This article throws light upon the six different modes of post-transcriptional modifications in proteins.
Polypeptide chains like RNA transcripts are also modified after their synthesis. This additional processing is termed as post transcriptional modification.
These types of post translational modifications are important in achieving the functional status specific to any given protein. Because the final 3D structure of the molecule is closely elated to its specific function, the folding of protein is also important.
These complex biochemical processes are briefly described here for an understanding about overall process:
1. The N-terminus and C-terminus amino acids are usually removed or modified. The initial and terminal formylmethionine residue in bacterial polypeptide is usually removed enzymatically. In eukaryotes, initial methionine residue is removed and the amino group of the N-terminal residue is chemically modified.
2. Individual amino acid residues are sometimes modified, e.g., phosphate may be added to the hydroxyl groups of certain amino acids such as tyrosine. The process of phosphorylation is extremely important in regulating several cellular activities and is a result of the action of enzymes called kinases. In other proteins, methyl group may be added enzymatically.
3. In some proteins, carbohydrate side chains are sometimes added. Covalently added carbohydrates form a class of molecules called glycoproteins having antigenic properties.
4. Sometimes polypeptide chains are trimmed to make active protein molecules, e.g., insulin is produced as a large molecule and then trimmed to 51 amino acid molecules.
5. At the end terminal end of some proteins, a sequence of up to 30 amino acids is found that plays an important role in directing the protein to the location in the cell where it becomes functional. This is called a signal sequence and it determines the final destination of protein in the cell.
6. Polypeptide chains often complexes with metals. The quaternary and tertiary levels of protein structure often include and are dependent on metal atoms. The function of the protein is thus dependent on the molecular complex that includes both polypeptide chains and metal atoms, e.g., haemoglobin containing four iron atoms.