Amino endopeptidases

Endothelin. The endothelin (ET) peptide family (50) comprises thiee peptides ET-1 (133), ET-2 (134), and ET-3 (135). ET-1, the most abundant, is a 21-amino acid peptide. A 203-amino acid peptide piecuisoi, piepioET, is cleaved after translation by endopeptidases to form a 38-amino acid proET which is converted to active ET by a putative endothelin-converting enzyme (ECE). ET-3 differs from ET-1 and ET-2 by sis amino acids. 

ECEs are metalloproteinases that are homologous to the neutral endopeptidase (NEP, E-24.11, neprilysin) unlike NEP, however, they form disulfide-bonded homodimers. In man, with ECE-1 and ECE-2, two isoforms are known, which are encoded by two separate genes. For ECE-1 four different variants have been identified (ECE-1 a-d), which are generated by the use of alternative promoters (Table 2). The ECE-1 isoforms only differ in their N-terminal amino acid sequence. For ECE-2, a single gene product has been described in... 

Neutral endopeptidase (NEP, nephrilysin) is an enzyme that preferentially catalyzes cleavage at the amino group of hydrophobic residues of the B-chain of insulin... 

The synthesis of the CG525155 (a neutral endopeptidase inhibitor) required a Pd catalyzed Tsuji-Trost reaction as the key step following the strategy described by Johnson. Starting from the optically active allenyl amino acid methylester 91 (synthesized in several steps from 90), the seco-derivative 92 as the crucial precursor was generated in several steps in high yield. The Pd (0)... 

There are two main classes of proteolytic digestive enzymes (proteases), with different specificities for the amino acids forming the peptide bond to be hydrolyzed. Endopeptidases hydrolyze peptide bonds between specific amino acids throughout the molecule. They are the first enzymes to act, yielding a larger number of smaller fragments, eg, pepsin in the gastric juice and trypsin, chymotrypsin, and elastase secreted into the small intestine by the pancreas. Exopeptidases catalyze the hydrolysis of peptide bonds, one at a time, fi"om the ends of polypeptides. Carboxypeptidases, secreted in the pancreatic juice, release amino acids from rhe free carboxyl terminal, and aminopeptidases, secreted by the intestinal mucosal cells, release amino acids from the amino terminal. Dipeptides, which are not substrates for exopeptidases, are hydrolyzed in the brush border of intestinal mucosal cells by dipeptidases. 

The end product of the action of endopeptidases and exopeptidases is a mixmre of free amino acids, di- and tripeptides, and oligopeptides, all of which are absorbed. Free amino acids are absorbed across the intestinal mucosa by sodium-dependent active transport. There are... 

Proteases, which can be classified as either peptidases or proteinases. These cleave polypeptide chains eventually into their component amino acids. Peptidases can be further classified as endopeptidases (which act on the main-chain amido groups along the polypeptide molecule) or as exopeptidases (which act only at terminal amino acid residues). 

As mentioned earlier, by far the largest number of zinc enzymes are involved in hydrolytic reactions, frequently associated with peptide bond cleavage. Carboxypeptidases and ther-molysins are, respectively, exopeptidases, which remove amino acids from the carboxyl terminus of proteins, and endopeptidases, which cleave peptide bonds in the interior of a polypeptide chain. However, they both have almost identical active sites (Figure 12.4) with two His and one Glu ligands to the Zn2+. It appears that the Glu residue can be bound in a mono- or bi-dentate manner. The two classes of enzymes are expected to follow similar reaction mechanisms. 

Chymotrypein is a proteolytic and milk-curdling enzyme of the pancreatic secretion. It is a protein endopeptidase which catalyses the hydrolysis of native proteins to peptones, polypeptides and amino acids, by breaking the peptide linkages of the carboxyl groups of tyrosine and phenylalanine. 

The NC-IUBMB classifies peptidases (EC 3.4) into exopeptidases (EC 3.4.11-19), which remove one or a few amino acids, and endopeptidases (proteinases, EC 3.4.21-99), which catalyze the cleavage of peptide bonds away from either end of the polypeptide chain (Fig. 2.1). Exopeptidases are further subdivided into enzymes that carry out hydrolysis at the N-terminus or the C-terminus (Figs. 2.1 and 2.2). Thus, aminopeptidases (EC 3.4.11) cleave a single amino acid from the N-terminus [3] those removing a dipep-... 

The evolutionary classification has a rational basis, since, to date, the catalytic mechanisms for most peptidases have been established, and the elucidation of their amino acid sequences is progressing rapidly. This classification has the major advantage of fitting well with the catalytic types, but allows no prediction about the types of reaction being catalyzed. For example, some families contain endo- and exopeptidases, e.g., SB-S8, SC-S9 and CA-Cl. Other families exhibit a single type of specificity, e.g., all families in clan MB are endopeptidases, family MC-M14 is almost exclusively composed of carboxypeptidases, and family MF-M17 is composed of aminopeptidases. Furthermore, the same enzyme specificity can sometimes be found in more than one family, e.g., D-Ala-D-Ala carboxypeptidases are found in four different families (SE-S11, SE-S12, SE-S13, and MD-M15). 

