Amino-acid residues valine

It is interesting to note that there is another carboxy-peptidase. carboxypeptidase B (CPB), in the same tissue that has an ainino acid sequence homologous to that of CPA but differs in substrate specificity. Whereas CPA prefers aromatic (phenylalanine, tyrosine, and tryptophan) or hydrophobic amino acid residues (valine, leucine, and... 

FIGURE 5.5 (a) The hydroxy amino acids serine and threonine are slowly destroyed during the course of protein hydrolysis for amino acid composition analysis. Extrapolation of the data back to time zero allows an accurate estimation of the amonnt of these amino acids originally present in the protein sample, (b) Peptide bonds involving hydrophobic amino acid residues snch as valine and isolencine resist hydrolysis by HCl. With time, these amino acids are released and their free concentrations approach a limiting value that can be approximated with reliability. 

By means of a procedure described above, Hanson and Fittkau (HI) isolated seventeen different peptides from normal urine. One of them, not belonging to the main peptide fraction, consisted of glutamic acid, and phenylalanine with alanine as the third not definitely established component. The remaining peptides contained five to ten different amino acid residues and some unidentified ninhydrin-positive constituents. Four amino acids, i.e., glutamic acid, aspartic acid, glycine, and alanine, were found in the majority of the peptides analyzed. Twelve peptides contained lysine and eight valine. Less frequently encountered were serine, threonine, tyrosine, leucine, phenylalanine, proline, hydroxyproline, and a-aminobutyric acid (found only in two cases). The amino acid composi-... 

Fig. 9 Correlation of (A) the second order rate constants (k2 = kcatIKM) and (B) the transition stabilization (pATS) with the hydrophobicity (it) of the substituent of the amino acid residue for the cleavage of /V-acetylamino acid methyl esters by a-chymotrypsin. The open symbols are for the points for two branched residues (valine and isoleucine). Data from Table A6.8.

Cyclosporin A contains II amino acids, joined in a cyclic strncture by peptide bonds. The structure is also stabilized by intramolecular hydrogen bonds. Only two of the amino acids, i.e. alanine and valine, are typical of proteins. The compound contains several A-methylated amino acid residues, together with the even less common L-a-aminobutyric acid and an Ai-methylated butenylmethylthreonine. There is one o-amino acid, i.e. o-alanine, and the assembly of the polypeptide chain is known to start from this residue. Many of the other natural cyclosporin structures differ only with respect to a single amino acid (the a-aminobutyric acid residue) or the number of amino acids that have the extra Ai-methyl group. 

This peptide is also referred to as apo Lp-Ser based on its COOH-terminal amino acid residue (H4, Ml), initially believed to be valine (B8, B9). Its NH2-terminal residue is threonine with the amino acid composition listed in Table 9. The sequence of this peptide announced recently (S34) has the noted feature (Fig. 6), also shared by apo LP-Gln II (B5) and apo LP-Ala (B6) of having a number of basic adjacent to acidic residues. 

Point mutations that give altered sterol profiles have been generated in A. thaliana and S. cerevisiae LSs. Matsuda and co-workers used a yeast expression system to select for spontaneous mutations in A. thaliana CS that restored sterol-independent growth to an LS-deficient mutant of yeast [67]. In this way they were able to identify a mutation from isoleucine to valine (at Ile481) that allowed synthesis of the sterols lanosterol and parkeol. Further studies have identified a number of other amino acid residues in A. thaliana CS and... 

Biopharmaceuticals based on natural proteins and peptides are often called by the same name as the biologic natural material despite differences in one or more amino-acid residues. For example, insulin, which regulates blood glucose and is used clinically to treat type 1 diabetes and some cases of type 2 diabetes, has several variants that are approved for human use. Insulin contains two polypeptides, A and B chains (Figure 1.2), that are linked together by two disulfide bridges to assume a biologically active conformation. Compared with human insulin, insulin extracted from beef tissue exhibits threonine alanine and isoleucine valine substitutions at posi-... 

Kirchgessner and Steinhart (52) studied the in vitro digestion of soy protein isolate with pepsin. They showed that a relatively higher percentage of the essential amino acids threonine, valine, isoleucine, leucine and phenylalanine were found in the undigested residue and therefore would be present in lower proportions in the hydrolysate. Myers et al. ( ) hydrolyzed a soy protein isolate with a fungal acid protease. They also found that the essential amino acid content of the hydrolyzate was lower than the isolate, but, as prepared by their continuous process, the hydrolyzate PER was not significantly reduced as compared to the isolate PER. 

It has been known for many years, however, that the (3-branched amino acids, especially valine and isoleucine, cause problems in synthesis,14,5] and special care and additional reaction time are required when -substituted amino acids are added to a growing peptide chain in synthesis. For example, in the synthesis of [2,4-diisoleucine]oxytocin efforts to couple the isoleucine to isoleucine by the azide method failed and only the rearranged product was obtained 61 Also, it is much more difficult to hydrolyze peptide bonds formed between two or more contiguous -substituted amino adds using standard 6M HC1 conditions. For example, in the hydrolysis of [2,4-diisoleucine]oxytocin (3 isoleucine residues adjacent to each other) complete hydrolysis takes 60 hours. 

The replacement of product amino acid residues by positional alteration of domains [102,103], The cysteine- and valine-specific activa-tion-thiolation didomains of P. chrysogenum ACV synthetase have been successfully inserted into the terminal position of surfactin synthetase in Bacillus subtilis [102], In the case of ACV synthetase, e.g., specific domains could be replaced to generate new tripeptides or to improve the efficiency of poorly incorporated amino acid analogues. 

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