Post-translational Control

Control of protein function by post-translational modification may also occur by altering the order of the amino acids in the protein backbone. These modifications may be promoted by other proteins or they may be self-directed.

Three mechanisms of post-translational control.

Certain proteins are synthesized as larger precursor proteins and are activated by cleavage of their peptide backbone by proteases. Many of these large protein precursors, called zymogens or proproteins, are synthesized with an N-terminal signal sequence that instructs the cell to export the protein. (Proteins have an amino group at one end and a carboxyl group at the other. The amino-group end is called the N-terminus, the carboxyl end is called the C-terminus.) The N-terminal signal sequences are then cleaved, but the exported protein may still be inactive until cleaved again by another protease. Proteins activated by this mechanism include digestive proteases such as trypsin, the activity of which must be controlled before export by the cell. Serum albumin is also processed in this manner, as are the peptide hormones insulin, vasopressin, and oxytocin. Post-translational cleavage is also responsible for controlling the process of blood clotting.