B-Cystine

The major differentiation of the cystine suggested by Middlebrook and Phillips was into fractions designated as A - - B and C - - D. The A + B cystine reacted with bisulfite solution and could be reduced by thioglycolate at pH 5, whereas the C -f- D fraction was supposedly completely inert under the conditions. If made to react under more drastic conditions (e.g., boiling bisulfite) the C - - D fraction reacted in a different and less well-understood fashion involving sulfur loss from the protein. 

In terms of amino acids bacterial protein is similar to fish protein. The yeast s protein is almost identical to soya protein fungal protein is lower than yeast protein. In addition, SCP is deficient in amino acids with a sulphur bridge, such as cystine, cysteine and methionine. SCP as a food may require supplements of cysteine and methionine whereas they have high levels of lysine vitamins and other amino acids. The vitamins of microorganisms are primarily of the B type. Vitamin B12 occurs mostly hi bacteria, whereas algae are usually rich in vitamin A. The most common vitamins in SCP are thiamine, riboflavin, niacin, pyridoxine, pantothenic acid, choline, folic acid, inositol, biotin, B12 and P-aminobenzoic acid. Table 14.4 shows the essential amino acid analysis of SCP compared with several sources of protein. 

D. B. Volkin and A. M. Klibanov, Thermal destruction processes in proteins involving cystine residues, J. Biol. Chem, 262, 2945 (1987). 

Harry B. Gray and Walther Ellis,13 writing in Chapter 6 of reference 13, describe three types of oxidation-reduction centers found in biological systems. The first of these, protein side chains, may undergo oxidation-reduction reactions such as the transformation of two cysteine residues to form the cystine dimer as shown in equation 1.28 ... 

Sanchez-Cano et al. have proposed paired synthesis for obtaining L-cysteic acid and L-cysteine from L-cystine which greatly improves the economical parameters [57], The global process-flow for the paired synthesis, with L-cystine and water as starting materials is shown in Fig. 3. Table 2 compares the results for the paired (B) and the individual syntheses (A, C). 

Figure 2-4 illustrates the minor pathway for metabolism of cyanide in mammalian systems in which cyanide chemically combines with the amino acid cystine. This chemical reaction yields cysteine and B-thiocyanoalanine that is further converted to form 2-aminothiazoline-4-carboxylic acid and its tautomer, 2-iminothiazolidiene-4-carboxylic acid. 

The second and minor metabolic pathway consists of the reaction of cyanide with cystine to yield cysteine and B-thiocyanoalanine (Wood and Cooley 1955). The latter is then converted to 2-imino-4-thiazolidinecarboxylic acid and excreted in urine. Cystine has not been used for the purpose of mitigation of cyanide effects because its contribution to detoxification via this pathway is minor. 

FIGURE 5.22 (A) Reaction of an Fmoc-amino acid with 2-chlorotrityl chloride resin.56 The ester bond formed is cleavable by the mild acid, which does not affect tert-butyl-based protectors. (B) Generation of a protected peptide containing cystine by detachment of a chain, deprotection of cysteine residues, and oxidation of the sulfhydryls by the reagent containing iodine. The cations produced are trapped by CF3CH2OH. 

FIGURE 6.20 Synthesis of cystine-containing peptides from cysteine-containing peptides by removal of other protectors followed by (A) deprotection of the sulfhydryls and their oxidation to the disulfide, and (B) formation of the disulfide bond by reaction of a liberated sulfhydryl with a sulfhydryl that is protected and activated by 3-nitro-2-pyridylsulfanyl (Npys).89... 

FIGURE 6.22 Disulfide interchange.92 (A) Discovered in synthesis when hydrazinolysis of an unsymmetrical derivative of cystine gave two symmetrical products instead of the expected monohydrazide at the urethane-protected cysteine moiety of the derivative.95 (B) Mechanism for interchange catalyzed by strong acid,94 which is suppressed by thiols. (C) Mechanism for interchange catalyzed by weak alkali, which is enhanced by thiols. 

