Question

Discuss the functions of molecular chaperones and compare Hsp70 family chaperones with chaperonins (i.e. Hsp60-family chaperones)

Describe the various types of posttranslational modifications (note: protein splicing will not be covered)

Compare cotranslational and posttranslational import, discussing where translation occurs in each case

Answer 1

Molecular chaperones we're first identified in bacteria E.coli but are present in both prokaryotes and eukaryotes. There are two type of chaperones 1- Hsp60 and 2-Hsp70 families. Function of molecular chaperones:

1: molecular chaperones mainly by preventing formation of incorrect structure rather than by promoting formation of correct sequence.

2: molecular chaperones also required to assist the refolding of stress denatured protein.

3: formation of oligomeric structure.

4: protein trafficking through membranes and assistance in protieolytic degradation.

Comparison between Hsp70 and Hsp60:

Hsp70: The members of Hsp70 ( Hsp70, Hsp40,Hsc70 and GrpE) act early in the life of many protein, binding to a string of about seven hydrophobic amino acids before the protien leaves the ribosome. Hsp70 polypeptide chain is divided into two functional regions , one that binds and hydrolyses ATP and second that binds hydrophobic segments of unfold polypeptide chain. Hsp70 is induced by stress (heat shock) whereas Hsc70 is contitutively expressed in cells. The ATP dependent reaction cycle of Hsp70 is regulated by the Hsp40.

Hsp 60: it is also called chapronins forms a large barrel shaped structure that acts later in a protien's life. Chapronins bind unfold, partly folded and acts incorrectly folded protein molecule but not protein in their native state. this type of chapronins forms an isolation chamber into which misfold protein are fed. They are form ring structure,forming a cylinder. The structure consisting 14 subunits that are arrenge in two heptameric rings stacked on top of each other in an inverted orientation.

Types of posttranslational modifications:

1:Acetylation - The transfer of acetyl group to nitrogen,ooccurs in all eukariyotic protein through both irreversible and reversible mecanism. Acetylation reduced the affinity of histone for DANA and possibly also reduces the interaction between the individual nucleosome that leads to formation of 30nm chromatin fibers.

2 Methylation: The trans of one carbon methyl group to nirtogen or oxygen to amino acid. Many lysine and arginine amino acids at N terminus of histone undergo metylation mediated by methyltransferase and sadenosyl methonine.

3 phosphorylation: phosphorylation residue modiefied on serine, theonine. Phosphorylation play role in transcription, repair, condentation, apoptosis, cell cycle.

4 Glycosylation: Glycosylation play important role in posttranslational modifications with significant effect on protien folding, distribution stablity. Carbohydrates in the form of aspargine linked or serine linked oligoschharide are major components of cell surface.

5 Nitrosylation : Nitric oxide and its chemical messenger reactreacts with free cystine residue. It is reversible reaction. Its play important role in the caspase activation and the induction of apoptosis.

6 Ubiqutination: ubiquitin is small protein present in cell tissues in eukariyotes. It is in an enzymatic process that involves binding in ubiquitin for ddegradation

comparison between co translation and posttranslation modifications

1: cotrancotranslation occur in endoplasmic reticulam

2: the nacent protein associated with translocation while it still being synthesize on ribosome.

3: cotranslation protien are molecular chaperone

Posttranslational:

1: it occurs in mitochondria , plasma membranes, peroxisome

2: The protein realesed from ribosome then translation has been completed . The completed protein tranlocate with appropriate membranes.

3. The post translation modifications included acetylation , Methylation , Glycosylation, Ubiqutination, nirtosylation ,lipidation, phosphorylation