Peptides other modifications

The lantibiotic cypemycin isolated from a Streptomyces sxi21tl exhibits bis-methylation at Alai (Me2N-Ala) and an L- //o-isoleucine at position 13. The gene for its precursor peptide has not been reported, and hence the amino acid that is modified to generate the allo-We is not known. If it is introduced posttranslationally like all other modifications in lantibiotics, its most likely precursors would be either He or Leu and the posttranslational modification may involve a radical mechanism as neither amino acid is activated at C/3 or C7 for heterolytic chemistry. 

Cleavage of the signal peptide and other modifications in the ER (formation of disulfide bonds, glycosylation, phosphorylation) give rise to the mature prohormone ( pro-POMC ). 

Before this "maturation" can occur there must be other modifications to procollagen. These begin while the peptide chains are still attached to ribosomes of the rough ER. Hydroxylases (Chapter 18) localized in the membranous vesicles of the ER convert some of the proline and lysine residues of the procollagen chains into 4-hydroxyproIine625 629 630 and hydroxylysine (Eqs. 8-6 and 8-7). Lesser amounts of 3-hydroxypro-line are formed. About 100 molecules of 4-hydrox-yproline and 50 of 5-hydroxylysine are created in each al chain. 

Other modifications of capillary electrophoresis techniques, described below, have been adopted for the improvement of separation of several substances, including proteins, peptides and aminoacids. Furthermore, new procedures are also being developed to avoid the adsorption of proteins and peptides onto the walls of the capillary and consequently improving their selectivities. 

Although bacterial hosts can be useful for protein production, E. coli and other prokaryotic expression systems have some limitations. For example, they are incapable of posttranslational modifications such as glycosylation and as a consequence many eukaryotic proteins produced in prokaryotic expression systems are nonfunctional (Rai and Padh, 2001). However, in the case of most peptides, including self-assembling peptides, such modifications are currently not required. Of course, this limitation may have implications for future... 

Various numbering methods have been used to indicate substitution or other modification in or of the residues of a peptide. The method now recommended by lUPAC and introduced into the Dictionary of Natural Products (see Section 1.2.1) uses numerical locants of the type 3, where 3 is the locant of the substituent in the amino acid residue and 2 is the amino acid position in the ring or chain, numbered from the N-terminal end, which is standard for all peptides). A/ -methyloxytocin would indicate a methyl substituent on N-5 of the glutamine residue at position 4 of oxytocin. 

Many proteins, particularly those from higher eukaryotes, carry modifications that have been introduced either co- or post-translationally (Coligan et al. 1995 Higgins and Hames 1999). Particularly noteworthy are phosphorylation, which is responsible for regulating protein activity, and glycosylation, which has important roles in the transport, localisation and stability of proteins. In addition to these, there are many other modifications which, for reasons of space, we are not able to discuss here (Table 5-4). These modifications are all potentially identifiable by mass spectrometry of isolated peptides (Meyer 1994 Higgins and Hames 1999 Kellner et al. 1999), but many can also be characterised by Edman sequencing, e.g., hydroxypro-line and methyllysine. 

The replacement of amide bonds by retro-in-verso amide replacements (71, 72) and other amide bond isosteres generates pseudopeptides (11). This process was first used to stabilize peptide hormones in vivo and later to prepare transition state analog (TSA) inhibitors. Systematic efforts to convert good in vitro inhibitors into good in vivo inhibitors became the driving force for further development of peptidomimetics. Figure 15.17 illustrates some of the peptide backbone modifications that have been made in an effort to increase bioavailability. Replacement of scissile amide (CONH) bonds with groups insensitive to hydrolysis (e.g., CHaNH) has been extensively practiced. Reviews of this work have appeared (11,73). Removal of the proton donors and... 

During enzymatic modification under appropriate reaction conditions, L-amino acids (generally in ester form) are partially covalently incorporated into the peptide chains of a protein hydrolysate. Thus these enzymatic modification reactions with amino acid enrichment would be expected to be more important for health aspects than other modification processes without covalent amino acid enrichment. 

Proteic materials used in traditional tempera painting, for instance, are subject to condensation and cross-linking reactions with other components such as lipids. Gas chromatography-mass spectrometry (GC-MS) investigations have shown that the oxidative degradation of amino acids such as cysteine, serine, and phenylalanine leads to the formation of amino malonic acid. The latter compound has been identified via the analysis of aged paint samples [7]. Other modifications are caused by the pH variation of the paint surface that contributes to the hydrolysis of peptide bonds and the partial dehydration of serine and threonine. 

The carrier transport pathway is stereospecific (Matthews, 1975, 1983 Boyd and Ward, 1982 Asatoor et al, 1973 Cheeseman and Smyth, 1972) peptides of the D-configuration are handled by the transporter (Boyd and Ward, 1982 Asatoor et al, 1973 Cheeseman and Smyth, 1972) but are poorly taken up and slowly hydrolyzed (Matthews, 1975, 1983). An a-pepti bond is preferred (Matthews, 1975, 1983), though not required (Bai et al, 1991) for carrier transport, whereas methylation, acetylation, or other modification of the N-terminal a-amino group (Addison et al, 1974 Das and Radhakrishnan, 1975 Rubino et al, 1971 Addison et al, 1975), as well... 

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