FIGURE 6.23 The synthesis of insulin, starting with a cystine-containing peptide. [Kamber et al., 1977]. Moc = methoxycarbonyl, Bpoc = biphenylisopropoxycarbonyl, Trt = trityl, Acm = acetamidomethyl. (a) HOBt-assisted carbodiimide-mediated coupling (b) removal of Trt by HC1 in CF3CH2OH-CH2Cl2 (9 1) at pH 3.5 (c) removal of Bpoc by CF3CH2OH-CH2 Cl2 (9 1) at 60°C (d) removal of Acm and oxidation by iodine. 

The A and B peptide chains in insulin are linked through disulfide bridges. Their presence was suspected from the change in molecular weight which followed the reduction of insulin. For quantitative analyses the S-S bridges had to be broken. Sanger, following the approach used by Toennies and Homiller (1942), oxidized the protein with performic acid, so that the half-cystines were converted to cysteic acid. After oxidation, insulin could be separated into its A and B chains, the A peptide with 20 amino acid residues and the B with 30. 

Nur wenigc davon kdnncn im intaktcn Protein quantitativ erfaBt wer-den. So z.B, Tyiosin nnd Tiyptoplian durch Messung der Ultraviolett-Absorption, Trj ptophan durch spezifische koloriinetrische Methoden, Cystin und Cystein durch Titration oder durch photometrischc Methoden (siehc z.B. Block (9), Light und Smith 69)). 

Figure 15 Examples of artificial collagen model peptide knots, (a) Regioselective artificial cystine knot, (b) dilysine scaffold,(c) c/s,c/s-1,3,5-trimethylcyclohexane-1,3,5-tricarboxylic acid Kemp tri acid , (d) TREN-(suc-OH)3, (Tris(2-aminoethyl)amine) succinic acid, (e) Fe + [2,2 -bipyridyl-peptide]3, " (f) monoalkyl chains, " (g) Boc-/3-Ala-TRIS-(OH)3 knot, and (h) N-terminal GFGEEG link. ...

Figure 2.11 LC analysis of cystine DCL. Trace A shows the DCL templated with CaM, and trace B shows the blank DCL. The amplified peaks ( ) are dipeptides ce and cc. Reproduced from Reference 23 with permission of Wiley-VCH Verlag GmbH Co. KGaA. Copyright Wiley-VCH Verlag GmbH Co. KGaA.

Kamber B, Hartmann A, Eisler K, Riniker B, Rink H, Sieber P, Rittel W. The synthesis of cystine peptides by iodine oxidation of 5-trityl-cysteine and S-acetamidomethyl-cysteine peptides. Helv Chim Acta 1980 63 899-915. 

The prototype of oxidizing agents that convert S-protected cysteine peptides directly into cystine peptides (Scheme 1, route B) is iodine. This procedure was initially introduced by Kamber and Rittel for the oxidation of 5-Trt cysteine peptides 66 and has since been extended to the 5-Acm/45,67 5-Tacm/68 5-Phacm/69-71 and 5-Tmb derivatives/64 It can also be... 

Peptide bonds are cleaved in a nonselective, but not in a completely random manner. Based on anchimeric side-chain assistance, steric factors, and bond strains, acid-labile peptide bonds are predicted to include sites containing Asp, Glu, Ser, Thr, Asn, Gin, Gly, and ProJ22l The disulfide topologies of circulin B and cyclopsychotride, backbone-cyclized peptides with three disulfide bonds, were determined by partial hydrolysis for 5 hours.[22 Occasionally, the bond between adjacent half-cystine residues is cleaved due to the nonselective nature of the mechanism of partial acid hydrolysis.[21] By this procedure, in all cases, a complex mixture of peptide fragments is produced which requires careful chromatographic separation by RP-HPLC for subsequent analysis by mass spectrometry (see Section 6.1.6.2.7). 

I 21.10 Predict whether the isoelectric points for the following a-amino acids are considerably acidic, slightly acidic, or basic (a) alanine, (b) lysine, (c) aspartic acid, (d) cystine, (e) tyrosine. (See Table 21-1 and Problem 21.6.) ... 

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