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Protein synthesis

Protein synthesis. How a nucleotide sequence is translated into amino acids. Show unusual bases. Fig. 11-26. AA-tRNA synthetases. Enzymes that allow the correct amino acid to bind to its cognate tRNA Considered the “second genetic code”. Aminoacyl-tRNA. tRNA “charged” with amino acid

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Protein synthesis

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  1. Protein synthesis How a nucleotide sequence is translated into amino acids

  2. Show unusual bases Fig. 11-26

  3. AA-tRNA synthetases • Enzymes that allow the correct amino acid to bind to its cognate tRNA • Considered the “second genetic code”

  4. Aminoacyl-tRNA • tRNA “charged” with amino acid • Carries out codon recognition through anticodon loop • Delivers amino acid to ribosomes • Amino acid has no role in codon recognition

  5. The adapter molecule-tRNA synthetase • 20 amino acids each with its own amino acid tRNA synthetase (aaRS) • Multiple tRNAs for each amino acid use same aaRS • Esterification reaction binds amino acid and cognate tRNA • Synthetase reaction • Activates amino acid for peptide bond formation • Bridges the info gap between amino acids and codons

  6. tRNA synthetases • All synthetases have two major domains • More ancient domain has catalytic site for binding minihelix portion of the tRNA • Divided into two classes • Second domain for interacts with anticodon in most cases • More recent in evolutionary time

  7. Two classes of aa-tRNA synthetases • Class I (2’ esterification) are monomeric • Arg, cys, gln, glu, ile, leu, met, trp, tyr, val • Class II (3’ esterification) are always oligomeric (usually homodimeric) • Ala, asn, asp, gly, his, lys, phe, pro, ser, thr

  8. tRNA recognition • Sequence elements in each tRNA are recognized by its specific synthetase including • 1) One or more of 3 bases in acceptor stem • 2) Base at position 73 “Discriminator base” • (3) In many, at least one anticodon base

  9. Alanyl-tRNAAla synthtase (alaRS) • Single, non-canonical base pair G3:U70 in the acceptor stem defines the recognition • All tRNAAlashave this base pair • If this base pair is altered, recognition does not occur • 24-base microhelix analog is also correctly aminoacylated by alanyl-tRNAAla synthtase

  10. Second genetic code • Sequence and structures of RNA oligos that mimic the acceptor stem and confer specific aminoacylations constitute an operational RNA code for amino acids • Such as code may have predated the genetic code

  11. Codon bias • Some codons are used more than others • In E. coli, occurrence of codons correlates with abundance of tRNAs that read them • Table 30.4

  12. Nonsense suppression • Mutations in gene that alter a sense codon to a stop codon (nonsense) result in premature termination of the protein • Leads to death in bacterial cells • Second mutations in tRNA genes are able to suppress these mutations so the cell can live • Nonsense suppression

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