Secrets of protiens

The major post translational modifications are:
I. Proteolytic Cleavage:
Most proteins undergo proteolytic cleavage following translation. The simplest form of this is the removal of the initiation methionine. Many proteins are syn­thesized as inactive precursors that are activated under proper physiological conditions by limited proteolysis. Inactive precursor proteins that are activated by removal of polypeptides are termed proproteins. Certain proteins particularly of the enzyme class are synthesized as inactive precursors called zymogens. Zymogens are activated by proteolytic cleavage such as is the situation for several proteins of the blood clotting cascade.
The preproprotein insulin secreted from the pancreas has a prepeptide. After cleavage of the 24 amino acid signal peptide the protein folds into proinsulin, which is further cleaved yielding active insulin, composed of two peptide chains linked together through disulfide bonds.
II. Chemical Modification:
The chemical modification mainly includes methylation, sul­fation, phosphorylation, lipid addition, and glycosylation.
(a) Glycosylation:
Many proteins, particularly in eukaryotic cells, are modified by the addi­tion of carbohydrates, a process called glycosylation. Glycosylation in proteins results in addition of a glycosyl group to asparagine, hydroxylysine, serine, or threonine.
(b) Acylation:
Acylation involves of the addition of an acyl group, usually at the N-terminus of the protein. In most cases the initiator methionine is hydrolyzed and an acetyl group is added to the new N-terminal amino acid. Acetyl-CoA is the acetyl donor for these reactions.
(c) Methylation:
The most common methylations are on the s-amine of lysine residues, oc­curs on nitrogen and oxygen. The activated methyl donor is S-adenosylmethionine (SAM). Methylation of the oxygen of the R-group carboxylates of gutamate and aspartate also takes place and forms methyl esters. Proteins can also be methylated on the thiol R- group of cysteine. Methylation of histones in DNA is an important regulator of chromatin structure and consequently of transcriptional activity.