Many proteins, in order to perform their role in the cell, must be further processed after they are synthesized on the ribosome. These post-translational modifications involve either proteolysis of the polypeptide backbone or chemical derivatization of protein functional groups.
Nearly all bacterial proteins are synthesized with a formyl–methionine as the first amino acid, as described above. Yet most proteins isolated from cell culture do not contain an N-terminal fMet or Met residue, indicating that the maturation process involves proteolytic removal of this amino acid. Some proteolytic events release a functional protein from a synthesized precursor, or proprotein. An example is insulin, which is released from its proinsulin precursor by excision of an internal 33-residue peptide. Another type of proteolytic processing is the removal of N-terminal signal peptides, which target some proteins for insertion into membranes or for secretion from the cell.
Proteins are also covalently modified by specific enzymes that act on side-chain functional groups or on the N- or C-termini. More than 150 types of side-chain modifications are known; these include glycosylations, methylations, and acetylations, among others. For instance, a variety of carbohydrates are added to proteins, primarily at asparagine residues, and serve as recognition markers on cell surfaces.
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