Lecture Connections 27 | Protein Metabolism © 2009 W H Freeman and Company CHAPTER 27 Protein Metabolism Key topics: – The genetic code – Synthesis of aminoacyl-tRNA – Translation: RNA-dependent protein synthesis Protein Synthesis is a Very Complex Process • Occurs in a large macromolecular assembly called the ribosome, which is made of many proteins and several RNA molecules into a functional ribosome • Uses a large number of tRNAs that interact with mRNA • Each tRNA is charged with one of the 20 common amino acids by 20 different aminoacyl tRNA synthetases • Requires a dozen or more additional protein factors for initiation, elongation, and termination of translation • Involves perhaps 100 additional enzymes that carry out a variety of modifications with many proteins Protein Synthesis Occurs on Ribosomes • In eukaryotes, most ribosomes are attached to the cytosolic face of endoplasmic reticulum Cricks’ Adaptor Hypothesis • It was recognized in early 1950s that protein sequence is encoded by the nucleotide sequence • The molecular nature of players in protein synthesis remained unknown • Crick proposed around 1954 the adaptor hypothesis – template is RNA (maybe rRNA) – the amino acid is carried to the template by an adaptor – adaptors may contain nucleotides Early History of Protein Synthesis Research • 1955: Paul Zamenick’s group identifies ribosome as the site of protein synthesis • 1956: Volkin and Astrachan find RNA that is like DNA but not realize the importance of this finding • 1958: Zamenick and Hoagland find that amino acids became attached to what is now known as tRNA • 1960: Sydney Brenner realizes at Good Friday meeting with Crick and Francois Jacob and that the DNA-like RNA of Volkin and Astrachan is the messenger RNA Several Protein Factors are Involved in Initiation First Elongation Step • Aminoacyl tRNA binds to the elongation factor Tu that also carries GTP • The EF-Tu-GTP complex with second amino acid on its tRNA binds to the aminoacyl site • After GTP hydrolysis EF-Tu-GDP leaves the ribosome Formation of the Peptide Bond • The 23S RNA ribozyme is the catalyst Termination of Protein Synthesis • When the stop codon (UAG) reaches ribosome a release factor binds to the A site • The linkage between nascent polypeptide and tRNA in the P site is cleaved • Protein, mRNA, and deacylatd tRNA dissociate from ribosome Coupling of Transcription and Translation in Bacteria Chapter 27: Summary In this chapter, we learned that: • The primary genetic code for protein synthesis is in triplets of nucleotides in mRNA that recognize the anticodon of tRNA • The secondary genetic code for protein synthesis is in recognition features between tRNA and the enzyme that attaches the correct amino acids to this tRNA • The mRNA binds to the ribosome and its codons are exposed to the aminoacyl-tRNA binding site • The protein synthesis from charged aminoacyl tRNA substrates is catalyzed by a ribozyme in the ribosome ...CHAPTER 27 Protein Metabolism Key topics: – The genetic code – Synthesis of aminoacyl-tRNA – Translation: RNA-dependent protein synthesis Protein Synthesis is a Very Complex... additional protein factors for initiation, elongation, and termination of translation • Involves perhaps 100 additional enzymes that carry out a variety of modifications with many proteins Protein. .. Hypothesis • It was recognized in early 1950s that protein sequence is encoded by the nucleotide sequence • The molecular nature of players in protein synthesis remained unknown • Crick proposed