<P> Post-translational modification (PTM) refers to the covalent and generally enzymatic modification of proteins following protein biosynthesis . Proteins are synthesized by ribosomes translating mRNA into polypeptide chains, which may then undergo PTM to form the mature protein product . PTMs are important components in cell signaling, as for example when prohormones are converted to hormones . </P> <P> Post-translational modifications can occur on the amino acid side chains or at the protein's C - or N - termini . They can extend the chemical repertoire of the 20 standard amino acids by modifying an existing functional group or introducing a new one such as phosphate . Phosphorylation is a very common mechanism for regulating the activity of enzymes and is the most common post-translational modification . Many eukaryotic proteins also have carbohydrate molecules attached to them in a process called glycosylation, which can promote protein folding and improve stability as well as serving regulatory functions . Attachment of lipid molecules, known as lipidation, often targets a protein or part of a protein attached to the cell membrane . </P> <P> Other forms of post-translational modification consist of cleaving peptide bonds, as in processing a propeptide to a mature form or removing the initiator methionine residue . The formation of disulfide bonds from cysteine residues may also be referred to as a post-translational modification . For instance, the peptide hormone insulin is cut twice after disulfide bonds are formed, and a propeptide is removed from the middle of the chain; the resulting protein consists of two polypeptide chains connected by disulfide bonds . </P>

Where do post translational modifications occur in the cell