co and post translation modification, by

1-Introduction
2-Protein modifications
[A] Folding
1-Chaperon mediated
2-Enzymatic
[B] Cleavage
[C] Glycosylation
[D] Attachment of Lipids
[E] Proteolysis
3-Conclusion
4-Reference

1. Co & post-translation modification By KAUSHAL KUMAR SAHU Assistant Professor (Ad Hoc) Department of Biotechnology Govt. Digvijay Autonomous P. G. College Raj-Nandgaon ( C. G. )

2. Synopsis 1-Introduction 2-Protein modifications [A] Folding 1-Chaperon mediated 2-Enzymatic [B] Cleavage [C] Glycosylation [D] Attachment of Lipids [E] Proteolysis 3-Conclusion 4-Reference

3. Introduction Protein modification can occur at any step in the "life cycle" of a protein. For example, many proteins are modified shortly after translation is completed to mediate proper protein folding or stability or to direct the nascent protein to distinct cellular compartments.

4. Protein modifications Translation completes the flow of genetic information within the cell. The sequence of nucleotides in DNA has now been converted to the sequence of amino acids in a polypeptide chain. The synthesis of a polypeptide, however, is not equivalent to the production of a functional protein. To be useful, polypeptides must fold into distinct three-dimensional conformations, and in many cases multiple polypeptide chains must assemble into a functional complex.

5. Chaperones and Protein Folding   The proper folding of proteins within cells is mediated by the activities of other proteins. Proteins that facilitate the folding of other proteins are called molecular chaperones. The term "chaperone" was first used by Ron Lackey and his colleagues. Chaperone binding stabilizes the amino-terminal portion in an unfolded conformation until the rest of the polypeptide chain is synthesized and the completed protein can fold correctly.

6. Chaperones Family Size Location Hsp 60 ~ 1 MDa Mitochondria, Chloroplast Hsp 70 ~ 70 kDa Cytoplasm, ER , Mitochondria, Chloroplast Hsp 90 ~ 90 kDa ER, cytosol , mitochondria Hsp 100 ~ 100 kDa Mitochondria, ER chloroplast

7. Exa - Mitochondria

8. [b]Enzymes that Catalyze Protein Folding Cells contain at least two types of enzymes that act as chaperon. 1 Peptidyl prolyl isomerase [PPI] 2 Protein Disulphide Isomerases (PDI) - Protein disulfide isomerase  or PDI is an enzyme in the ER that catalyzes the formation and breakage of disulfide bonds between  cysteine  residues within proteins as they fold. 

9. [B]Protein Cleavage Cleavage of the polypeptide chain (proteolysis) is an important step in the maturation of many proteins. (1)Removal of the signal peptide

10. Proteolytic trimming Many proteins (insulin, collagen) are initially synthesized as larger inactive precursor proteins which are proteolytically trimmed to produce their active final forms. This process is also called as protein splicing.

11. (a) Glycosylation   Many proteins, particularly in eukaryotic cells, are modified by the addition of carbohydrates, a process called glycosylation . The proteins to which carbohydrate chains have been added (called glycoproteins) are usually secreted or localized to the cell surface, although many nuclear and cytosolicproteins are also glycosylated.  

12. (b)Attachment of Lipids   Some proteins in eukaryotic cells are modified by the attachment of lipids to the polypeptide chain. Three general types of lipid additions N- myristoylation , prenylation , and palmitoylation are common in eukaryotic proteins associated with the cytosolic face of the plasma membrane.   A fourth type of modification GPI anchored , the addition of glycolipids , plays an important role in anchoring some cell surface proteins to the extracellular face of the plasma membrane.  

13. Fig- Glycosylphosphatidylinositol (GPI)

14. Protein degradation Levels of protein within cells are determined not only by rates of synthesis but also by rates of degradation. In eukaryotic cells, two major pathways—the ubiquitin-proteasome  pathway and lysosomal  proteolysis—mediate protein degradation.

15. A] The Ubiquitin-Proteasome Pathway Ubiquitin is a 76-amino-acid polypeptide that is highly conserved in all eukaryotes (yeasts, animals, and plants). Proteins are marked for degradation by the attachment of  ubiquitin  to the amino group of the side chain of a lysine residue.

16. Conclusion Protein modifications play a key role in functional proteomics, because they regulate activity, localization and interaction with other cellular molecules such as proteins, nucleic acids, lipids, and cofactors.

17. Cell & Molecular Biology 5 th edition Gerald Karp The Cell A Molecular Approach 4 th edition Geoffrey M Cooper Internet sources- in wikipedia.org