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PROTEIN SYNTHESIS ! Translation of nucleic acid information RNA components of protein synthesis ! Messenger RNA (mRNA): gene-specific information ! Transfer RNA (tRNA): codon decoders ! Ribosomal RNA (rRNA): peptide bond synthesis Transfer RNA (tRNA) ! Transcribed from special tRNA genes (about 40; one for each codon, one or more for each amino acid). ! Matches codons with amino acids (called “adaptor” or “translator” molecule). ! Generally a cloverleaf with secondary and tertiary structure; has “anticodon” at one end (3 bases complementary to codon), corresponding amino acid hooked onto the other end. Three bases make a “codon.” (Degenerate, but not ambiguous) Charging the transfer RNAs ! Activating enzyme; aa-tRNA synthetase ! Joins amino acids to tRNAs with the correct anticodons ! Provides some of the energy to form the peptide bond How many activating enzymes would you expect a cell to have? Ribosome structure Each ribosome consists of a large and small subunit Each subunit consists of 1 or 2 RNA molecules and 35-45 proteins Ribosome structure Each ribosome consists of a large and small subunit Ribosome structure On an exam, you will probably See the ribosome with only two binding sites--the A and P sites Initiation Translation begins with the formation of an initiation complex Elongation The growing polypeptide chain is added to each new amino acid; then the ribosome moves one codon forward. Two steps skipped here Crystal structures of the 70S ribosome with translation elongation factors EF-Tu and EF-G. During protein synthesis, EF-Tu (periwinkle blue) delivers an aminoacyl tRNA (green) to the ribosome for each amino acid indicated by the mRNA. EF-G helps move the mRNA and tRNAs through the ribosome. What enzyme forms the peptide bond between the two amino acids? The RNA of the large subunit is the catalyst! Termination When the ribosome reaches a stop codon, a release factor disconnects the complex. Polysomes Simultaneous protein synthesis by several ribosomes using one mRNA Many proteins are modified after translation Common modifications: •Proteolysis: ER import, enzyme activation •Glycosylation: of secreted proteins •Phosphorylation: enzyme activation or inactivation Compartmentation Summary •mRNA provides the information (code) for a protein •tRNA translates the code, codon by codon, amino acid by amino acid •rRNA (in ribosomes) holds the mRNA and tRNAs together and forms the peptide bonds •Post-translational processing may be needed to produce a finished protein •Where do the proteins go? Compartmentation is an another--and important--issue, especially in eukaryotes