Trimming Many proteins destined for secretion from the cell are initially made as large, precursor molecules that are not functionally active. Portions of the protein chain must be removed by specialized endoproteases, resulting in the release of an active molecule.

The cellular site of the cleavage reaction depends on the protein to be modified. For example, some precursor proteins are cleaved in the ER or the golgi complex. Zymogens are inactive precursors of secreted enzymes. They become activated through cleavage once they have reached their proper site of action.

Covalent alterations Proteins, both enzymatic and structural, may be activated or inactivated by covalent attachment of a variety of chemical groups.

Hydroxylation Proline and lysine residues of the X -chain of collagen are extensively hydroxylated in the ER. In this process an OR group is added to the amino acid. This hydroxylation reaction requires molecular oxygen and a reducing agent such as vitamin C.

Sulphation This involves addition of sulphur group to tyrosine residue.

Myristylation This involves addition of acetyl group to the N terminal of protein or asparagine.