Structure crosslinked amino acids

Nakamura F, Yamazaki K and Suyama K (1992) Isolation and structural characterization of a new crosslinking amino acid, cyclopentenosine, from the acid hydrolysate of elastin. Biochem Biophys Res Comm 186, 1533-1538. 

Figure 7. Postulated structures of crosslinked amino acids derived from interaction of protein functional groups (NH, NH2, OH, SH) with the double bond of dehydroalanine and methyl dehydroalanine side chains. Asymmetric centers are designated by asterisks.

The structural versatility of pseudopoly (amino acids) can be increased further by considering dipeptides as monomeric starting materials as well. In this case polymerizations can be designed that involve one of the amino acid side chains and the C terminus, one of the amino acid side chains and the N terminus, or both of the amino acid side chains as reactive groups. The use of dipeptides as monomers in the manner described above results in the formation of copolymers in which amide bonds and nonamide linkages strictly alternate (Fig. 3). It is noteworthy that these polymers have both an amino function and a carboxylic acid function as pendant chains. This feature should facilitate the attachment of drug molecules or crosslinkers,... 

A small number of proteins, and again insulin is an example, are synthesized as pro-proteins with an additional amino acid sequence which dictates the final three-dimensional structure. In the case of proinsulin, proteolytic attack cleaves out a stretch of 35 amino acids in the middle of the molecule to generate insulin. The peptide that is removed is known as the C chain. The other chains, A and B, remain crosslinked and thus locked in a stable tertiary stiucture by the disulphide bridges formed when the molecule originally folded as proinsulin. Bacteria have no mechanism for specifically cutting out the folding sequences from pro-hormones and the way of solving this problem is described in a later section. 

The modification of amino acids in proteins and peptides by oxidative processes plays a major role in the development of disease and in aging (Halliwell and Gutteridge, 1989, 1990 Kim et al., 1985 Tabor and Richardson, 1987 Stadtman, 1992). Tissue damage through free radical oxidation is known to cause various cancers, neurological degenerative conditions, pulmonary problems, inflammation, cardiovascular disease, and a host of other problems. Oxidation of protein structures can alter activity, inhibit normal protein interactions, modify amino acid side chains, cleave peptide bonds, and even cause crosslinks to form between proteins. 

These studies suggest that fine structural and mechanistic information can be obtained from site-specific photocrosslinking using genetically incorporated unnatural amino acids. The potential of photocrosslinkers to identify biomolecular interactions more specifically than general formaldehyde crosslinking, combined with its... 

Proteins crosslinked by formaldehyde are important in photography, the leather industry and in bio-medical sciences. Due to the complex structure of the gelatin molecules (consisting of approximately 20 Afferent kinds of amino acids) and the very low crosslink density, it is not possible to detect crosslink resonances under normal conditions. In order to overcome this problem a 13C enriched formaldehyde is used. By comparison with the chemical shifts of model crosslink compounds it is concluded that the predominant crosslink is formed between the lysine and arginine components in gelatin. A possible mechanism for the reaction between these two amino acid components and the formaldehyde has been proposed 154>. 

The structures of the Type III and V collagen telopeptides have been less studied. However, the NMR study of Type III telopeptides has been reported, and the 22-amino-acid G-terminal telopeptide is extended with a tight turn involving residues 8-11 (Liu et al., 1993). Crosslink analysis reveals connectivity between the G-terminal telopeptide of Type III collagen and the N-terminal helical region of another Type III molecule (Henkel, 1996). 

Two types of crosslinking domains exist in tropoelastin those rich in alanine (KA) and those rich in proline (KP). Within the KA domains, lysine residues are typically found in clusters of two or three amino acids, separated by two or three alanine residues. These regions are proposed to be Q-helical with 3.6 residues per turn of helix, which has the effect of positioning two lysine sidechains on the same side of the helix, although there is no direct structural evidence (Brown-Augsburger et al., 1995 Sandberg et al, 1971), and facilitating the formation of desmosine crosslinks. Desmosine crosslinks are formed by the condensation of two allysine... 

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