Nothing in nature is permanent. Just as cells and organisms eventually die, proteins stop functioning. In most cases, a cell outlives its proteins. So, when the proteins stop functioning, the cell can
recycle many of the amino acids in the polypeptide(s) making up the protein. Proteins stop functioning often by folding into one or more stable, but useless shapes. An important method for recycling misfolded or unused proteins, called protein degradation, reduces
proteins to individual amino acids for use in building new proteins Cells use three different mechanisms to degrade proteins confined to a specific compartment of the cell. Proteosomes degr proteins in the cytoplasm and in the nucleus. These proteins are tagged for destruction by the covalent attachment of many copies of a small protein called ubiquitin. Lysosomes degrade proteins in the endomembrane system, including those captured by endocytosis Metalloproteinases degrade proteins in the extracellular space allowing remodelling of this space in multicellular organisms.
