Endopeptidases serine endopeptidase

The mechanism of hydrolysis of cysteine peptidases, in particular cysteine endopeptidases (EC 3.4.22), shows similarities and differences with that of serine peptidases [2] [3a] [55 - 59]. Cysteine peptidases also form a covalent, ac-ylated intermediate, but here the attacking nucleophile is the SH group of a cysteine residue, or, rather, the deprotonated thiolate group. Like in serine hydrolases, the imidazole ring of a histidine residue activates the nucleophile, but there is a major difference, since here proton abstraction does not appear to be concerted with nucleophilic substitution but with formation of the stable thiolate-imidazolium ion pair. Presumably as a result of this specific activation of the nucleophile, a H-bond acceptor group like Glu or Asp as found in serine hydrolases is seldom present to complete a catalytic triad. For this reason, cysteine endopeptidases are considered to possess a catalytic dyad (i.e., Cys-S plus H-His+). The active site also contains an oxyanion hole where the terminal NH2 group of a glutamine residue plays a major role. 

Cysteine endopeptidases, like serine endopeptidases, can also catalyze peptide synthesis under preparative conditions [66-68]. Thus, papain has been used to synthesize enkephalins and angiotensin. 

The /3-lactam structure can also react with active-serine hydrolases other than PBPs and /3-lactamases. It has been shown that appropriately substituted cephalosporins (e.g., 5.18) are potent mechanism-based inactivators of human leukocyte elastase (HLE, EC 3.4.21.37), a serine endopeptidase involved in the pathogenesis of pulmonary emphysema and other connective tissue diseases [57-60]. Subsequent work has demonstrated that substituted /3-lactams such as 5.19 or 5.20 are more stable HLE inhibitors and have improved potencies [61-63]. 

Serine-endopeptidase Cholecystokinin-C-terminal octapeptide Met3-Gly4... 

The proteolytic enzymes are classified into endopeptidases and exopeptidases, according to their site of attack in the substrate molecule. The endopeptidases or proteinases cleave peptide bonds inside peptide chains. They recognize and bind to short sections of the substrate s sequence, and then hydrolyze bonds between particular amino acid residues in a relatively specific way (see p. 94). The proteinases are classified according to their reaction mechanism. In serine proteinases, for example (see C), a serine residue in the enzyme is important for catalysis, while in cysteine proteinases, it is a cysteine residue, and so on. 

Prolyl endopeptidase (PEP, EC 3.4.21.26) is the only serine protease which is known to cleave a peptide substrate in the C terminal side of a proline residue... 

Glutamyl endopeptidase [EC 3.4.21.19] (also known as staphylococcal serine proteinase, V8 proteinase, protease V8, and endoproteinase Glu-C), a member of the peptidase family S2B, catalyzes the hydrolysis of Asp-Xaa and Glu-Xaa peptide bonds. In appropriate buffers, the specificity of the bond cleavage is restricted to Glu-Xaa. Peptide bonds involving bulky side chains of hydrophobic aminoacyl residues are hydrolyzed at a lower rate. 

This enzyme [EC 3.4.21.62], a serine endopeptidase that evolved independently of chymotrypsin, contains no cys-teinyl residues. This enzyme catalyzes the hydrolysis of peptide bonds in proteins and has a broad specificity, with a preference for a large uncharged aminoacyl residue in the PI subsite. 

This endopeptidase [EC 3.4.21.4], a member of the peptidase family SI, hydrolyzes peptide bonds at Arg—Xaa and Lys—Xaa. See Chymotrypsin Catalytic Triad Acyl-Serine Intermediate... 

Replacement of the peptide bond by a thioester group has been extensively applied in synthetic peptides used as enzyme substrates. Several classes of enzymes such as serine peptidases and metallo-endopeptidases are able to cleave efficiently thioester-modified amino acids and peptides. 61... 

The proteases may be conveniently classified according to their activities and functional groups. The serine proteases are endopeptidases that have a reactive... 

This text is a good source of information on the chemical mechanisms underlying the different modes of peptidase catalysis. Three important enzymes are covered subtilisin, a serine endopepti-dase papain, a cysteine endopeptidase and chymosin, an aspartic endopeptidase. 

Chymotrypsin, a serine endopeptidase, most readily reacts at the carboxyl group of the aromatic amino acid residue of proteins and polypeptides (or N-acyl aromatic amino acid esters) to form first a tetrahedral intermediate which then collapses into an acyl-enzyme (7+ 8- I>). The acyl-enzyme is then hydrolyzed by water to furnish the M-acylated aromatic amino acid again through the formation of a tetrahedral intermediate (JO Jl -> J2) (1-4). 

Proteases can be subdivided into two major groups exopeptidases cleaving the peptide bond proximal to the amino or carboxy terminal of the substrate, and endopeptidases cleaving distant from the termini (Rao et al., 1998). According to the functional group at the active site, proteases are further classified into four groups serine proteases, aspartyl proteases, cysteine proteases and metalloproteases. Based on the pH optimal for their functioning, proteolytic enzymes can be characterised as alkaline, neutral or acidic proteases. 

Birk serine PIPs (BBPIPs) [137-141] Hordeum (barley) lipid transfer proteins (LTPs) that inhibit malt cysteine endopeptidases [169] and Solanum (potato) Kunitz Pi-type cysteine Pis [185-188] (Table 5). 

Dietary proteins, with very few exceptions, are not absorbed rather they must be digested into amino acids, or di- and tripeptides. Protein digestion begins in the stomach, where proenzyme pepsinogen is autocatalytically converted to pepsin A. Most proteolysis takes place in the duodenum via enzymes secreted by the pancreas, including trypsinogen, chymotrypsinogen and pro-carboxypeptidase A. These serine and zinc proteases are produced in the form of their respective proenzymes they are both endopeptidase and exopeptidase, and their combined action leads to the production of amino acids, dipeptides and tripeptides. 

Du/tripeptidyl-peptidases Peptidyl-dipeptidases Serine carboxypeptidases MetaUocarboxypeptidases Cysteine carboxypeptidases Omega peptidases Serine endopeptidases Cysteine endopeptidases Aspartic endopeptidases MetaUoendopeptidases Threonine endopeptidases Other endopeptidases... 

Bode, W., and Huber, R., 1986. Crystal structure of pancreatic serine endopeptidases. InMolecular and Cellular Basis of Digestion (pp. 213 - 234), edited by P. Desnuelle, H. Sjostrom, and O. Noren. Elsevier. 

Alcalase, a serine endopeptidase from Bacillus licheniformis and whose major component is subtUisin A (subtihsin Carlsberg)[ l has been employed to selectively saponify peptide methyl and benzyl esters (Scheme 16) in a solvent system consisting of 90% tert-butyl alcohol and 10% buffer (pH 8.2) leaving the N-terminal Fmoc group intact. A selective classical alkaline saponification of methyl esters would have been impossible due to the base sensitivity of the Fmoc group. 

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