After forming the
polypeptide, to become biologically active, it has to undergo a folding process
and have to attain its 3d structure.
This folding process
occurs with the help of a group of specialized proteins called Chaperons.
DEFINITION :
Before or After the
folding process, the polypeptide undergoes through an enzymatic process in
which there may be the removal of a chemical group or addition of a chemical
group, which will affect the protein activity.
These enzymatic
alterations collectively are called as POST TRANSLATIONAL
MODIFICATIONS.
TYPES OF POST TRANSLATIONAL
MODIFICATIONS
1.AMINO
TERMINAL MODIFICATION: The formyl group and amino methionine group
present at the beginning of the polypeptide are removed enzymatically
2. LOSS
OF SIGNAL SEQUENCE:
a group of 15-30 amino acid residues are required to direct the protein to its
destination in the cell called signal sequences.
Specific peptidases remove such sequences.
3.COVALENT
MODIFICATION OF PROTEINS: It can occur by any processes like glycosylation,
phosphorylation, carboxylation, hydroxylation, methylation, and addition of
prosthetic groups.
i)Glycosylation: attachment of
glycoproteins or proteoglycans to the polypeptide
ii)Phosphorylation: covalent addition of phosphate
group to the hydroxyl group of serine, threonine, and tyrosine
iii)Carboxylation: covalent addition of carboxyl group to the
glutamic acid residue of prothrombin
iv)Hydroxylation: covalent addition of
hydroxyl group to proline and lysine to hydroxyproline & hydroxy lysine
during collagen formation.
v)Methylation: covalent addition of
methyl group to lysine like in cytochrome c and muscle proteins
vi)Addition of Prosthetic group: attachment of non-protein substance to protein
through a covalent bond.
E.g., attachment of biotin to a Heme group of
cytochrome.
4.PROTEOLYTIC
PROCESSING:
Trimming of the precursors so that the protein becomes biologically active.
E.g., the trimming of the large inactive precursors
in insulin makes it biologically active.
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