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A carboxypeptidase is a protein that is involved in the digestion of proteins from foods. There are a number of carboxypeptidases found in the body, with differing roles and preferences for substrates. They have numerous other roles in cellular metabolism, including the maturation of hormones.
First, to define a carboxypeptidase, it is necessary to define a protein. These kinds of molecules are made up of long chains of amino acids, with each pair of amino acids being held together by a peptide bond. The order of the amino acids varies in different proteins.
Each amino acid has an N-terminal group in the beginning and a C-terminal group at the end. The peptide bonds that connect the amino acids join the C-terminal group of one amino acid to the N-terminal group of the next. Enzymes called proteases cleave peptide bonds by adding a molecule of water to the bond, causing a break in the chain of amino acids. This results in large fragments of proteins known as polypeptides, or shorter fragments known as peptides.
There are proteases that cleave in the middle of the chain known as endopeptidases. They leave large protein fragments that are digested by exopeptidases. An example of an exopeptidase is a carboxypeptidase that cleaves the final amino acid from the C-terminal, or carboxy group, of the peptide or polypeptide. Multiple carboxypeptidases can act in concert to totally degrade a protein or polypeptide.
The first carboxypeptidases to be studied were those involved in the digestion of proteins from food. These proteins need to be broken down into amino acids, or very small peptides, so the body can use them to construct the proteins it needs for growth and metabolism. Carboxypeptidase A and B are synthesized in the pancreas as inactive, larger forms, then transported to the small intestine and activated by the cleavage of another protease. This prevents them from damaging the tissue in which they are produced.
These two enzymes differ in the structures they will cleave. Each prefers different amino acids at the C-terminal group of the peptide. The final products of the enzymes are free amino acids, and small peptides of two or three amino acids. These products are absorbed by the epithelial cells of the small intestines.
Another carboxypeptidase of great significance in mammalian physiology is carboxypeptidase E, also known as carboxypeptidase H. This enzyme activates peptide hormones, such as insulin and neurotransmitters, that are peptides. It does this by cleaving off C-terminal amino acids that are basic. Other carboxypeptidases have functions that range from wound healing to blood clotting.
Carboxypeptidases can have medical uses. The toxic compound methotrexate is sometimes used to treat people with cancer. When people suffer toxic effects from the disease, such as kidney failure, carboxypeptidase G2 is used to treat the toxicity. It degrades the methotrexate to an inactive form.