A protease is a member of a very large group of enzymes that have a variety of functions in the body. A primary one is as a digestive enzyme to process protein. Without protease, the body would not be able to digest the protein in food. Other types of proteases are involved in the regulation of cellular events, such as blood clotting. These are also called proteolytic enzymes or proteinases.
Proteins are long chains of amino acids that are held together by peptide bonds. Small fragments of proteins are known as peptides, and larger fragments are referred to as polypeptides. Enzymes that break down peptides are called peptidases.
Proteases are types of proteins that accelerate the degradation of others. They differ in the manner in which they carry out this activity. Exopeptidases cleave off terminal amino acids and nibble away at proteins. They break down peptide bonds to release amino acids. In contrast, endopeptidases act within the protein, and also cleave peptide bonds, producing polypeptides as the result of their activities.
There are several classes of proteases, depending on the type of amino acid at the site where the reaction occurs, and any additional molecule needed for activity. For instance, many proteins require a metal atom to be active. They are known as metalloproteinases. Other proteases have an amino acid known as serine at their active site, and are known as serine proteases.
The initial studies of proteases, in human physiology, were done to discern their role in digestion in the gastrointestinal system. The goal of enzymatic digestion is to break larger molecules into smaller ones. Several proteases work in concert with peptidases to degrade the proteins in foods to small peptides and amino acids. Such small molecules can be absorbed by the intestinal cells and used as fuel or to build new protein molecules.
One thing all of these digestive proteases have in common is that they are synthesized as larger, inactive forms to prevent the tissue that contains them from enzymatic damage. Such precursors are known as zymogens. Another feature they share is that they are all endopeptidases, although they vary in their preference for which part of proteins they will cleave. This substrate specificity is based on the location of specific amino acids in the target proteins.
The stomach contains the digestive protease pepsin, which is stimulated by the stomach’s hydrochloric acid. Pepsin breaks the proteins into polypeptides, which travel to the intestine. In this location, they are broken into even smaller pieces by the additional digestive proteases trypsin and chymotrypsin. All of these enzymes are serine proteases.
Other types of protease act to regulate the activity of other proteins. By cleaving a specific site on a protein, they can either turn them on or off. This can be part of a mechanism for signaling a physiological change. Another function of proteases is to help in the processing of proteins that are produced in larger forms, such as the amyloid precursor protein. Other proteases degrade proteins that are no longer needed for cellular function.