Biochemistry: Riboflavin (Vitamin B2), 2 most important enzymes dependent on FAD and FMN.

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Riboflavin (Vitamin B2)

  • Riboflavin was the first B complex component to be isolated in a pure state.
  • This vitamin is synthesized by green plants and microorganisms.

Structure of Riboflavin

  • Riboflavin has a dimethyl isoalloxazine ring to which a ribitol is attached.
  • Ribitol is the alcohol of ribose sugar.
  • Riboflavin is converted to its active co-enzyme forms (FMN and FAD) with the help of ATP.
  • Riboflavin is heat stable.

Co-enzyme Activity of Riboflavin

  1. Riboflavin exists in tissues tightly bound (but not covalently) with enzymes.
  2. Enzymes containing Riboflavin are called Flavoproteins.
  3. The two co-enzymes are FMN (Flavin mono nucleotide) and FAD (Flavin adenine dinucleotide).
  • During the oxidation process, FAD accepts two hydrogen atoms from substrate.
  • In turn, FAD is reduced to FADH2.
  • The two nitrogen atoms of isoalloxazine nucleus accept the hydrogen atoms.
FMN-dependent Enzymes
  • During the amino acid oxidation, FMN is reduced.
  • It is re-oxidized by molecular oxygen to produce hydrogen peroxide.
  • In the respiratory chain, the NADH dehydrogenase contains FMN.
  • The electrons are transported in the following manner: 

                      NAD+ →FMN→ CoQ

FAD-dependent Enzymes

  • FADH2 when oxidized in the electron transport chain will generate 1.5 ATP molecules.
  1. Succinate to fumarate by succinate. dehydrogenase
  2. Acyl CoA to alpha-beta unsaturated acyl CoA by acyl CoA dehydrogenase.
  3. Xanthine to uric acid by xanthine oxidase.
  4. Pyruvate to acetyl CoA by pyruvate dehydrogenase.
  5. Alpha ketoglutarate to succinyl CoA by alpha ketoglutarate dehydrogenase.

For Complete lesson, please visit Vitamins in “Biochemistry I” from the home page

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