Protein synthesis

Reading Frame

an RNA sequence can be translated in 1 of 3 of these, depending on where the decoding (synthesis) process begins

aminoacyl-tRNA synthetases

• loads the correct amino acids onto tRNAs

• covalently couples each amino acid to its appropriate set of tRNAs

• uses the hydrolysis of ATP

Redundancy of the genetic code

most amino acids are encoded for by more than one codon

• some amino acids have more than one tRNA

• most tRNAs only require accurate base-pairing at the 1st two positions

• the third position can tolerate a mismatch (wobble) at the third position

Ribosome

• preforms protein synthesis

• complex catalytic machine

• comprised of 1 large and one small subunit that fir together to form a function al ribosome

• the subunits remain separate until they join together on an mRNA to initiate protein synthesis

4 RNA binding sites of Ribosome

• 1 is for mRNA

• 3 is for tRNA

  • E-site binds the exiting tRNA

  • P-site binds the peptidyl tRNA

  • A-site binds to the incoming aminoacyl tRNA

Stages of Protein Synthesis (general 3 steps)

• initiation

  • ribosome binds to mRNA start codon

• elongation

  • polypeptide chain elongates by successfully making amino acids

• termination

  • when a stop codon is encountered, polypeptide is released and ribosome dissociates

Initiation

Binding the initiator tRNA to the small Ribosomal subunit

• the initiator always carries a methionine

• protein synthesis can be modulated by the regulation/availability of the levels of eIF2-GTP

• The small subunit binds to the 5’ end of the mRNA

• small subunit moves along the mRNA searching for the appropriate AUG codon

• additiona; initiation factors that act as ATP powered helicases help the ribsosome negotiate the secondary structure present in the mRNA

• nucleotides surrounding an AUG codon influence whether it will be recognized as the initiation codon

• the prefered context of the AUG is the Kozak sequence ACCAUGG. if the sequence largly differs from this, the small subunit will move to the next AUG

• once the AUG site has been recognized, initiation factors are relaeased and the large subunit associates

• the tRNA is bound at the P site. the A site is vacant and the ribosome is ready for protein synthesis

eIF2-GTP