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Origins of Sugars in the Prebiotic World

Origins of Sugars in the Prebiotic World. One theory: the formose reaction (discovered by Butterow in 1861) Mechanism?. Con’t. Today, similar reactions are catalyzed by thiazolium, e.g., Vitamin B 1 (TPP), another cofactor: Cf Exp. 7: Benzoin condensation e.g.

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Origins of Sugars in the Prebiotic World

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  1. Origins of Sugars in the Prebiotic World • One theory: the formose reaction (discovered by Butterow in 1861) Mechanism?

  2. Con’t

  3. Today, similar reactions are catalyzed by thiazolium, e.g., Vitamin B1 (TPP), another cofactor: • Cf Exp. 7: Benzoin condensation • e.g. Mechanism? Uses thiazolium

  4. We have seen how the intermediacy of the resonance-stablized oxonium ion accounts for facile substitution at the anomeric centre of a sugar • What about nitrogen nucleophiles? Many examples: Could this process have occurred in the prebiotic world?

  5. Reaction of an oxonium ion with a nitrogenous base: NUCLEOSIDES! • Nucleosides are quite stable: • Weaker anomeric effect: N< O < Cl (low electronegativity of N) • N lone pair in aromatic ring  hard to protonate

  6. 1) Anomeric effect: Cl > O > N (remember the glycosyl chloride prefers Cl axial 2)

  7. These effects stabilize the nucleoside making its formation possible in the pre-biotic soup • Thermodynamics are reasonably balanced • However, the reaction is reversible • e.g. deamination of DNA occurs ~ 10,000x/day/cell in vivo • Deamination is due to spontaneous hydrolysis & by damage of DNA by environmental factors • Principle of microscopic reversibility: spontaneous reaction occurs via the oxonium ion

  8. Ribonucleosides & Deoxyribonucleosides Ribonucleosides • Contain ribose & found in RNA: Deoxyribonucleosides • Contain 2-deoxyribose, found in DNA

  9. Ribonucleosides Deoxyribonucleosides

  10. Important things to Note: • Numbering system: • The base is numbered first (1,2, etc), then the sugar (1’, 2’, etc) • Thymine (5-methyl uracil) replaces uracil in DNA • Confusing letter codes: • A represents adenine, the base • A also represents adenosine, the nucleoside • A also represents deoxyadenosine (i.e., in DNA sequencing, where “d” is often omitted) • A can also represent alanine, the amino acid

  11. Nucleoside + phoshphate  nucleotide • In the modern world, enzymes (kinases) attach phosphate groups In the pre-RNA world, how might this happen?

  12. Observation: • Surprisingly easy to attach phosphate without needing an enzyme • One hypothesis: cyclo-triphosphate (explains preference for triphosphate

  13. If correct, this indicates a central role for triphosphates of nucleosides (NTPs) in early evolution of RNA (i.e., development of the RNA world) • NTPs central to modern cellular biology

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