Carboxypeptidase

From Wikipedia, the free encyclopedia

Carboxypeptidase is an enzyme that hydrolyzes the carboxy-terminal (C-terminal) peptide bond of proteins and peptides thus releasing the last amino acid of the chain. Humans, animals, and plants contain several types of carboxypeptidases with diverse functions ranging from catabolism to protein maturation.

The first carboxypeptidases studied were those involved in the digestion of food (pancreatic carboxypeptidases A1, A2, and B). However, most of the known carboxypeptidases are not involved in catabolism; they help to mature proteins or regulate biological processes. For example, the biosynthesis of neuroendocrine peptides such as insulin requires a carboxypeptidase. Carboxypeptidases also function in blood clotting, growth factor production, wound healing, reproduction, and many other processes.

1 Classification
2 Activation
3 Carboxypeptidase E (CPE)
4 Carboxypeptidase A (CPA)
5 References
6 See also
7 External links

[edit] Classification

Carboxypeptidases are usually classified into one of several families based on their active site mechanism. Enzymes that use a metal in the active site are called "metallo-carboxypeptidases". Other carboxypeptidases that use active site serine residues are called "serine carboxypeptidases" and those that use an active site cysteine are called "cysteine carboxypeptidase" (or "thiol carboxypeptidases"). These names do not refer to the selectivity of the amino acid that is cleaved.

Another classification system for carboxypeptidases refers to their substrate preference. In this classification system, carboxypeptidases that have a stronger preference for those amino acids containing aromatic or branched hydrocarbon chains are called A-like enzymes (A for aromatic/aliphatic). Carboxypeptidases that cleave positively charged amino acids are called B-like enzymes (B for basic). A metallo-carboxypeptidase that cleaves a C-terminal glutamate from the peptide N-acetyl-L-aspartyl-L-glutamate is called Glutamate Carboxypeptidase. A serine carboxypeptidase that cleaves the C-terminal residue from peptides containing the sequence -Pro-Xaa (Pro is proline, Xaa is any amino acid on the C-terminus of a peptide) is called prolyl Carboxypeptidase.

[edit] Activation

Some, but not all, carboxypeptidases are initially produced in an inactive form; this precursor form is referred to as a procarboxpeptidase. In the case of pancreatic carboxypeptidase A, the inactive zymogen form, pro-carboxypeptidase A, is converted to its active form - carboxypeptidase A - by the enzyme enteropeptidase. This mechanism ensures that the cells wherein pro-carboxypeptidase A is produced are not themselves digested.

[edit] Carboxypeptidase E (CPE)

(also called: carboxypeptidase H, enkephalin convertase)

Carboxypeptidase E is found in neuroendocrine cells and in adrenal gland chromaffin cells. The glycoprotein can be both membrane-associated or soluble. At the C-end of the molecule lies an amphiphilic α-helix which might be responsible for the membrane localisation. Carboxypeptidase E is found in all species of vertebrates that have been examined, and is also present in many other organisms that have been studied (nematode, sea slug). Interesting, carboxypeptidase E is not found in the fruit fly, and another enzyme (presumably carboxypeptidase D) fills in for carboxypeptidase E in this organism.

Carboxypeptidase E appears to have several functions. The active form of carboxypeptidase E was shown to be in secretory vesicles, where it acts as an exopeptidase to activate neuropeptides. It does that by cleaving off basic C-terminal amino acids, producing the active form of the peptide. Products of carboxypeptidase E include insulin, enkephalin, and most other neuroendocrine peptides.

It has also been proposed that membrane-associated carboxypeptidase E acts as a sorting signal for regulated secretory proteins in the trans-Golgi network of the pituitary and in secretory granules; regulated secretory proteins are mostly hormones and neuropeptides. However, this role for carboxypeptidase E remains controversial, and some evidence shows that this enzyme is not necessary for the sorting of regulated secretory proteins.

Mice with mutant carboxypeptidase E, Cpe^fat, display endocrine disorders like obesity and infertility. In some strains of mice, the fat mutation also causes hyperproinsulinemia in adult male mice, but this is not found in all strains of mice. The obesity and infertility in the Cpe^fat mice develop with age; young mice (<8 weeks of age) are fertile and have normal body weight. Peptide processing in Cpe^fat mice is impaired, with a large accumulation of peptides with C-terminal lysine and/or arginine extensions. Levels of the mature forms of peptides are generally reduced in these mice, but not completely eliminated. It is thought that a related enzyme (carboxypeptidase D) also contributes to neuropeptide processing and gives rise to the mature peptides in the Cpe^fat mice.

[edit] Carboxypeptidase A (CPA)

Carboxypeptidase A usually refers to the pancreatic exopeptidase which hydrolyzes peptide bonds of C-terminal residues with aromitic or aliphatic side chains. Most scientists in the field now refer to this enzyme as CPA-1, and to a related pancreatic carboxypeptidase as CPA-2. In addition, there are 4 other mammalian enzymes named CPA-3 through CPA-6, and none of these are expressed in the pancreas. Instead, these other CPA-like enzymes have diverse functions. CPA-3 (also known as mast-cell CPA) is involved in the digestion of proteins by mast cells. CPA-4 (previously known as CPA-3, but renumbered when mast-cell CPA was designated CPA-3) may be involved in tumor progression, but this enzyme has not been well studied. CPA-5 and CPA-6 have also not been well studied. Interestingly, a human mutation of CPA-6 is known to cause Duane's syndrome (abnormal eye movement). This, together with the localization of CPA-6 to embryonic eye muscle (in addition to other limited locations) suggests a role for CPA-6 in the control of neuronal migration/axonal guidance.

CPA-1 and CPA-2 (and presumably all other CPAs) contain a zinc atom at the active site. Loss of the zinc leads to loss of activity, which can be replaced easily by zinc, and also by some other divalent metals (cobalt, nickel).