KE7001 Biochemistry Lectures

Protein synthesis and control of gene expression

Reading material: Stryer, Chapters 29 and 31

Abstract: Amino acids are activated and linked to particular tRNA molecules by specific aminoacyl tRNA synthetases. Protein synthesis takes place on ribosomes, which are composed of rRNA (2/3) and many proteins. The ribosome contains three binding sites for tRNA, the A (aminoacyl) P (peptidyl) and E (exit) sites. The aminoacyl tRNA is delivered to the A site of the ribosome by the GTP form of elongation factor Tu. A peptide bond is made to the peptide attached to the tRNA in the P site by peptidyltransferase. This is formed by the 23S rRNA. The peptide then sits at the tRNA in the A site. In the translocation step the tRNA molecules and the mRNA are moved so that the free tRNA at the P site moves to the E site and the tRNA with the peptide in the A site moves to the P site. Then another cycle in the peptide synthesis can take place.

Peptide synthesis is initiated in prokaryotes by the binding of formylmehionyl-tRNA to the P site of the ribosome. In eukaryotes, methionine bound to a special tRNA participates in the initiation. Protein synthesis is terminated by the binding of release factors to stop codons of the mRNA.

Prokaryotic genes are often arranged in operons. The replication of operons are regulated by binding of repressors to specific DNA sequences that are preventing RNA polymerase binding. Activator proteins also participate in gene expression. The situation is much more complex for eukaryotes, where a number of transcription factors regulate gene expression.

Key concepts:
Translation
tRNA
Codon/anticodon
Aminoacyl tRNA synthetases
Ribosome
30S, 50S subunits
rRNA, proteins
A (aminoacyl) P (peptidyl) and E (exit) sites
Elongation factor Tu
Peptidyltransferase
Translocation
Initiation
Termination

Operon
Repressor
Activator
Transcription factors

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