10 - Biological oxidation – general concept, redox systems and redox potential. Оxidoreductases. Substrates of the enzymes of biological oxidation and final hydrogen acceptors. Redox systems with biological significance: NAD+/NADH, NADP+/NADPH, FMN/FMN.H2, FAD/FADH2, ubiquinone/ubiquinol, cytochromes (heme), lipoate, ascorbate.

sections

• redox, oxidation, reduction

• redox properties

• characteristics of biological oxidation

• substrates of biological oxidation and final hydrogen acceptors

• oxidoreductases

• redox systems

• redox, oxidation, reduction

• redox = oxidation + reduction

• Oxidation = loss of electrons

• Reduction = gain of electrons

• Capabilities to lose electrons can be measured using redox potential using galvanic cell

• redox properties

• redox potential of standard hydrogen redox system = 0

• RP is measured in volts

• When conc. of oxidised and reduced are equal, ph = 7

• the bigger the difference between the redox potentials = the more free energy released

• characteristics of biological oxidation

• change in oxidation state, that’s measured through galvanic cell

• Oxidation is losing, Reduction if gaining electrons → they’re forming redox pair

• They have different capabilities to release electrons to measure this we use redox potential

• substrates of biological oxidation and final hydrogen acceptors

• substrates are OH -, C=O, NH2, S-H, -CH2-CH2, dipheriols

• Final acceptors are O2 → H2O/H2O2. also lactate, ethanol and metabolic redox systems

• oxidoreductases

• Dehydrogenation: removes 2 e⁻ + 2 H⁺

  • Enzyme: Dehydrogenase

  • Cofactors: FAD, FMN

• Oxygenases: insert oxygen into substrates

  • Dioxygenases: insert 2 O atoms

  • Monooxygenases: insert 1 O atom

  • Cofactors: CoQ, Cytochrome C

• redox systems

• NAD⁺/NADH + H⁺

  • Redox potential: –0.320 to –0.400 V

  • Cannot be oxidised directly by O₂

• FAD/FADH₂

  • Can be directly oxidised by O₂ → H₂O

• Cytochromes (b, c₁, a, a₃)

  • Transfer e⁻ via Fe³⁺ ↔ Fe²⁺

  • Can be reduced by O₂ → H₂O

• Thiol redox systems

  • Include: Glutathione, Lipoate (in PDH), Thioredoxin

• Quinones (CoQ, Coenzyme Q)

  • Transfer 2 e⁻ + 2 H⁺ in electron transport

  • Can form semiquinone radicals (1 e⁻ transfer)

• Ascorbate (Vitamin C)

  • Can gain/lose 2 H⁺ + 2 e⁻