Exopeptidases

An aminopeptidase activity has been demonstrated to cleave the N-terminal Arg residue from bovine /3-lipotropin [49]. This activity appears to be specific for basic residues, but is blocked by the presence of proline residues on the carboxyl side of the N-terminal arginine [22]. However, it is not yet clear whether this aminopeptidase is involved in precursor processing in situ. 

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Proteolytic enzymes - hydrolyse proteins selectively, either on terminal groups (exopeptidases) or internal linkages (endopeptidases), eg... 

An exopeptidase that sequentially releases an amino acid from the N-terminus of a protein or peptide. Examples include cystinyl aminopeptidase (MEROPS M01.011), which removes a terminal cysteine from the biologically important peptides oxytocin and vasopressin, and methionyl aminopeptidase (M24.001), which removes the initiating methionine from cytosolic... 

An exopeptidase that can only degrade a dipeptide. Examples are carnosine dipeptidase I (MEROPS M20.006), which degrades carnosine (beta-Ala-His), and membrane dipeptidase (MEROPS Ml9.001), which is important in the catabolism of glutathione, degrading the dipeptides Cys-Gly. Dipeptidases are included in Enzyme Nomenclature sub-subclass 3.4.13. 

There are several different types of exopeptidases aminopeptidases, carboxypeptidases, dipeptidyl-peptidases, tripeptidy 1-peptidases, peptidyl-... 

Gener ally, a family of peptidases contains either exopeptidases or endopeptidases, but there are exceptions. Family Cl contains not only endopeptidases such as cathepsin L, but also the aminopeptidase bleomycin hydrolase. Some members of this family can act as exopeptidases as well as endopeptidases. For example, cathepsin B also acts as a peptidyl-dipeptidase, and... 

An exopeptidase that does not cleave standard peptide bonds. An example is pyroglutamyl-peptidase I (MEROPS C15.010), which releases an N-terminal pyroglutamyl from hormones such as thyrotropinreleasing hormone and luteinizing hormone. Omega peptidases are included in Enzyme Nomenclature subsubclass 3.4.19. 

An exopeptidase that sequentially releases dipeptides from the C-terminus of a protein or peptide. An example is angiotensin-converting enzyme (also known as peptidyl-dipeptidase A MEROPS XM02-001), which plays an important role in the control of blood pressure by converting angiotensin I to angiotensin II. Peptidyl-dipeptidases are included in Enzyme Nomenclature sub-subclass 3.4.15. 

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... 

Metallo proteases Exopeptidase group Peptidyl dipeptidase-A (ACE) Aminopeptidase-M Carboxypeptidase-A... 

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. 

The disposition of insulin was shown to be susceptible to non-absorptive losses to metabolism and mucociliary clearance. Modification of the deposition profile of insulin in the lung showed that higher absorption rates were obtained for more peripheral deposition and co-administration of a metabolic inhibitor reduced losses to exopeptidase metabolism [101], It is acknowledged by the investigators that the IPL technique and the dosing technique of Byron and coworkers are not widely accessible and have therefore not been widely adopted [119], Active absorption has also been studied in this system as described in Sect. 6.2.43. 

Wall DA, Lanutti AT (1993) High levels of exopeptidase activity are present in rat and canine bronchoalveolar lavage fluid. Int J Pharm 97 171-181. 

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 NC-IUBMB has introduced a number of changes in the terminology following the proposals made by Barrett, Rawlings and co-workers [7] [8]. The term peptidase should now be used as a synonym for peptide hydrolase and includes all enzymes that hydrolyze peptide bonds. Previously the term peptidases was restricted to exopeptidases . The terms peptidase and protease are now synonymous. For consistency with this nomenclature, the term proteinases has been replaced by endopeptidases . To complete this note on terminology, we remind the reader that the terms cysteine endopeptidases and aspartic endopeptidases were previously called thiol proteinases and acid or carboxyl proteinases , respectively [9],... 

One of the general principles of the Nomenclature Committee is that enzymes should be classified and named according to the reaction they catalyze. However, the overlapping specificities of and great similarities in the action of different peptidases render naming solely on the basis of function impossible [10]. For example, some enzymes can act as both endo- and exopeptidases. Thus, cathepsin H (EC 3.4.22.16) is not only an endopeptidase but also acts as an aminopeptidase (EC 3.4.11), and cathepsin B (EC 3.4.22.1) acts as an endopeptidase as well as a peptidyl-dipeptidase (EC 3.4.15). The actual classification of peptidases is, therefore, a compromise based not only on the reaction catalyzed but also on the chemical nature of the catalytic site, on physiological function, and on historical priority. 

SB 1 Family of endo- and exopeptidases (including proprotein convertases) Asp, His, Ser Parallel /3-sheet... 

SC 6 Families of endopeptidases (oligopeptidases) and exopeptidases (including lysosomal carboxypeptidase A in family S10) Ser, Asp, His a,/3-Hydrolase... 

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. 

Peptide hydrolases (peptidases or proteases, i.e., enzymes hydrolyzing peptide bonds in peptides and proteins, see Chapt. 2) have received particular attention among hydrolases. As already described in Chapt. 2, peptidases are divided into exopeptidases (EC 3.4.11 -19), which cleave one or a few amino acids from the N- or C-terminus, and endopeptidas-es (proteinases, EC 3.4.21-99), which act internally in polypeptide chains [2], The presentation of enzymatic mechanisms of hydrolysis in the following sections will begin with peptidases and continue with other hydrolases such as esterases. 

The metal ion is held in place by amino acid residues, generally His, Glu, Asp, or Lys. In many metallopeptidases, which may be exopeptidases or en-dopeptidases, only one zinc ion is required. In all Co2+ or Mn2+-dependent, and in some Zn2+-dependent metallopeptidases, two metal ions are present and act cocatalytically these enzymes are exopeptidases [2][73][74],... 

Increased permeability is just one prerequisite in the development of useful peptide prodrugs. Another condition is that efficient bioactivation must follow absorption. Mucosal cell enzymes able to hydrolyze peptides include exopeptidases such as aminopeptidases and carboxypeptidases, endopepti-dases, and dipeptidases such as cytosolic nonspecific dipeptidase (EC 3.4.13.18), Pro-X dipeptidase (prolinase, EC 3.4.13.4), and X-Pro dipeptidase (prolidase, EC 3.4.13.9). For example, L-a-methyldopa-Pro was shown to be a good substrate for both the peptide transporter and prolidase. This dual affinity is not shared by all dipeptide derivatives, and, indeed, dipeptides that lack an N-terminal a-amino group are substrates for the peptide transporter but not for prolidase [29] [33] [34],... 

The in vitro hydrolysis of insulin has been shown to be catalyzed by exopeptidases and endopeptidases. Carboxypeptidase A (EC 3.4.17.1) cleaves the C-terminus of the B-chain (ThrB3°) and that of the A-chain (AsnA21) [145], Leucyl aminopeptidase (EC 3.4.11.1) cleaves the N-terminus of the B-chain (PheB1) and can continue to shorten it. But, leucyl aminopeptidase appears also able to cleave the N-terminus of the A-chain (GlyA1). In addition to these exopeptidases, entire insulin is also cleaved by endopeptidases of the... 

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