Thiol histidine

The reaction of 1-amino-1-deoxyketoses, and their N-alkyl and N-aryl derivatives, with alkyl or aryl isothiocyanates (Huber et al, 1960) was studied in more detail, and new 4-(alditol-l-yl)-l-alkyl(aryl)-3-alkyl(aryl)-l,3-dihydro-2H-imidazole-2-thiones were obtained. These compounds were used as starting materials for the synthesis of OL-histidines, DL-histidine-2-thiol, and other imidazole derivatives of biological interest. 

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. 

The mechanism for the lipase-catalyzed reaction of an acid derivative with a nucleophile (alcohol, amine, or thiol) is known as a serine hydrolase mechanism (Scheme 7.2). The active site of the enzyme is constituted by a catalytic triad (serine, aspartic, and histidine residues). The serine residue accepts the acyl group of the ester, leading to an acyl-enzyme activated intermediate. This acyl-enzyme intermediate reacts with the nucleophile, an amine or ammonia in this case, to yield the final amide product and leading to the free biocatalyst, which can enter again into the catalytic cycle. A histidine residue, activated by an aspartate side chain, is responsible for the proton transference necessary for the catalysis. Another important factor is that the oxyanion hole, formed by different residues, is able to stabilize the negatively charged oxygen present in both the transition state and the tetrahedral intermediate. 

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). 

A sulfonyl chloride group rapidly reacts with amines in the pH range of 9-10 to form stable sulfonamide bonds. Under these conditions, it also may react with tyrosine —OH groups, aliphatic alcohols, thiols, and histidine side chains. Conjugates of sulfonyl chlorides with sulf-hydryls and imidazole rings are unstable, while esters formed with alcohols are subject to nucleophilic displacement (Nillson and Mosbach, 1984 Scouten and Van der Tweel, 1984). The only stable derivative with proteins therefore is the sulfonamide, formed by reaction with e-lysine... 

Ergothioneine is the betaine derived from 2-thiol histidine (i.e. the trimethyl-ammonium derivative). It can be written as a thiol 40 or thione 41 42 the... 

Affinity chromatography and related techniques (e.g., thiol chromatography and IMAC) are widely used for preparative isolation because they enable a single protein or class of proteins to be selectively purified from very complex mixtures. They may be occasionally used as analytical tools. For example, protein A affinity chromatography has been used for quantitative analysis of immunoglobulins in ascites fluid.45 Information about surface-accessible histidine and phosphate groups may be obtained using IMAC. 

In addition to the lack of sequence homology, ULPs have little structural homology to other DUB classes except in the active site. The structure of ULPl (see Figure 8.2) in complex with the C-terminal aldehyde of yeast SUMO (SMT3) illustrates that, like most other DUBs, ULPs are thiol proteases, utilizing a conserved catalytic triad consisting of an active-site cysteine, histidine, and aspartate [40]. 

Some amino acids have additional ionizable groups in their side-chains. These may be acidic or potentially acidic (aspartic acid, glutamic acid, tyrosine, cysteine), or basic (lysine, arginine, histidine). We use the term potentially acidic to describe the phenol and thiol groups of tyrosine and cysteine respectively under physiological conditions, these groups are unlikely to be ionized. It is relatively easy to calculate the amount of ionization at a particular pH, and to justify that latter statement. 

The major metal-binding amino acid side chains in proteins (Gurd and Wilcox, 1956 see Voet and Voet, 1990) (Table II) are carboxyl (aspartic acid and glutamic acid), imidazole (histidine), indole (tryptophan), thiol (cysteine), thioether (methionine), hydroxyl (serine, threonine, and tyrosine), and possibly amide groups (asparagine and glutamine, although... 

Monoamine oxidase (MAO) (E.C. 1.4.3.4) is an enzyme found in all tissues and almost all cells, bound to the outer mitochondrial membrane. Its active site contains flavine adenine dinucleotide (FAD), which is bound to the cysteine of a -Ser-Gly-Gly-Cys-Tyr sequence. Ser and Tyr in this sequence suggest a nucleophilic environment, and histidine is necessary for the activity of the enzyme. Thiol reagents inhibit MAO. There are at least two classes of MAO binding sites, either on the same molecule or on different isozymes. They are designated as MAO-A, which is specific for 5-HT (serotonin) as a substrate, and MAO-B, which prefers phenylethylamine. Similarly, MAO inhibitors show a preference for one or the other active site, as discussed below. 

The cleavage mechanism of the caspases is shown schematically in Fig. 15.5. They use a typical protease mechanism with a catalytic diad for cleavage of the peptide bond. The nucleophilic thiol of an essential Cys residue forms a covalent thioacyl bond to the substrate during the catalysis. The imidazole ring of an essential histidine is also involved in catalysis and this facilitates hydrolysis of the amide bond in the sense of an acid/base catalysis. 

In enzymes, the most common nucleophilic groups that are functional in catalysis are the serine hydroxyl—which occurs in the serine proteases, cholinesterases, esterases, lipases, and alkaline phosphatases—and the cysteine thiol—which occurs in the thiol proteases (papain, ficin, and bromelain), in glyceraldehyde 3-phosphate dehydrogenase, etc. The imidazole of histidine usually functions as an acid-base catalyst and enhances the nucleophilicity of hydroxyl and thiol groups, but it sometimes acts as a nucleophile with the phos-phoryl group in phosphate transfer (Table 2.5). 

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