In this process, the C-terminal carboxyl group of ubiquitin becomes attached to the e-amine of a lysine residue of the substrate protein through an isopeptide
The biological functions of the isopeptide
and ester cross-links appear to be conferral of incredible mechanical, chemical and thermal stability, while thioester cross-link function in adhesion to host tissue via formation of a strong covalent bond.
Baker, "Stabilizing isopeptide
bonds revealed in gram-positive bacterial pilus structure," Science, vol.
Abbreviations DTT: Dithiothreitol Cbs: Cystathionine [beta]-synthase Cse: Cystathionine [gamma]-lyase Mthfr: Methyltetrahydrofolate reductase Cys: Cysteine Hcy: Homocysteine HHcy: Hyperhomocysteinemia HPLC: High-performance liquid chromatography Hcy-protein: Hcy linked to a protein via an amide (pep tide or isopeptide
) bond Met: Methionine NAC: N-acetyl cysteine N-Hcy-protein: Hcy linked to a protein via an isopeptide
bond OPA: o-Phthaldialdehyde S-Hcy-protein: Hcy linked to a protein via a disulfide bond SDS: Sodium dodecyl sulfate TCEP: Tris(2-carboxyethyl)phosphine.
Fibrin cross-linking is a highly specific acyl transfer reaction consisting of two steps: (1) glutamine of the substrate forms a binary complex with the active site cysteine of the enzyme through the thioester linkage, accompanied by ammonia release; (2) the acyl group of the binary complex is transferred to the acyl acceptor amine and form an isopeptide
, releasing the enzyme at the same time.[sup] Fibrin included two a-chains, two [sz]-chains, and two a-chains, linked by the disulfide bond.
Generally, calcium ion released from those compounds induces endogenous transglutaminase (TGase), which catalyzes the formation of [epsilon]-([gamma]-glutamyl) lysine cross-links (isopeptide
covalent bonds) between myofibrillar proteins during setting, and thus improves the texture of surimi gel .
UBZ2E functions as a ubiquitin-conjugating enzyme to activate ubiquitin-like modifier (UBL), which is transferred onto the target protein with an isopeptide
bond formation .
FXIIIa is a transglutaminase that catalyzes the formation of isopeptide
bonds between the free amine group of a lysine residue and the acyl group at the end of the side chain of a glutamine residue .
These chains are formed by the successive attachment of monomers by an isopeptide
bond, most frequently formed between the side chain of Lys-48 in one ubiquitin and the carboxyl group of the C-terminal Gly-76 of a neighbouring ubiquitin.
(37) The additional carbon in the homocysteine side chain permits it to exist as a thiolactone, which reacts with the Lys residues to form isopeptide
Johnson, "Rapid, single-step procedure for the identification of transglutaminase-mediated isopeptide
crosslinks in amino acid digests," Journal of Chromatography B, vol.
Simon et al., "Controlled in situ preparation of A[beta](1-42) oligomers from the isopeptide
"iso-A[beta]i(1-42)", physicochemical and biological characterization," Peptides, vol.