Peptidyl lysine

For purposes of this manuscript, we wish to concentrate only on the steps leading to the formation of desmosines, amino acids found predominantly in elastin. With respect to their formation, the following suggests their spontaneous formation from peptidyl lysine and the oxidation product, peptidyl allysine. Narayanan et al. (28,29) have shown that when purified lysyl oxidase and non-crosslined elastin, specifically tropoelastin, are incubated together, the desmosines are formed. Desmosine formation, however, only occurs at temperatures that favor fibrillar arrangements of tropoelastin. Subsequently, it is felt that the maturation of non-crosslinked elastin into cross-linked elastin appears to involve only two major steps, namely insolublization through the formation of fibrils and fixation of the fibrils by crosslinking. 

To form the desmosines, three peptidyl allysine molecules and a molecule of peptidyl lysine must condense. The steps in condensation probably involve the formation of 1,2-dihydropyridines and 1,4-dihydropyridines as shown in Figure 4 (19-24,46,48). Several kinds of chemical evidence (46,48) suggest that the hydropyridines are easily oxidized under normal oxygen tension to corresponding pyridinium ions, such as the desmosines (isodesmosine or desmosine). The exact pathway by which the desmosines are formed, however, is still not clear. 

Lysine tyrosylquinone (LTQ) (Figure 3) is the protein-derived cofactor of mammalian lysyl oxidase, an important enzyme in the metabolism of connective tissue. Lysyl oxidase catalyzes the posttranslational modification of elastin and collagen. It oxidizes selected peptidyl lysine residues to peptidyl a-aminoadipic -semialdehyde residues. This initiates formation of the covalent cross-linkages that insolubilize these extracellular proteins. This enzyme also contains copper as a second prosthetic group. 

To summarize, the defects in connective tissue development that accompany copper-deficiency result from a decrease in the activity of the copper-dependent enzyme, lysyl oxidase. Lysyl oxidase catalyzes the formation of aldehydes (allysine) from peptidyl lysine in pre-elastin and pre-collagen. These aldehyde residues condense to form the covalent cross-links in collagen and elastin. In copper-deficient tissues, production of the intermediate residues, allysine, is impaired cross-linking is prevented and the result is a fragility and loss of strength in elastin and... 

B. L, (1970) Oxidation of peptidyl lysine by an amine oxidase from bovine aorta. Biochem. Biophys. Res. Commun, 40 1391. 

Peptidyl lysine-5-hydorxylase Ty mine-5 -oxygenase 4-Hydroxylphenylpyruvate dioxygenase... 

Peptidyl hydroxyprohne and hydroxylysine are formed by hydroxylation of peptidyl proline or lysine in reactions catalyzed by mixed-function oxidases that require vitamin C as cofactor. The nutritional disease scurvy reflects impaired hydroxylation due to a deficiency of vitamin C. 

The modified aldehyde group can spontaneously condense with vicinal peptidyl aldehydes or with e-amino groups of peptdidyl lysine. An example of such a cross-link found in collagen is shown in Figure 12. 

This enzyme [EC 1.4.3.13], also referred to as protein-lysine 6-oxidase, catalyzes the reaction of a peptidyl-L-lysyl-peptide with dioxygen and water to produce a peptidyl-allysyl-peptide, ammonia, and hydrogen peroxide. The enzyme will also act on 5-hydroxylysyl residues in proteins. 

Fluoromethyl ketones are one of the most widely used classes of peptidyl a-fluoroalkyl ketones, second only to trifluoromethyl ketones. Peptidyl fluoromethyl ketones are very effective as irreversible inhibitors of cysteine proteases the first reported use of a fluoromethyl ketone compound was the use of Z-Phe-Ala-CH2F as an irreversible inhibitor of cathepsin BJ2,31 Today, many lysine and arginine derivatives have been synthesized as potential inhibitors for trypsin and trypsin-like enzymesJ3 There are four basic methods for the synthesis of peptide fluoromethyl ketones (1) the reaction of HF with peptide diazomethyl ketones (Section 15.1.4.1.1), (2) a halogen-exchange reaction with a chloro-, bromo-, or iodomethyl ketone (Section 15.1.4.1.2), (3) a Henry nitro-aldol condensation reaction (Section 15.1.4.1.3), and (4) a modified Dakin-West acylation reaction (Section 15.1.4.1.4). 

These enzymes require a metal for activity and are inhibited by metal-chelating compounds. They are exopeptidases and include carboxypeptidase A (peptidyl-L-amino-acid hydrolase) and B (peptidyl-L-lysine hydro-... 

Figure 3. Lysyl oxidase. The enzyme, lysyl oxidase, appears to seek out lysyl residues in alanyl- and lysyl-rich regions in the pro fibrillar forms of elastin. The presence of an aromatic amino acid residue adjacent to lysine appears to block its oxidation. The product of oxidation is peptidyl a-aminoadipic-S-semialdehyde. Assays for the enzyme against elastin involve first the preparation of an elastin-rich pellet containing 3H-lysyl residues labeled in the 6 or 4,5 position. This is usually accomplished by incubating embryonic chick aortas in medium containing 3H-lysine plus f3-aminopropionitrile (BAPN) to inhibit endogenous lysyl oxidase activity. BAPN is then removed leaving behind an elastin-rich residue in which the profibrillar forms of elastin labelled with 3H-lysine are only partially crosslinked. When lysyl oxidase preparations are added to this residue the release of tritium represents the assay for activity. It has also been demonstrated that tropoelastin, when incubated with lysyl oxidase, forms a-aminoadipic-S-semialdehyde and eventually crosslinks as shown in Figure 4.

Ascorbic acid has specific and weU-deflned roles in two classes of enzymes the copper-containing hydroxylases (such as dopamine /3-hydroxylase and peptidyl glycine hydroxylase) and the 2-oxoglutarate-linked iron-containing hydroxylases, of which the best studied are the proline and lysine hydroxylases involved in maturation of connective tissue (and other) proteins. 

On the other hand, the inhibitory effect of erythromycin, a 14-membered-ring macrolide, on such a peptidyltransferase reaction is markedly diminished in terms of the character of a substrate. Erythromycin inhibits poly(A)-dependent polymerization of a transferred substrate such as lysine residue linked to tRNA but not other oligonucleotide-dependent polymerization of an amino acid linked either to tRNA or to oligonucleotides such as CACCA and UACCA. It has been shown that the transfer of A-acylaminoacyl residues to puromycin (puromycin reaction) is usually stimulated by erythromycin [88, 89, 95]. Igarashi et al. [96] have also confirmed these findings. That is to say, they found that erythromycin inhibits the release of a deacylated tRNA from the P site of ribosome. The release of such a deacylated tRNA from the P site and the translocation of peptidyl-tRNA from the A site to the P site of ribosome occurs concomitantly when EF-G catalyzes the GTP-dependent movement of the ribosome and the codon-anticodon-linked mRNA-peptidyl-tRNA complex. 

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