The proteolytic activity of some multicatalytic peptidases is stimulated by ATP, whereas that of others is not influenced by ATP [32], The ATP-dependent proteolytic system first found in reticulocytes requires the presence of a heat-stable polypeptide called ubiquitin, one of the roles of which is to mark particular proteins for subsequent degradation [33. ATP-Indepen-dent multicatalytic peptidases can degrade proteins that have a free amino or an /V-acctylatcd terminus, as well as oxidatively altered or phosphorylat-ed proteins [34], The small peptides generated are resistant to multicatalytic peptidases and are further degraded by cytoplasmic endopeptidases and exopeptidases. 

Another structurally simple modification involves replacement of a Lys residue with Lys(NH2) (6.83). Here, the amino group of lysine is replaced with a hydrazino group, which is less-basic by ca. 3 pKa units. This modification allows model peptides to be stabilized against endopeptidases such as trypsin and thrombin [207], Thus, the peptides Tyr-Gly-Xaa-Gly-Tyr-Ala-NH2 with Xaa = Lys or Arg were very rapidly hydrolyzed by trypsin (half-... 

Both HFC and HNC are endopeptidases that contain one Zn(II) atom located at the active site (Springman, E., Birkedal-Hansen, H. and Van Wart, H.E., unpublished observations) and require Ca(II) for stability [84]. The protein chains of both are very similar in their sequence and construction. The HFC and HNC preproenzymes consist of 468 and 467 amino acids, of which the first 18 and 20, respectively, represent the signal peptides [27,29,30]. Thus, the zymogens pro-HFC and pro-HNC consist of 450 and... 

Protein digestion occurs in two stages endopeptidases catalyse the hydrolysis of peptide bonds within the protein molecule to form peptides, and the peptides are hydrolysed to form the amino acids by exopeptidases and dipeptidases. Enteropeptidase initiates pro-enzyme activation in the small intestine by catalysing the conversion of trypsinogen into trypsin. Trypsin is able to achieve further activation of trypsinogen, i.e. an autocatalytic process, and also activates chymotrypsinogen and pro-elastase, by the selective hydro-... 

These proteolytic enzymes are all endopeptidases, which hydrolyse links in the middle of polypeptide chains. The products of the action of these proteolytic enzymes are a series of peptides of various sizes. These are degraded further by the action of several peptidases (exopeptidases) that remove terminal amino acids. Carboxypeptidases hydrolyse amino acids sequentially from the carboxyl end of peptides. They are secreted by the pancreas in proenzyme form and are each activated by the hydrolysis of one peptide bond, catalysed by trypsin. Aminopeptidases, which are secreted by the absorptive cells of the small intestine, hydrolyse amino acids sequentially from the amino end of peptides. In addition, dipeptidases, which are structurally associated with the glycocalyx of the entero-cytes, hydrolyse dipeptides into their component amino acids. 

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. 

Proteins are first denatured by the stomach s hydrochloric acid (see p. 270), making them more susceptible to attack by the endopeptidases (proteinases) present in gastric and pancreatic juice. The peptides released by endopeptidases are further degraded into amino acids by exopeptidases. Finally, the amino acids are resorbed by the intestinal mucosa in cotransport with Na"" ions (see p. 220). There are separate transport systems for each of the various groups of amino acids. 

These zinc-dependent endopeptidases (meprin A [EC 3.4.24.18] and meprin B [EC 3.4.24.63] ) are members of the peptidase family M12A. They catalyze the hydrolysis of peptide bonds in proteins and peptide substrates. Meprin A, a membrane-bound enzyme that has been isolated from mouse and rat kidney and intestinal brush borders as well as salivary ducts, acts preferentially on carboxyl side of hydrophobic amino acyl residues. Meprin A and B are insensitive to inhibition by phosphora-midon and thiorphan. 

This zinc-dependent endopeptidase [EC 3.4.24.11] catalyzes the hydrolysis of peptide bonds and exhibits preferential cleavage at the amino group of hydrophobic residues in proteins and polypeptides. Neprilysin is a membrane-bound glycoprotein that is inhibited by phos-phoramidon and thiorphan. 

This endopeptidase [EC 3.4.22.2], a member of the Cl peptidase family hydrolyzes peptide bonds in proteins, exhibiting a broad specificity for those bonds. There is a preference for an amino acyl residue bearing a large hydrophobic side chain at the P2 position and the enzyme does not accept a valyl residue at Pi. 

Metabolism of angiotensinogen by renin produces the decapeptide angiotensin I. This relatively inactive peptide is acted on by a dipeptidase-converting enzyme to produce the very active octapeptide angiotensin II. In addition to converting enzyme, angiotensin I can be acted on by prolyl endopeptidase, an enzyme that removes the first amino acid to form angiotensin 1-7, a peptide primarily active in the brain. ACE has been... 

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