proteins in a dry state at neutral pH results in the formation of isopeptide bonds between the e-amino groups of lysine residues and the P- or y-carboxamide groups of asparagine and glutamine residues ... 

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