Carboxypeptidases amino acid residues

Carboxypeptidases are zinc-containing enzymes that catalyze the hydrolysis of polypeptides at the C-terminal peptide bond. The bovine enzyme form A is a monomeric protein comprising 307 amino acid residues. The structure was determined in the laboratory of William Lipscomb, Harvard University, in 1970 and later refined to 1.5 A resolution. Biochemical and x-ray studies have shown that the zinc atom is essential for catalysis by binding to the carbonyl oxygen of the substrate. This binding weakens the C =0 bond by... 

ENZYMATIC ANALYSIS WITH CARBOXYPEPTIDASES. Carboxypeptidases are enzymes that cleave amino acid residues from the C-termini of polypeptides in a successive fashion. Four carboxypeptidases are in general use A, B, C, and Y. Carboxypeptidase A (from bovine pancreas) works well in hydrolyzing the C-terminal peptide bond of all residues except proline, arginine, and lysine. The analogous enzyme from hog pancreas, carboxypeptidase B, is effective only when Arg or Lys are the C-terminal residues. Thus, a mixture of carboxypeptidases A and B liberates any C-terminal amino acid except proline. Carboxypeptidase C from citrus leaves and carboxypeptidase Y from yeast act on any C-terminal residue. Because the nature of the amino acid residue at the end often determines the rate at which it is cleaved and because these enzymes remove residues successively, care must be taken in interpreting results. Carboxypeptidase Y cleavage has been adapted to an automated protocol analogous to that used in Edman sequenators. 

The introduction of redox activity through a Co11 center in place of redox-inactive Zn11 can be revealing. Carboxypeptidase B (another Zn enzyme) and its Co-substituted derivative were oxidized by the active-site-selective m-chloroperbenzoic acid.1209 In the Co-substituted oxidized (Co111) enzyme there was a decrease in both the peptidase and the esterase activities, whereas in the zinc enzyme only the peptidase activity decreased. Oxidation of the native enzyme resulted in modification of a methionine residue instead. These studies indicate that the two metal ions impose different structural and functional properties on the active site, leading to differing reactivities of specific amino acid residues. Replacement of zinc(II) in the methyltransferase enzyme MT2-A by cobalt(II) yields an enzyme with enhanced activity, where spectroscopy also indicates coordination by two thiolates and two histidines, supported by EXAFS analysis of the zinc coordination sphere.1210... 

Nakagawa, S., and H. Umeyama. 1981. Molecular Orbital Study of the Effects of Ionic Amino Acid Residues on Proton Transfer Energetics in the Active Site of Carboxypeptidase A. Chem. Phys. Letters 81, 503-507. 

Carboxypeptidase A"" (CPA, EC 3.4.17.1) is a proteolytic enzyme that cleaves C-terminal amino acid residues with hydrophobic side chains selectively. Several X-ray structures are available" The active site of CPA consists of a hydrophobic pocket (primary substrate recognition site) that is primarily responsible for the substrate specificity, a guanidinium moiety of Argl45 that forms hydrogen bonds to the carboxylate of the substrate, and Glu270, whose carboxylate plays a critical role, functioning either as a nucleophile to attack the scissUe carboxamide carbonyl carbon of the substrate or as a base to activate the zinc-bound water molecule, which in turn attacks the scissile peptide bond ". However, semiempirical calculations had shown that the direct attack of... 

The considerable detail to which we now can understand enzyme catalysis is well illustrated by what is known about the action of carboxypeptidase A. This enzyme (Section 25-7B and Table 25-3) is one of the digestive enzymes of the pancreas that specifically hydrolyze peptide bonds at the C-terminal end. Both the amino-acid sequence and the three-dimensional structure of carboxypeptidase A are known. The enzyme is a single chain of 307 amino-acid residues. The chain has regions where it is associated as an a helix and others where it is associated as a /3-pIeated sheet. The prosthetic group is a zinc ion bound to three specific amino acids and one water molecule near the surface of the molecule. The amino acids bound to zinc are His 69, His 196, and Glu 72 the numbering refers to the position of the amino acid along the chain, with the amino acid at the /V-terminus being number l. The zinc ion is essential for the activity of the enzyme and is implicated, therefore, as part of the active site. 

