Topic: Synthesis and modification of proteins
Question: Write a note on post translational modification of protein.
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Post-translational modifications (PTMs) are chemical changes made to proteins after they have been synthesized. They play an important role in regulating protein function and activity. Common types of PTMs include:
1. Phosphorylation: Addition of phosphate groups to serine, threonine or tyrosine residues. Regulates enzyme activity, protein-protein interactions, and signaling pathways. Catalyzed by kinases. Reverse reaction involves phosphatases.
2. Glycosylation: Addition of carbohydrate groups to proteins. Important for protein folding, localization, stability and cell-cell recognition. Two main types:
– N-linked: Carbohydrate attached to asparagine. Occurs in ER and Golgi.
– O-linked: Carbohydrate attached to serine or threonine. Occurs in Golgi.
3. Ubiquitination: Addition of ubiquitin molecules to lysine residues. Targets proteins for degradation by the proteasome. Involves ubiquitin-activating (E1), -conjugating (E2) and -ligating (E3) enzymes. Can be reversed by deubiquitinating enzymes.
4. Acetylation: Addition of acetyl groups to lysine residues. Regulates protein stability, localization and protein-protein interactions. Catalyzed by histone acetyltransferases (HATs). Can be reversed by histone deacetylases (HDACs).
5. Methylation: Addition of methyl groups to lysine or arginine residues. Regulates gene transcription, DNA repair and protein-protein interactions. Catalyzed by methyltransferases. Can be reversed by demethylases.
6. Palmitoylation: Addition of palmitate lipids to cysteine residues. Anchors proteins to cell membranes and regulates protein trafficking and signaling. Added by palmitoyl acyltransferases. Can be reversed by acylprotein thioesterases.
7. Oxidation: Addition of oxygen groups. Can activate or inactivate proteins. Example is the oxidation of cysteine thiols into sulfenic acid or disulfide bonds between cysteine residues. Reversed by antioxidant systems like the thioredoxin system.
In summary, PTMs provide an additional level of control over protein function in cells. By making small chemical attachments at specific sites on proteins, their activity, stability, localization, and interactions can be modulated in response to various stimuli. Defects in PTM enzymes or reactions are linked to many diseases.
