USMLE step 1 biochemistry

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Mahshid Mir, M.D. [2] Ribosomes are made of proteins and rRNA

  • Eukaryotes → 60 and 40s = 80s
  • Prokaryotes → 50 s and 30 s = 70 s
    • Have 23s in 50s

Translation

  • Initiation
    • IF1, IF2, If3
      • Assist in assembly of smaller ribosomal subunit to first trna molecule
      • Methionine is always the start
      • f-Methionine in prokaryotes
      • IF-2 first binds to 30s and then to methionine tRNA. Then when 50s comes along, it hydrolyzes GTP on IF2 and allows 50s to attach to 30s
    • A site → Incoming aminoacyl TRNA binds
    • P site → Polypeptide binds (Growing chain)
      • First tRNA binds here
    • E site → Free tRNA (exit)
  • Elongation
    • Incoming charged aminoacyl TRNA binds to A site
    • Elongation factor help incoming trna to bind to A site (Uses GTP)
    • 50s has peptidyl transferase transfers AA from p site to A site
      • In prokaryotes, activity is in 23s subunit of 50s rRNA
    • Translocation
      • Ribosome complex moves 3 nucleotides
      • tRNA + Peptide is moved from A site to P site
      • Newly uncharged tRNA from P side to E side
      • EF-G  → Eukaryotes
      • EF-2 in eukaryotes
        • Diptheria and exotoxin (Pseudomonas) inhibit this
  • Termination
    • Stop codons
      • UGA, UAA and UAG
      • Signal to STOP
      • No new TRNA coming
      • Release factor binds to MRNA and hydrolyzes GTP and new polypeptide is released.
  • Antibiotics
    • Aminoglycosides → Inhibit 30s subunit before initiation (No pairing with TRNA)
    • Linezolid → Inhibits initiation by binding to 50s subunit
    • Tetracycline
      • Bind to 30s subunit
      • Prevent aminoacyl TRNA from binding to A site
    • Chloramphenicol
      • Inhibits peptidyl transferase
      • Binds to 50s
    • Macrolides
      • Inhibits translocation by binding to 50s subunit
    • Clindamycin
      • Same as macrolides
    • Lincamycin
      • Bind to 50s and inhibit translocation
    • Streptrogranin
      • Bind to 50s and inhibit translocation
    • Buy AT 30 ,CCELL at 50
  • Post translational modification
    • Trim terminals
    • Covalent modification
      • Glycosylation, hydroxylation, phosphorylation
      • Collagen → Hydroxylation of proline and lysine
    • Disulfide bonds
    • Protein folding
      • Primary structure → Chain
      • Secondary structure → Beta and alpha pleated
      • Tertiary structure