Free SH groups

The reaction of thioethers with ethyleneimine in the presence of acid yields sulfonium compounds. The reaction is reversible under alkaline conditions (125). Compounds in which double-bonded sulfur can exist in tautomerism with a form having a free SH group, such as thiourea (126,127), thiocarboxyhc acids (128), and thiophosphates (129), react to give aminoaLkylated products. The P-aminoethyl thiocarboxylate rearranges to give the amide. 

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

The amino acid residues in neurotoxins which are important for neurotoxic action are still not entirely clarified. Some neurotoxins contain one free SH group, while others do not. From this fact, it would be logical to assume the sulfhydryl group is not essential. This was actually proven to be the case. 

Fungal cutinases show no free SH groups but have 4 Cys residues, indicating that they are in disulfide linkage [119]. The reaction of the native enzyme with DTE was extremely slow but in the presence of SDS at its CMC rapid reduction could be observed [102]. Reduction of the disulfide bridge resulted in irreversible inactivation of the enzyme and the protein tended to become insoluble. CD spectra of cutinase in the 205-230 nm region, before and after DTE reduc-... 

Prepare the sulfhydryl column by washing with a disulfide reducing agent. Apply 10 ml of freshly made lOmM DTT solution (15.4mg of DTT dissolved in 10ml of equilibration buffer). This treatment converts the immobilized ligands into a fully reduced form (free —SH groups). 

In the previous studies using inhibitors and additives, it became clear that AMDase requires no cofactors, such as biotin, coenzyme A and ATP. It is also suggested that at least one of four cysteine residues plays an essential role in asymmetric decarboxylation. One possibility is that the free SH group of a cysteine residue activates the substrate in place of coenzyme A. Aiming at an approach to the mechanism of the new reaction, an active site-directed inhibitor was screened and its mode of interaction was studied. Also, site-directed mutagenesis of the gene coding the enzyme was performed in order to determine which Cys is located in the active site. 

In the first step, thioredoxin reductase reduces a small redox protein, thioredoxin, via enzyme-bound FAD. This involves cleavage of a disulfide bond in thioredoxin. The resulting SH groups in turn reduce a catalytically active disulfide bond in nucleoside diphosphate reductase ( ribonucleotide reductase ). The free SH groups formed in this way are the actual electron donors for the reduction of ribonucleotide diphosphates. 

Smjndly, the gold cluster described above was prepared in such a way that it could be bound to accessible -SH groups. Since this cluster is rather bulky, its accessibility was increased by the addition of spacers of various lengths to the cluster and to the free -SH groups on the ribosomal particles. 

Activation of Proteins with Traut s Reagent Yielding Proteins with Additional Free SH Groups... 

Regulation of the Calvin Cycle Iodoacetate reacts irreversibly with the free —SH groups of Cys residues in proteins. 

While on the HPLC column, Fab(SH) fragments can reoxidize to give F(ab)2. Alkylation of free SH-groups by the addition of 5 pL of 50 mM lodoacetamide in 1MNTE8 to the 45 pL of peak sample prevents this Incubate for 10 min at room temperature before applying a 10-pL sample to the HPLC column. 

Over the years it has been established that the usual samples of RNase have 4 disulfide groups and no free SH groups. In the middle 1950 s Ledoux claimed that there were SH groups present [one of the latest papers (168)]. Much of his work could not be repeated at that time, and claims for the importance of SH to activity do appear to be wrong. However, more recent work has shown that some reduction of the SS groups can occur with little or no loss of activity and criticisms of some parts of the earlier work may be unjustified. 

Reversible blocking of SH groups can be obtained through the formation of unsymmetrical disulfides with a variety of sulfenyl halides, for example, 4-nitrophenylsulfenyI chloride (177), SS cleavage with trisodium phosphorothioate (178), or protection with mercury compounds such as p-mercury benzoic acid (179). In each case the free SH groups can be regenerated with mercaptans. 

At the same time, some case are known when it is possible to carry out complex-formation reactions with N,S-donor chelating systems having a free SH group, for example (3.51) [61], (3.52) [62], (3.53) [63], and (3.54) [64] ... 

We pay special attention to the reaction (2.12). If we take into account the tautomeric equilibrium of thiazoline and mercaptoazomethine forms, it is possible to consider the ligand 461 as a compound with free SH group in this case also. Among the syntheses (2.12), we note the recently reported reaction of 2-phenyl-benzthiazoline 617 with cobalt and nickel acetates (3.54), resulting in structurally characterized chelates of the type 618 [64] ... 

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