Peptide cysteine

All the complexes consist of several subunits (Table 2) complex I has a flavin mononucleotide (FMN) prosthetic group and complex II a flavin adenine dinucleotide (FAD) prosthetic group. Complexes I, II, and III contain iron-sulphur (FeS) centers. These centers contain either two, three, or four Fe atoms linked to the sulphydryl groups of peptide cysteine residues and they also contain acid-labile sulphur atoms. Each center can accept or donate reversibly a single electron. 

The general conclusion to be drawn from the experience reported with these various oxidative methods is that with increasing reactivity of the oxidizing agents more side reactions are to be expected at sensitive amino acid residues, in particular at Met, Trp, and Tyr. In this context, the azodicarboxylic acid derivatives could represent a valid alternative 54,55 since these reagents are devoid of any side reaction at these sensitive residues 56 The reaction of di-ferf-butyl azodicarboxylate (1) with a peptide cysteine thiol leads to the intermediate formation of a sulfenohydrazide adduct 2 that reacts with the second thiol to generate the... 

In roasted coffee similar reactions seem to be involved forming methylmercaptan by the Strecker degradation of free methionine and forming H S from peptide cysteine. Figure 8 presents additional flavor contributing constituents of roasted coffee. 3-Thiolanone 6 and 2-methyl-3-thiolanone 1 were identified by Stoll et al. (19) and patented as coffee flavors. The two thiolanones 6, T are formed as major constituents in erythrose and xylose/ cysteine model systems, respectively. 

Figure 11 Sequences of inducer pheromones of class II bacteriocin (a) and competence-stimulating peptides of streptococci (b) and their leader peptides. Cysteine residues putatively formed disulfide bridge are boxed. The name in parentheses represents the bacteriocin induced by the pheromone.

ThiyI radicals are important reactants in several enzymes and form in vivo during conditions of oxidative stress [6]. They have been considered for a long time as rather unreactive species. However, recently several reactions of thiyi radicals with biomolecules have been described (catalysis olcis-trans isomerization of unsaturated fatty acids, addition to the pyrimidine bases C5-C6 double bonds, and hydrogen abstraction from polyunsaturated fatty acid, thymine and peptide C -H and side chain C-H bonds) [7]. More recently, the intramolecular addition of peptide cysteine thiyi radicals (CysS ) to phenylalanine (Phe) yielding alkylothio-substituted cyclohexadienyl radicals was demonstrated in the peptides Phe-Cys and Phe-Gly-Cys-Gly (Fig. 2) [8]. 

Linkage of peptide cysteine groups to silicon surfaces using long-chain trichlorosilanes Peptide antibodies [4]... 

The peptides cystein and methionine contain sulfur in their side groups. Sulfides bind easily to metal ions, such as Fe U Cu +, Zn +, and Pb ". In the case of Fe, Fe-S complexes are formed, which are capable of easy reduction or oxidation. Pb + is poisonous and binds strongly at Zn + sites, but the structure is very different from the tetrahedral zinc complex. 

Gupta, K Kumar, M., and Balaram, P. (2010) Disulfide bond assignments by mass spectrometry of native natural peptides cysteine pairing in disulphide bonded conotoxins. Anal. Chem., 82, 8313-8319. 

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... 

Disulfide bridge (Section 27 7) An S—S bond between the sulfur atoms of two cysteine residues in a peptide or pro tein... 

The Dim ester was developed for the protection of the carboxyl function during peptide synthesis. It is prepared by transesterification of amino acid methyl esters with 2-(hydroxymethyl)-l,3-dithiane and Al(/-PrO)3 (reflux, 4 h, 75°, 12 torr, 75% yield). It is removed by oxidation [H2O2, (NH4)2Mo04 pH 8, H2O, 60 min, 83% yield]. Since it must be removed by oxidation it is not compatible with.sulfur-containing amino acids such as cysteine and methionine. Its suitability for other, easily oxidized amino acids (e.g., tyrosine and tryptophan) must also be questioned. It is stable to CF3CO2H and HCl/ether. - ... 

During the synthesis of peptides that contain 4-methoxybenzyl-protected cysteine residues, sulfoxide formation may occur. These sulfoxides, when treated with HF/ anisole, form thiophenyl ethers that cannot be deprotected therefore, the peptides should be subjected to a reduction step prior to deprotection. ... 

Thiazolidines have been prepared from /3-aminothiols—for example, cysteine—to protect the —SH and — NH groups during syntheses of peptides, including glu-tathione. Thiazolidines are oxidized to symmetrical disulfides with iodine they do not react with thiocyanogen in a neutral solution. 

H2/Pd-C. If hydrogenation is carried out in the presence of (BOC)20, the released amine is directly converted to the BOC derivative. H2/Pd-C, NH3, —33°, 3-8 h, quant.When ammonia is used as the solvent, cysteine or methionine units in a peptide do not poison the catalyst. Pd-C or Pd black, hydrogen donor, solvent, 25° or reflux in EtOH, 15 min-2 h, 80-100% yield. Several hydrogen donors, including cyclohex-... 

Polypeptides. These are a string of a-amino acids usually with the natural 5(L) [L-cysteine is an exception and has the R absolute configuration] or sometimes "unnatural" 7f(D) configuration at the a-carbon atom. They generally have less than -100 amino acid residues. They can be naturally occurring or, because of their small size, can be synthesised chemically from the desired amino acids. Their properties can be very similar to those of small proteins. Many are commercially available, can be custom made commercially or locally with a peptide synthesiser. They are purified by HPLC and can be used without further purification. Their purity can be checked as described under proteins. 

FIGURE 9.19 Proteins containing the C-terminal sequence CAAX can undergo prenylation reactions that place thioether-linked farnesyl or geranylgeranyl groups at the cysteine side chain. Prenylation is accompanied by removal of the AAX peptide and methylation of the carboxyl group of the cysteine residue, which has become the C-terminal residue. 

The farnesylation and subsequent processing of the Ras protein. Following farnesylation by the FTase, the carboxy-terminal VLS peptide is removed by a prenyl protein-specific endoprotease (PPSEP) in the ER, and then a prenylprotein-specific methyltransferase (PPSMT) donates a methyl group from S-adenosylmethionine (SAM) to the carboxy-terminal S-farnesylated cysteine. Einally, palmitates are added to cysteine residues near the C-terminus of the protein. 

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