Isopeptide proteins, structure

During processing and storage of foods, numerous reactions lead to protein structure modification. As well as denaturation and addition reactions, which lead to the formation of disulfide bridges and reversible oxidation of side chains of bound amino acids, isopeptide... 

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. 

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.

Fig. 1. A schematic diagram outlining the hierarchic structure of the ubiquitin system. In an ATP-dependent manner a thioester bond is formed between the C-terminus of ubiquitin and an internal cystein residue of the ubiquitin-activating enzyme. Subsequently, ubiquitin is transferred to a member of the family of ubiquitin-conjugating enzymes, which are also able to form a thioester bond with ubiquitin. The third class of enzymes, the ubiquitin ligases, direct ubiquitin to the proteolytic substrates. Different families of this class of enzymes are known, some of which are also able to form a thioester intermediate with ubiquitin (HECT-domain ligases). The final ubiquitin-substrate linkage is an isopeptide bond between the C-terminus of ubiquitin and internal lysine residues in the substrate proteins...

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.

Following the work on fibrin, the next protein to be extensively studied as far as the isopeptide was concerned was the heterogeneous structural protein keratin. In their work, Asquith et al. ( ) identified and isolated G-L from native wool keratin and subsequently isolated the aspartyl analogue e-(-B-ASPARTIC) LYSINE (A-L) from the same source (26). Other workers went on to identify the isopeptides in human hair, guinea pig hair, porcupine quills and protein from hair follicles (27, ). Other tissues where isopeptides have been located include avian and mouse muscle ( ), human stratum corneum ( ), erthyrocyte membranes... 

The function of the isopeptides in the various tissues is not clear, although they may contribute directly to the structure of the particular protein. For example, prior to keratinization or by simply increasing the molecular weight of a protein system confers enhanced stability, decreased solubility and increased resistance to enzymic hydrolysis. 

FIGURE 5.6 Model structure proposed for the fiber cuticle surface membrane. The exterior surface of the cuticle consists of a monolayer comprising C21 branched-chain fatty acids linked covalently via thioester bonds to the proteins comprising the Allworden-induced resistant membrane (epicuticle). The epicuticle consists of an inert protein matrix containing isopeptide cross-links and is linked via an unknown mechanism to the underlying a-layer of the exocuticle. 

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