Cysteine negative charges

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

Also important for stabilizing a protein s tertiary stmcture are the formation of disulfide bridges between cysteine residues, the formation of hydrogen bonds between nearby amino acid residues, and the presence of ionic attractions, called salt bridges, between positively and negatively charged sites on various amino acid side chains within the protein. 

BSA possesses a total of 59 lysine e-amine groups (with only 30-35 of these typically available for derivatization), 1 free cysteine sulfhydryl (with an additional 17 disulfides buried within its three-dimensional structure), 19 tyrosine phenolate residues, and 17 histidine imidazole groups. The presence of numerous carboxylate groups gives BSA its net negative charge (pi 5.1). 

MTSES carries a net negative charge (Karlin and Akabas, 1998) and is membrane impermeant (Holmgren et al., 1996 Seal et al., 1998). The half-life of MTSES in buffer (pH 7.5, 22 C) is 20min (Karlin and Akabas, 1998). The length of MTSES that covalently links to the cysteine sulfhydryl is 4.8A (Holden and Czajkowski, 2002). 

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. 

Fig. 8.17. Mechanism of hydrolysis of phosphotyrosine residues by tyrosine phosphatases. Cleavage of phosphate from phosphotyrosine residues takes place by an in-line attack of a nucleophilic cysteine thiolate of the tyrosine phosphatase at the phosphate of the phosphotyrosine residue. The negative charge on the thiolate is stabilized by the positive charge of a conserved Arg residue. In the course of the reaction, an enzyme-Cys-phosphate intermediate is formed, which is hydrolytically cleaved to phosphate and enzyme-Cys-SH. The figure shows selected interactions. Other interactions in the active center involved in substrate binding and catalysis are not shown. According to Tainer and Russel, (1994). R substrate protein.

At pH 7 the weakly basic imidazole group of histidine may be partially protonated. Both the -SH group of cysteine and the phenolic -OH of tyrosine are weakly acidic and will dissociate and thereby acquire negative charges at a sufficiently high pH. 

Here the charges shown are those on the cluster. The cysteine ligands from the protein each add an additional negative charge. The Chromatium HIPIP and the... 

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