Serine histidine

Other serine hydrolases such as cholinesterases, carboxylesterases, lipases, and fl-lactamases of classes A, C, and D have a hydrolytic mechanism similar to that of serine peptidases [25-27], The catalytic mechanism also involves an acylation and a deacylation step at a serine residue in the active center (see Fig. 3.3). All serine hydrolases have in common that they are inhibited by covalent attachment of diisopropyl phosphorofluoridate (3.2) to the catalytic serine residue. The catalytic site of esterases and lipases has been less extensively investigated than that of serine peptidases, but much evidence has accumulated that they also contain a catalytic triad composed of serine, histidine, and aspartate or glutamate (Table 3.1). 

The active site of lipases is characterized by a triad composed of serine, histidine, and aspartate, and acyl-enzyme complexes are the crucial intermediates in all lipase-catalyzed reactions 117,118) (Fig. 13). 

The occurrence of alanine in proteins was first shown by Schutzen-berger, who did not actually identify his product with the synthetical one Weyl in l88i obtained it as a decomposition product of silk and showed that his preparation was similar in properties to Strecker s synthetical alanine. He thus established it as a constituent of a protein molecule. The researches of Emil P ischer have shown that alanine is a constant constituent of all proteins. It is worthy of note that of the eighteen definitely determined units of a protein molecule, six of them, namely, isoleucine, phenylalanine, tyrosine, serine, histidine and tryptophane, are derivatives of a-aminopropionic acid. 

Principal functional groups Serine, histidine Cysteine, histidine Zinc, glutamic acid tyrosine Aspartic acid ... 

The active site of serine proteases is characterized by a catalytic triad of serine, histidine, and aspartate. The mechanism of lipase action can be broken down into (i) adsorption of the lipase to the interface, responsible for the observed interfacial activation (ii) binding of substrate to enzyme (iii) chemical reaction and (iv) release of product(s). 

Serine proteases are characterized by a catalytic triad of Ser, His, and Asp. They work in pairs of Ser-His and His-Asp. Replacement of Asp by a second His revealed the auxiliary nature of aspartate, probably in orienting the histidine with respect to the catalytic serine. All of the catalytic steps are performed by the dyad serine-histidine. 

Several pharmaceutical enzymes belong to the group of serine-histidine estero-proteolytic enzymes (serine proteases), which display their catalytic activity with the aid of an especially reactive serine residue, whoso p-hydroxyi group forms a covalent bond with the substrate molecule. This reaction takes place by cooperation with the imidazole base of histidine. The specificity of the enzymes is achieved by the characteristic strocture of their substrate-binding centers, which in these proteases are built according to the same principle. They consist of a hydrophobic slit formed by apolar aide chains of amino acids and a dissociated side chain-located carboxyl group of an aspartic add residue at the bottom. 

A similar concept was used in the development of artificial chymotrypsin mimics [54]. The esterase-site was modeled by using the phosphonate template 75 as a stable transition state analogue (Scheme 13.19). The catalytic triad of the active site of chymotrypsin - that is, serine, histidine and aspartic acid (carboxy-late anion) - was mimicked by imidazole, phenolic hydroxy and carboxyl groups, respectively. The catalytically active MIP catalyst 76 was prepared using free radical polymerization, in the presence of the phosphonate template 75, methacrylic acid, ethylene glycol dimethacrylate and AIBN. The template removal conditions had a decisive influence on the efficiency of the polymer-mediated catalysis, and best results were obtained with aqueous Na2CC>3. 

Sheehan and co-woricers prepared a pentapeptide, L-Thr-L-AlarL er-L-His-L-Asp as an esterase model, from a viewpoint that three amino acids (serine, histidine and aspartic acid) are involved at the actne site (131,132). The rate constant for the penti ptide f 7,1.5 sec (pH 7.73,25.5 ) is greater than 0.25 M sec for... 

Pancreatic lipase can be considered a model for all other lipases. Most if not all of these enzymes seem to be nonspecific carboxyl ester hydrolases of the serine histidine type. Their specificity consists, by definition, in their ability to hydrolyze insoluble substrates, but apart... 

Enzymes that hydrolyze lysophospholipids are found in nearly all tissues and organisms. They seem to be non-specific esterases of the serine-histidine type (25) and hardly deserve the name lysophospholipase because they also hydrolyze esters other than phospholipids. They should probably be considered together with such enzymes as cholesterol esterases and monoglyceride lipases as amphiphilic carboxyl ester hydrolases. These non-specific esterases have a preference for amphiphilic (hydrophilic-lipophilic) substrates. Such an enzyme may perhaps hydrolyze lysophospholipis, monoglycerides, diglycerides, and cholesterol esters. 

Residues of serine, histidine, and aspartic acid will be designated Ser-195, His-57, and Asp-102 in the following discussion. The numbers refer to their location in the amino acid sequence of chymotrypsinogen (22), but they have no absolute significance for other enzymes and would occur in a different order if they had been based on the subtilisin sequence. 

Amino acids identified in both total DOM (as THAA) and HMWDOM (as DCAA) include aspartic acid, glutamic acid, serine, histidine, glycine, threonine, alanine, arginine, tyrosine, valine, isoleucine, phenylalanine, leucine, and tryptophan (e.g., McCarthy et al., 1996 Yamashita and Tanoue, 2003). Figure 3.5A shows the mole percent distribution of these amino acids in DOM and HMWDOM. AU... 

Ammonium ions are produced by the catabolism of a number of amino acids. Glutamate dehydrogenase is the major source of ammonium ions in the body. Ammonium ions are also produced from the catabolic pathways of serine, histidine, tryptophan, glycine, glutamine, and asparagine. L-Amino acid oxidase and... 

Figure 34-18 Reactivation of phosphoryiated acetylcholinesterase by pralidoxime formation of aged phosphoryiated enzyme, which does not reactivate.The active site catalytic triad of serine, histidine, and glutamate is depicted by —OH, =NH—, and a negative charge, respectively.

Mechanistic and structural investigations have revealed that while some esterase antibodies promote direct attack by hydroxide on the ester function [ 15], others unexpectedly may also use a covalent mechanism involving a serine-histidine dyad strongly reminiscent of the catalytic triad of serine proteases [16]. 

The action of OP nerve agents on the nervous system results from their effects on enzymes, particularly esterases. The most notable of these esterases is acetylcholinesterase. The active site of acetylcholinesterase comprises a catalytic triad of serine, histidine and glutamic acid residues and other important features of the enzyme are a gorge connecting the active site to the surface of the protein and a peripheral anionic site (Bourne etal., 1995,1999 Sussman etal., 1991 Thompson and Richardson, 2004), The OPs phosphy-late1 the serine hydroxyl group in the active site of the enzyme. 

Sequence patterns Protein sequence is not well conserved, because protein function is often attributed to only a small set of residues, such as the catalytic triad Serine-Histidine-Aspartate in serine protease enzymes. Special algorithms have been developed to find these patterns or motifs, of which the constituting parts are most often discontinuous parts of a protein sequence. 

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