Biological Oxidation and Reduction Notes

Biological Oxidation and Reduction

Oxidoreductase Activity and Half Reactions

  • Many key enzymes in ATP synthesis and other biochemical pathways exhibit oxidoreductase activity.
  • A key skill in biochemistry is dividing oxidation-reduction reactions into half-reaction components to determine the number of electrons transferred.
  • Example: Lactic acid fermentation
    • Pyruvate and NADH are converted to lactate and NAD+NAD^+ by lactate dehydrogenase.
    • Overall reaction: C<em>3H</em>4O<em>3+NADH+H+→C</em>3H<em>6O</em>3+NAD+C<em>3H</em>4O<em>3 + NADH + H^+ \rightarrow C</em>3H<em>6O</em>3 + NAD^+
    • Reduction half-reaction: C<em>3H</em>4O<em>3+2H++2e−→C</em>3H<em>6O</em>3C<em>3H</em>4O<em>3 + 2H^+ + 2e^- \rightarrow C</em>3H<em>6O</em>3
    • Oxidation half-reaction: NADH→NAD++H++2e−NADH \rightarrow NAD^+ + H^+ + 2e^-
  • Spontaneous oxidation-reduction reactions have a negative ΔG\Delta G and a positive electromotive force (E).

Electron Carriers

  • In the cytoplasm, several soluble molecules act as high-energy electron carriers:
    • NADH
    • NADPH
    • FADH2FADH_2
    • Ubiquinone
    • Cytochromes
    • Glutathione
  • Some of these carriers (e.g. NADH, FADH2FADH_2, ubiquinone, cytochromes) are used by the mitochondrial electron transport chain, which leads to the oxidative phosphorylation of ADP to ATP.
  • As electrons pass down the electron transport chain, they give up free energy to form the proton-motive force across the inner mitochondrial membrane.
  • In addition to soluble electron carriers, there are membrane-bound electron carriers embedded within the inner mitochondrial membrane.
    • Flavin mononucleotide (FMN) is bound to complex I of the electron transport chain but can also act as a soluble electron carrier.
  • Proteins with prosthetic groups containing iron-sulfur clusters are well-suited for electron transport.

Flavoproteins

  • Flavoproteins contain a modified vitamin B2B_2 (riboflavin).
  • They are nucleic acid derivatives, generally flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN).
  • Flavoproteins are notable for their presence in mitochondria and chloroplasts as electron carriers.
  • Flavoproteins are involved in modifying other B vitamins to their active forms.
  • Flavoproteins function as coenzymes for enzymes involved in:
    • Oxidation of fatty acids
    • Decarboxylation of pyruvate
    • Reduction of glutathione