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18. Angive at proteiner sythetiseres i retningen N mod C og at de kan modificeres og udskilles posttranskriptionelt
 Devlin, s.255-271
Color atlas of biochemistry, s. 210

 

Proteins are synthesized from their NH2 –terminal towards their COOH terminal. The initiation amino acid, methionine is bound with its –COOH-terminal to the OH-group of the acceptor stem’s ribose (on addenine) by an ester bond.  This ester bond is broken by the large subunits peptidyltransferase activity and replaced with a peptide bond with the following amino acid encoded by the mRNA.

So, the first amino acid in a polypeptide chain has a free –NH2 terminal and its –COOH terminal makes a peptide bond with the following amino acid.

 

The last amino acid in the polypeptide chain must therefore have a free COOH-terminal, created by the hydrolysis of the ester bond between the amino acid and the tRNA. This reaction is catalysed by a termination factor that mimics the adjacent tRNA and provides the water molecule.  

Some proteins emerge from the ribosome ready to function, while others undergo a variety of posttranscriptional modifications:

 

Generally, proteins that are going to stay in the same cell are synthesized in the free ribosomes in the cytoplasm, while proteins that are destined to leave the cell are synthesized in the rER. An N-terminal seqeunce of 16-20 a.a. determines which proteins are synthesized in rER and which in the cytoplasm. This part of the protein is called signal-peptide.

Information that determines the posttranscriptional fate of the protein resides in its structure. The amino acid sequence and confirmation of the polypeptide determine whether a protein will be a substrate for a modifying enzyme and/or identify it for direction to a subcellular or extracellular location.

 

Th first step in the posttranscriptional modification is the removal of the signal peptide by signal peptidease.

N-glycosylation - transfer of an oligosaccharide from a long-chain isoprenoid molecule called dolichol to a specific asparagine residue.

Proteins for export follow the secretory pathway, where they undergo hydrolysis, glycosylation and proteolysis while they are passing through rER, Golgi-apparatus towards the secretory vesicles.

Amino acids can also be modified posttranscriptionally. This leads to more then 100 different amino acid derivates. The amino acids can be methylated, phosphorylated, formylated, adenylated, oxidised etc.

 

Several examples:

Proteins can become phosphorylated, and glycosidases or glycosyltransferases may either remove sugars from protein-bound oligosaccharides or attach further sugars to them.

Finally, peptide fragments may be removed from proteins in the secretory vesicles prior to the release of their contents by exocytosis. These sequence specific, protease-catalysed cleaveage processes often serve to convert protein to its active form.

Chaperone proteins help proteins attain their correct confirmation, maintain their unfolded structure so they can pass through membranes, help unfold misfolded proteins, prevent formation of incorrect intermediates and prevent inappropriate interactions with other proteins.

 

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