Chemical methods for carboxyl end-group determination are considerably less satisfactory. Treatment of the peptide with anhydrous hydrazine at 100°C results in conversion of all the amino acid residues to amino acid hydrazides except for the carboxyl-terminal residue, which remains as the free amino acid and can be isolated and identified chro-matographically. Alternatively, the polypeptide can be subjected to limited breakdown (proteolysis) with the enzyme carboxypeptidase. This results in release of the carboxyl-terminal amino acid as the major free amino acid reaction product. The amino acid type can then be identified chroma-tographically. 

The carboxypeptidases are released from their inactive precursors in the pancreatic juice of animals. The most studied example is bovine carboxypeptidase A, which contains one mole of zinc per protein molecular weight of 34 500. These enzymes cleave the C-terminal amino acid residue from peptides and proteins, when the side-chain of the C-terminal residue is aromatic or branched aliphatic of l configuration. At least the first five residues in the substrate affect the activity of the enzyme. The enzyme also shows esterase activity. Esters and peptides inhibit each other competitively, indicating that the peptidase and esterase sites overlap, even if they are not the same. 

Carboxypeptidase A is a metalloenzyme (containing Zn+ ) which hydrolyzes the C-terminal peptide bond in polypeptide chains (1-4). The hydrolysis occurs most readily when the terminal amino acid residue has an aromatic (or a large aliphatic) R group (cf. 38-> 39+40). 

Aprotinin is a polypeptide consisting of 58 amino acid residues derived from bovine lung tissues and shows inhibitory activity toward various proteolytic enzymes including chymo-trypsin, kallikrein, plasmin, and trypsin. It was also one of the first enzyme inhibitors used as an auxiliary agent for oral (poly)peptide administration. The co-administration of aprotinin led to an increased bioavailability of peptide and protein drugs [5,44,45], The Bowman-Birk inhibitor (71 amino acids, 8 kDa) and the Kunitz trypsin inhibitor (184 amino acids, 21 kDa) belong to the soybean trypsin inhibitors. Both are known to inhibit trypsin, chymotrypsin, and elastase, whereas carboxypeptidase A and B cannot be inhibited [7,46],... 

The acid carboxypeptidase from A. saitoi releases the carboxyterminal phenenylalanine-amide (-Phe-NH2) from the carboxy-terminal amidated peptides, such as gastrin-related peptide (/-amyloxycarbonyl (Aoc)-Trp-Met-Asp-Phe-NH2, Aoc-WMDF-NH2) and molluscan cardioexcitatoiy neuropeptide (Phe-Met-Arg-Phe-NH2, FMRF-NH2) [86], The summarized data are shown in Table 12. When gastrin-related peptide was used as a substrate, the enzyme acted only as a carboxyamidase, because of the presence of the hydrophobic amino acid residue, tryptophan, in the P3 [12] position. 

Table 12. Hydrophobicity of side chains of amino acid residue adjacent to carboxyterminal bond to be split by the carboxypeptidase from A. saitoi...

Comparison of the deduced sequence of A. saitoi carboxypeptidase with other known serine carboxypeptidase sequences shows that they share a low degree of similarity 32% with wheat carboxypeptidase II, 32.3% with malt carboxypeptidase II and 26.2% with yeast carboxypeptidase Y (Figure 19) [88], However, all of the sequences conserve the catalytic domains (indicated by boxes II to IV in Figure 19) and the domain (box I in the Figure 19) which contains the amino acid residues recognizing the C-terminal carboxylate group of peptide substrates. There are also present in the sequence ten potential sites for N-linked glycosylation. 

