|Content of Protein Synthesis
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.