Amino acid cysteine

The side groups of the amino acids vary markedly in size and chemical nature and play an important role in the chemical reactions of the fiber. For example, the basic groups (hisidine, arginine, and lysine) can attract acid (anionic) dyes, and in addition the side chains of lysine and hisidine are important sites for the attachment of reactive dyes. The sulfur-containing amino acid cysteine plays a very important role, because almost all of the cysteine residues in the fiber are linked in pairs to form cystine residues, which provide a disulfide bridge —S—S— between different polypeptide molecules or between segments of the same molecules as shown ... 

Draw tetrahedral representations of the two enantiomers of the amino acid cysteine, HSCfi CHfNH lCX H, and identify each as R or S. 

The naturally occurring form of the amino acid cysteine (Problem 9.48) has the S configuration at its chirality center. On treatment with a mild oxidizing agent, two cysteines join to give cystine, a disulfide. Assuming that the chirality center is not affected by the reaction, is cystine optically active ... 

The amino acid cysteine, C3H7NO2S, is biosynthesized from a substance called cystathionine by a multistep pathway. 

Example Optically active aldehyde (10) was needed for a synthesis of biotin. The compound has a 1,1-dlX disconnection (10a) clearly available and a C-N (amide) disconnection leaving (11) which has the same skeleton as the amino acid cysteine (12). 

Two amino acids—cysteine and tyrosine—can be synthesized in the body, but only from essential amino acid ptecutsots (cysteine from methionine and tyrosine from phenylalanine). The dietary intakes of cysteine and tytosine thus affect the requirements for methionine and phenylalanine. The remaining 11 amino acids in proteins are considered to be nonessential or dispensable, since they can be synthesized as long as there is enough total protein in the diet—ie, if one of these amino acids is omitted from the diet, nitrogen balance can stiU be maintained. Howevet, only three amino acids—alanine, aspartate, and glutamate—can be considered to be truly dispensable they ate synthesized from common metabolic intetmediates (pyruvate, ox-... 

The enormous diversity of protein stmcture and function comes from the many ways in which 20 amino acids can combine into polypeptide chains. Consider how many tetrapeptide chains can be made using Just two amino acids, cysteine and aspartic acid ... 

The amino acid cysteine piays a unique roie in tertiary protein stmcture. The —SH groups of two cysteine side chains can cross-iink through an S—S bond caiied a disulfide bridge, as shown below. 

The synthesis in Scheme 13.38 is based on an interesting kinetic differentiation in the reactivity of two centers that are structurally identical, but diastereomeric. A bis-amide of we.w-2,4-dimethylglutaric acid and a chiral thiazoline was formed in Step A. The thiazoline is derived from the amino acid cysteine. The two amide carbonyls in this to-amide are nonequivalent by virtue of the diastereomeric relationship established... 

A fundamental question for all reactions which could have been involved in the early phase of chemical evolution is that of the origin of the reduction equivalents necessary for the autotrophic synthesis. For example, the synthesis of one molecule of glucose from carbon dioxide requires 24 electrons, while the synthesis of the amino acid cysteine requires as many as 26 electrons per molecule of amino acid ... 

Many substances which are necessary (and even essential) for life functions contain sulphur for example, the amino acids cysteine and methionine, the tripeptide glutathione or coenzyme A (CoA), with the latter containing the SH group of cys-teamine as the terminal functional group. CoA acts as a coenzyme in all important biochemical acylations. The cysteamine SH group bonds to carboxylic acids to give thioesters ... 

A particular interest for clinical applications was a possibility for detection of dopamine by its oxidation on nickel [19], cobalt [65], and osmium [66] hexacyanofer-ates. Except for oxidation of dopamine, cobalt and osmium hexacyanoferrates were active in oxidation of epinephrine and norepinephrine. For clinical analysis it is also important to carry out the detection of morphine on cobalt [67] and ferric [68] hexacyanoferrates, as well as the detection of oxidizable amino acids (cystein, methionine) by manganous [69] and ruthenium [70] hexacyanoferrate-modified electrodes. In general, oxidation of thiols was first shown for Prussian blue [71] and nickel hexacyanoferrate [72], This approach has been used for the detection of thiols in rat striatum microdialysate [73], Alternatively, the detection of thiocholine with Prussian blue was employed for pesticide determination in acetylcholine-esterase test [74],... 

Therefore the problem of searching retardants for these chain reactions of free radicals is critical. For instance, it is known that sulfur-containing amino acid (cysteine) attracts unpaired electrons of protein [2,3], Similar properties are reported about selenium, the element of the same subgroup Vl-a of the System as sulfur [4],... 

B. Sulfur-containing amino acids Cysteine Methionine... 

Within organisms, organic sulfur is present predominantly as the amino acids cysteine and methionine, and the algal and bacterial osmolyte, dimethylsulfoniopropionate (DMSP). The latter also serves as an antioxidant and cryoprotectant. Small amounts of organosulfur are also present in some polysaccharides, lipids, vitamins, enzymes, and in the iron-sulfur protein ferrodoxin. Cell lysis and microbial degradation releases... 

There are a number of metals that are irreversible inhibitors of enzymes. Those that are found in increasing quantities in the environment and hence are causing concern include lead, mercury, cadmium and copper. Enzymes that possess the amino acid cysteine in their active site are most vulnerable, since the sulphydryl group (-SH) in this site readily reacts with the metal ions (Appendix 3.7). 

Figure 8.3 A summary of pathways involved in the synthesis of non-essential amino acids. Glutamate is produced from ammonia and oxoglutarate. Glutamate is the source of nitrogen for synthesis of most of the amino acids. Cysteine and tyrosine are different because they require the essential amino acids (methionine and phenylyalanine) for their synthesis. These two amino acids are, therefore, conditionally essential, i.e. when there is not sufficient methionine or phenylyalanine for their synthesis, they are essential (Details are in Appendix 8.2).

The aliphatic amino acids (class 1) include glycine, alanine, valine, leucine, and isoleucine. These amino acids do not contain heteroatoms (N, 0, or S) in their side chains and do not contain a ring system. Their side chains are markedly apolar. Together with threonine (see below), valine, leucine, and isoleucine form the group of branched-chain amino acids. The sulfurcontaining amino acids cysteine and methionine (class 11), are also apolar. However, in the case of cysteine, this only applies to the undissociated state. Due to its ability to form disulfide bonds, cysteine plays an important role in the stabilization of proteins (see p. 72). Two cysteine residues linked by a disulfide bridge are referred to as cystine (not shown). 

At 0° to 100° C. part of the oxygen adsorbed on charcoal is endowed with catalytic properties, thus it will readily oxidise oxalic acid, amino acids, cystein and other similar compounds, whilst we would anticipate similar reactions if the oxygen molecule were now adsorbed in such a way as to cause a breakage of one of the two bonds 0=0 —> 0—0. At high temperatures, as is well known. 

---