Fig. 1 Blocks of multiple sequence alignment of protein sequences of carboxypeptidases from B. taurus, Mus musculus, Rattus norvegicus, Neurospora crassa, Schizosaccharomyces pombe, Drosophila melanogaster, and Homo sapiens along with protein sequence from H. pylori (Uniprot accession code HPAG1 0372 from strain HPAG1). Numbers on the top correspond to amino acid residue number of the carboxypeptidase enzyme from B. taurus. Gray vertical columns indicate conserved residues. Amino acid residues corresponding to Glu-182 and His-306, which coordinate to zinc, are conserved, whereas another Zn-coordinating amino acid residue corresponding to His-179 is substituted by Gin in the Helicobacter sequence. Functionally important residues corresponding to Arg-237 are also conserved...

His-306 of carboxypeptidase from Bos taurusarc conserved. These amino acid residues are implicated in zinc coordination. Yet another functionally important residue that binds to substrate corresponding to Arg-237 of bovine enzyme is also conserved in the H. pylori sequence (Fig. 1) (30-32). 

The remaining prohormone contains stretches of amino acids sequences unrelated to biological activity. These sequences are removed by proteolytic enzymes such as endopeptidases which act on dibasic or monobasic amino acid residues, and exopeptidases (e.g., carboxypeptidases and aminopeptidases) which remove amino acid residues from the C-terminal and N-terminal ends, respectively. A single prohormone may yield one or more biologically active and distinct hormones. 

Requirements of Active Sites in Enzymes Carboxypeptidase, which sequentially removes carboxyl-terminal amino acid residues from its peptide substrates, is a single polypeptide of 307 amino acids. The two essential catalytic groups in the active site are furnished by Arg145 and Glu270. 

Because only 40 to 60 amino acid residues can be determined by the Edman procedure, additional methods are needed for larger proteins. Determination of the C-terminal amino acid can be accomplished by treating the protein with carboxypeptidase. This enzyme selectively catalyzes the hydrolysis of the C-terminal amino acid. After the first amino acid has been removed, the enzyme begins to cleave the second amino acid, and so forth. By following the rates at which the amino acids appear, it is possible to determine the first few amino acids at the C-terminal end of the protein by employing this enzyme. However, because the enzyme hydrolyzes different peptide bonds at different rates, it is possible to identify only a few amino acids before the reaction mixture becomes too complex. 

Carboxypeptidase A small intestine6 all amino acid residues except arginine, lysine and proline... 

Carboxyl-terminal amino acid residues can be identified using exopeptidases called carboxypeptidase A and B. These are enzymes normally found in mammalian small intestine. Carboxypeptidase A causes the removal of amino acids, other than proline and basic amino acids, from the C terminus of peptides and proteins. The C-terminal amino acid appears first, followed by the next in line, and so forth. Carboxypeptidase B removes basic amino acids from the C termini of peptides and proteins. 

Carboxypeptidase A (EC 3.4.17.1) is a pancreatic digestive enzyme consisting of a single polypeptide chain of 307 amino acids with a total Mr of 36,000. It catalyzes the cleavage of amino acid residues from C termini of polypeptides. Importantly, for its mechanisms of action,... 

Fig. 8-4 The mechanism of covalent catalysis of the hydrolysis of a C-terminal amino acid residue from a peptide by carboxypeptidase A. The reaction is (a) —> (d), and the bold line structure is the peptide substrate. The C-terminal tyrosine side chain of the substrate shown in (a) is denoted by in (ft), (c), and (d).

Chymotrypsinogen, a single polypeptide chain of 245 amino acid residues, is converted to a-chymotrypsin, which has three polypeptide chains linked by two of the five disulfide bonds present in the primary structure of chymotrypsinogen. tt- and S-chymotrypsin also have proteolytic activity. In contrast, the conversion of procarboxypeptidase to carboxypeptidase involves the hydrolytic removal of a single amino acid. 

Two recently isolated serine proteases have quite different specificities. One is a protease of Staphylococcus aureus which has a high specificity for glutamic acid residues at Pi 16, 17). The other is the yeast carboxypeptidase listed in Table I 11), As the name indicates, it degrades polypeptides by cleaving amino acid residues from the C-terminal end of the chain—a most unexpected specificity for a serine protease. Unlike Carboxypeptidase A, it is stable, is capable of removing proline residues, and would seem to be an ideal enzyme for determining C-terminal sequences. 

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