Protein cysteine residues

The most common chemoselective conjugation is achieved through thiol coupling to free cysteine (C) groups via maelimide chemistry. In proteins, cysteine residues make up only 1.7% of amino acids, and because they tend to... 

The methodology of peptide mapping by RP-HPLC is straightforward. The protein is first denatured and reduced with urea and dithiothreitol (DTT), respectively, to unfold the protein. Cysteine residues are then blocked... 

Pantothenic acid has a central role in energy-yielding metabolism as the functional moiety of coenzyme A (CoA), in the biosynthesis of fatty acids as the prosthetic group of acyl carrier protein, and through its role in CoA in the mitochondrial elongation of fatty acids the biosynthesis of steroids, porphyrins, and acetylcholine and other acyl transfer reactions, including postsynthetic acylation of proteins. Perhaps 4% of all known enzymes utilize CoA derivatives. CoA is also bound by disulfide links to protein cysteine residues in sporulating bacteria, where it may be involved with heat resistance of the spores, and in mitochondrial proteins, where it seems to be involved in the assembly of active cytochrome c oxidase and ATP synthetase complexes. 

It is tempting to speculate that reduction of the Schiff base might occur by oxidation of protein cysteine residues. Experimental work is... 

Olsson, M., and Lindahl, T. (1980). Repair of alkylated DNA in Escherichia coli. Methyl group transfer from 06-methylguanine to a protein cysteine residue. J. Biol. Chem. 255, 10569-10571. 

In proteins, cysteine residues are not always readily accessible, since they are often involved in disulfide bridges within the complex three-dimensional biomacromolecular structures. Therefore, only a small number of cysteine residues can be used for bioconjugation reactions. The ligation of polymer bound dibromomaleimides takes advantage of this circumstance, as it allows for the functionalization of disulfide moieties. Haddleton and coworkers demonstrated the applicability of this reaction for bioconjugation of salmon calcitonin (sCT). ... 

The primary structure of a peptide is given by its ammo acid sequence plus any disulfide bonds between two cysteine residues The primary structure is determined by a systematic approach m which the protein is cleaved to smaller fragments even individual ammo acids The smaller fragments are sequenced and the mam sequence deduced by finding regions of overlap among the smaller peptides... 

For deterrnination of tryptophan, 4 M methanesulfonic acid hydrolysis is employed (18). For cystine, the protein is reduced with 2-mercaptoethanol, the resultant cysteine residue is carboxymethylated with iodoacetic acid, and then the protein sample is hydroly2ed. Also, a one-pot method with mercaptoethanesulfonic acid has been developed for tryptophan and cystine (19). 

Metallothioneins. The metaHothionekis, a group of low (<10,000) molecular weight proteins containing - 30% cysteine residues, are efficient... 

The ability to identify and quantify cyanobacterial toxins in animal and human clinical material following (suspected) intoxications or illnesses associated with contact with toxic cyanobacteria is an increasing requirement. The recoveries of anatoxin-a from animal stomach material and of microcystins from sheep rumen contents are relatively straightforward. However, the recovery of microcystin from liver and tissue samples cannot be expected to be complete without the application of proteolytic digestion and extraction procedures. This is likely because microcystins bind covalently to a cysteine residue in protein phosphatase. Unless an effective procedure is applied for the extraction of covalently bound microcystins (and nodiilarins), then a negative result in analysis cannot be taken to indicate the absence of toxins in clinical specimens. Furthermore, any positive result may be an underestimate of the true amount of microcystin in the material and would only represent free toxin, not bound to the protein phosphatases. Optimized procedures for the extraction of bound microcystins and nodiilarins from organ and tissue samples are needed. 

Two cysteine residues in different parts of the polypeptide chain but adjacent in the three-dimensional structure of a protein can be oxidized to form a disulfide bridge (Figure 1.4). The disulfide is usually the end product of air oxidation according to the following reaction scheme ... 

Lysozyme from bacteriophage T4 is a 164 amino acid polypeptide chain that folds into two domains (Figure 17.3) There are no disulfide bridges the two cysteine residues in the amino acid sequence, Cys 54 and Cys 97, are far apart in the folded structure. The stability of both the wild-type and mutant proteins is expressed as the melting temperature, Tm, which is the temperature at which 50% of the enzyme is inactivated during reversible beat denat-uration. For the wild-type T4 lysozyme the Tm is 41.9 °C. 

Disulfide bridge (Section 27.7) An S—S bond between the sulfur atoms of two cysteine residues in a peptide or protein. 

In recent years, biochemists have developed an arsenal of reactions that are relatively specific to the side chains of particular amino acids. These reactions can be used to identify functional amino acids at the active sites of enzymes or to label proteins with appropriate reagents for further study. Cysteine residues in proteins, for example, react with one another to form disulfide species and also react with a number of reagents, including maleimides (typically A ethylmaleimide), as shown in Figure 4.11. Cysteines also react effectively... 

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