Threonine peptides, cleavage

Moser et al. (1968) (one of the co-authors was Clifford Matthews) reported a peptide synthesis using the HCN trimer aminomalonitrile, after pre-treatment in the form of a mild hydrolysis. IR spectra showed the typical nitrile bands (2,200 cm ) and imino-keto bands (1,650 cm ). Acid hydrolysis gave only glycine, while alkaline cleavage of the polymer afforded other amino acids, such as arginine, aspartic acid, threonine etc. The formation of the polymer could have occurred according to the scheme shown in Fig. 4.9. 

This example illustrates that the reactivity of peptide bonds involving serine residues is not due solely to the presence of serine other, as yet poorly understood factors must also play a role. Threonine residues also show this particular type of reactivity [9]. Here, we examine the case of cyclosporin A (CsA), a cyclic undecapeptide that contains some unusual amino acids (6.57, Fig. 6.23), whose major breakdown reaction is hydrolytic cleavage at a threonine analogue. Position 1 is occupied by 4-[( )-but-2-enyl]-A,4-dimethyl-threonine, whose OH group reacts as a nucleophile at the carbonyl C-atom of the adjacent A-methylvalinc (position 11) [86],... 

Figure 4 The biosynthesis of nisin A as a representative example of the posttranslational maturation process of lantibiotics. Following ribosomal synthesis, NisB dehydrates serine and threonine residues in the structural region of the prepeptide NisA. NisC subsequently catalyzes intramolecular addition of cysteine residues onto the dehydro amino acids in a stereo- and regioselective manner. Subsequent transport of the final product across the cell membrane by NisT and proteolytic cleavage of the leader sequence by NisP produces the mature lantibiotic. For the sequence of the leader peptide, see Figure 6. Adapted with permission from J. M. Willey W. A. van der Donk, Annu. Rev. Microbiol. 2007, 61, 477-501.

Figure 7. Complete proteolytic stability of all types of P-and y-peptides towards a variety of peptidases. The P-peptides ranged in size from dimer to ISmer. The enzymes include all common types of peptidases (endo/exo, metallo, serine, threonine, and aspartyl proteases). After 40 hours there was no observable cleavage of any of the homologated peptides and no inhibition of the enzymes [41].

In general the thiol proteases catalyze the hydrolysis of a variety of peptide, ester, and amide bonds of synthetic substrates. Employing the general formula R —NH—CHR—CO—X, cleavage of the —CO—X— bond has been demonstrated when R represents the side chain of glycine, threonine, methionine, lysine, arginine, citrulline, leucine, and tyrosine. 

The assistance from a hydroxyl group in the hydrolysis of an amide bond either via the (largely reversible) hydroxyoxazolidine or the (largely irreversible) lactone mechanism leads to preferential cleavage of peptide bonds next to serine and threonine, on the one hand, and to lactones of... 

In the bicyclic peptide phalloidine where there are three eligible hydroxyl groups, one /3-hydroxyl in a threonine residue, and a y- and 5-hydroxyl group in the 7,5-dihydroxy-L-leucine residue, the controlled action of sulfuric acid leads to formation of the y-lactone only with selective cleavage of one peptide bond (Wieland and Schopf, 1959). 

The oxidation of trypsin and trypsinogen was carried out in aqueous 0.1 M acetate buffer solutions at room temperature. In this particular case and under these conditions no significant cleavage of peptide bonds next to tryptophan residues occurred. Careful analysis of hydrolyzates of NBS-oxidized trypsinogen and trypsin confirmed the selectivity of the oxidative modification of the protein, as Table XXIV shows. There is no significant loss of tyrosine, histidine, serine, threonine, or cystine, although all of these amino acids will react with NBS but considerably less rapidly than tryptophan. 

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