Cysteine thiol side chain

In line with the above-mentioned reactivity of cysteine thioesters is the occurrence of a S, N-acyl shift of the palmitoyl group from the thiol side chain to the a-amino group, when this amino group is present as a free amine. 

The sulfur atom is a favorable zinc ligand because of its size and polarizability. The thiol side chain of cysteine (p/Cg —8.5) is negatively charged as it complexes a metal ion in a protein in addition to metal coordination, the cysteine thiol may simultaneously accept hydrogen bonds from other protein residues (Adman et al, 1975 Ippolito et al, 1990). Hydrogen bond networks with cysteine metal ligands are discussed further in Section I11,B. 

Cu(II) has been shown to be bound by two cysteines [32]. The main limitation of the use of cysteine or glutathione for detecting metal ions is the thiol side chain which having a high affinity for soft metals is being employed to immobilise the peptide and hence is not available for coordination with the metal ion. 

Several different amino acid side chains can act as nucleophiles in enzyme catalysis. The most powerful nucleophile is the thiol side chain of cysteine, which can be deproto-nated to form the even more nucleophilic thiolate anion. One example in which cysteine is used as a nucleophile is the enzyme glyceraldehyde 3-phosphate dehydrogenase, which uses the redox coenzyme NAD+. As shown in Fig. 10, the aldehyde substrate is attacked by an active site cysteine, Cys-149, to form a hemi-thioketal intermediate, which transfers hydride to NAD+ to form an oxidized thioester intermediate (7). Attack of phosphate anion generates an energy-rich intermediate 3-phosphoglycerate. 

Figure 5-13 A thioester at the C-terrninus of one peptide serves as the electrophile for bimolecular condensation with the thiol side chain of an N-terminal cysteine residue. The S,N-acy intramolecular rearrangement forms the native amide bond.

Thioester ligation involves two segments, each containing a thioester and a Cys that results in the production of a cysteine residue at the ligation site. In this approach, the first step is formation of a covalent thioester-linked intermediate by nucleophilic attack of the thiol side chain of an AT-cysteine peptide on a thioester segment. An intramolecular S — N acyl transfer to form an amide bond then completes the ligation reaction (Figure 6). 

The correct pairing of half-cystine residues is shown to be dependent upon specific noncovalent bonds 17). With this finding in mind, oxidation of a pair of associating thiols (7 and 2) was chosen as a model reaction. Thiol 7 has the same group as cysteine side chain (HSCH2), 2 being a derivative of cysteamine. 

The 20 common amino acids can be further classified as neutral, acidic, or basic, depending on the structure of their side chains. Fifteen of the twenty have neutral side chains, two (aspartic acid and glutamic acid) have an extra carboxylic acid function in their side chains, and three (lysine, arginine, and histidine) have basic amino groups in their side chains. Note that both cysteine (a thiol) and tyrosine (a phenol), although usually classified as neutral amino acids, nevertheless have weakly acidic side chains that can be deprotonated in strongly basic solution. 

In contrast to the lability of certain dN adducts formed by the BHT metabolite above, amino acid and protein adducts formed by this metabolite were relatively stable.28,29 The thiol of cysteine reacted most rapidly in accord with its nucleophilic strength and was followed in reactivity by the a-amine common to all amino acids. This type of amine even reacted preferentially over the e-amine of lysine.28 In proteins, however, the e-amine of lysine and thiol of cysteine dominate reaction since the vast majority of a-amino groups are involved in peptide bonds. Other nucleophilic side chains such as the carboxylate of aspartate and glutamate and the imidazole of histidine may react as well, but their adducts are likely to be too labile to detect as suggested by the relative stability of QMs and the leaving group ability of the carboxylate and imidazole groups (see Section 9.2.3). 

Figure 28.21 The reactions of R u (11) pby 3 + are catalyzed by light at 452 nm that begins by forming an excited state intermediate. In the presence of persulfate, a sulfate radical is formed concomitant with the oxidative product Ru(III)bpy33+. This form of the chelate is able to catalyze the formation of a radical on a tyrosine phenolic ring that can react along with the sulfate radical either with a nucleophile, such as a cysteine thiol, or with another tyrosine side chain to form a covalent linkage. The result of this reaction cascade is to cause protein crosslinks to form when a sample containing these components is irradiated with light.

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