Isopeptides

Endothelins comprise a family of three vasoactive isopeptides of 21 amino acids that have an essential role in the regulation of the vascular and bronchiolar tone and the control of natriuresis in the kidney. Endothelin peptides are also involved in nociception and have a critical role in the progression of prostate and ovarian cancer. 

The endothelin receptor subtypes show differences in their signal transduction, ligand binding and tissue distribution. The ETA receptor is isopeptide-selective and binds ET-1 and ET-2 with the same and ET-3 with 70-100-fold lower affinity. The ETB receptor binds all three isoforms with the same affinity. 

Small tfbiquitin-like modifier represents a family of evolutionary conserved proteins that are distantly related in amino-acid sequence to ubiquitin, but share the same structural folding with ubiquitin proteins. SUMO proteins are covalently conjugated to protein substrates by an isopeptide bond through their carboxyl termini. SUMO addition to lysine residues of target proteins, termed SUMOylation, mediates post-transla-tional modification and requires a set of enzymes that are distinct from those that act on ubiquitin. SUMOylation regulates the activity of a variety of tar get proteins including transcription factors. 

Small Ubiquitin-like modifier (SUMO) is a conserved protein that is ubiquitously expressed in eukaryotes and is essential for viability. It serves as a reversible posttranslational modifier by forming an isopeptide bond with lysine residues in many target proteins, in a catalytic process termed SUMOylation. SUMOylation of proteins results in altered inter- or intramolecular interactions of the modified target (Fig. 1). 

Modification by SUMO is a reversible and often highly dynamic process. Cleavage of the isopeptide bond between SUMO and its targets is accomplished by SUMO specific cysteine proteases of the Ulp/SENP family (Fig. 2). Six members were identified in humans,... 

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... 

Y Sohma, M Sasaki, Y Hayashi, T Kimura, Y Kiso. Novel and efficient synthesis of difficult sequence-containing peptides through O-N intramolecular acyl migration reaction of O-acyl isopeptides. Chem Commun 124, 2004. 

The amide bonds in peptides are usually called peptide bonds, and they are formed between C(l) of one amino acid and N(2) of another (sometimes called eupeptide bonds). Peptides may also include compounds linked by other amide bonds (sometimes called isopeptide bonds). 

Another degradation reaction observed in suspension was the formation of covalent insulin dimers [134][136], These involve isopeptide links between two insulin molecules, that result from a transamidation reaction mainly between the B-chain N-terminus of one insulin molecule, and one of the four amide side chains in the A-chain (principally AsnA21) of the second insulin molecule. 

Goldknope, I. L. and Busch, H. Isopeptide linkage between nonhistone and histone 2A polypeptides of chromosomal conjugate-protein A24. Proc. Natl. Acad. USA, 1977, 74, 864-868. 

UCHs are cysteine proteases in that the critical residue in the catalytic site is a cysteine. In addition, histidine and aspartate residues are critical for catalytic activity. All UCHs contain these residues even if they do not share a high degree of homology elsewhere in the sequence. For example, the Aplysia UCH (Ap-uch) critical for the induction oflong-term facilitation has only 39% homology to its human counterpart UCH-Lf. Ap-uch and UCH-Ll both contain the catalytic cysteine, histidine, and aspartate residues at similar positions in the molecule. UCHs cleave small peptide chains linked to the C-terminus of ubiquitin. UBPs can cleave the isopeptide bond between ubiquitins in a polyubiquitin chain and the isopeptide bond between the ubiquitin and the substrate. 

Figure 7 Multiple roles of the deubiquitinating enzymes. Deubiquitinating enzymes (DUBs) of the UCH type (dark scissors) process ubiquitin precursors. UCH-L1 generates monoubiquitins from tandemly linked ubiquitin gene product. UCH-L3 acts on ubiquitin synthesized as a protein fused to small ribosomal subunits. DUBs of the UBP type (shaded scissors) process ubiquitins linked in isopeptide linkage in polyubiquitin chains. DUBs also reverse the ubiquitination on erroneously targeted substrates (editing). Another important function of DUBs is disassembly of polyubiquitin chains as the ubiquitinated substrate is degraded. Ubiquitin attached to substrates after activation are indicated as lollipop-like structures with filled circles. Free ubiquitin or ubiquitin unit in precursor is shown with open circles.

The studies on specificity of UBPs with respect to isopeptide linkages of ubiquitin do not address the physiological substrate specificity, however. Although diverse substrates are polyubiquitinated, the nature of the isopeptide linkages of ubiquitin is the same in all substrates. Therefore, physiological specificity must arise from noncatalytic parts of the UBP molecules that interact with the substrates. A recent study shows that two... 

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. 

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