Cleavage of Precursor Proteins
Many proteins and hormones are synthesized in the form of their precursors - zymogens, proenzymes and prehormones. These proteins are cleaved to form their final active structures. Insulin, for example, is synthesized as preproinsulin which yields proinsulin after the signal peptide has been cleaved. To form the mature insulin, the proinsulin is then cleaved at two positions to yield two polypeptide chains linked by 2 disulphide bonds. Proinsulin is necessary for the folding of the polypeptide chain as the 2 polypeptide chains of insulin may not correctly assemble into the correct form while its precursor proinsulin do.
Proteases in particular are synthesized in the inactive form so that they may be safely stored in cells, and ready for released in sufficient quantity when required. This is to ensure that the protease is only activated in the correct location or context as inappropriate activation of these proteases can be very destructive for an organism. Proteolysis of the zymogen yields an active protein; for example, when trypsinogen is cleaved to form trypsin, a slight rearrangement of the protein structure occurs which completes the active site of the protease, thereby activating the protein.
Proteolysis can therefore be a method of regulating biological processes by turning inactive proteins into active ones. A good example is the blood clotting cascade whereby an initial event triggers a cascade of sequential proteolytic activation of many specific proteases, resulting in blood coagulation. The complement system of the immune response also involves a complex sequential proteolytic activation and interaction that result in an attack on invading pathogens.
Famous quotes containing the words proteins and/or cleavage:
“Civilization means food and literature all round. Beefsteaks and fiction magazines for all. First-class proteins for the body, fourth-class love-stories for the spirit.”
—Aldous Huxley (18941963)
“Jesus abolished the very concept of guiltMhe denied any cleavage between God and man. He lived this unity of God and man as his glad tidings ... and not as a prerogative!”
—Friedrich Nietzsche (18441900)