Riboflavin is required for the production of  FAD and FMN. Below are some of the functions of FAD and FMN¹:

1. Citric Acid Cycle
2. Electron Transport Chain
3. Fatty Acid Oxidation
4. Niacin Synthesis
5. Vitamin B6 Activation
6. Neurotransmitter Catabolism
7. Antioxidant Enzymes

1. Citric Acid Cycle – FAD is reduced to FADH₂ in the citric acid cycle when succinate is converted to fumarate by succinic dehydrogenase as circled below.

Figure 10.411 The citric acid cycle requires FAD²

2. Electron Transport Chain – Under aerobic conditions, the electron transport chain is where the FADH2 is used to produce ATP. Complex I of the electron transport chain includes an FMN molecule. The electron transport chain is shown below.

Figure 10.412 Complex I in the electron transport chain contains FMN³

3. Fatty Acid oxidation – During fatty acid oxidation FAD is converted to FADH2 as shown below.

Figure 10.413 Fatty acid oxidation requires FAD

4. Niacin synthesis – As you will hear more about in the niacin section, niacin can be synthesized from tryptophan as shown below. An intermediate in this synthesis is kynurenine, and one of the multiple steps between kynurenine to niacin requires FAD.

Figure 10.414 Niacin synthesis from tryptophan requires FAD⁴

5. Vitamin B6 Activation – The enzyme that creates the active form of vitamin B6 (pyridoxal phosphate) requires FMN.

Figure 10.415 Vitamin B6 activation requires FMN^5,6

6. Neurotransmitter Catabolism – The enzyme monoamine oxidase (MAO) requires FAD. This enzyme shown below is important in the catabolism of neurotransmitters such as dopamine and serotonin.

Figure 10.416 Catabolism of dopamine involves monoamine oxidase, an enzyme that requires FAD⁷

Figure 10.417 Catabolism of serotonin involves monoamine oxidase, an enzyme that requires FAD⁸

7. Antioxidant Enzymes – The antioxidant enzymes glutathione reductase and thioredoxin reductase both require FAD as a cofactor. Thioredoxin reductase is a selenoenzyme. The function of glutathione reductase is shown in the following link. Glutathione reductase can reduce glutathione that can then be used by the selenoenzyme glutathione peroxidase to convert hydrogen peroxide to water.

In addition to the functions listed above, FAD is also used in folate activation, choline catabolism, and purine metabolism¹.

References & Links

1. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
2. http://en.wikipedia.org/wiki/File:Citric_acid_cycle_with_aconitate_2.svg
3. http://en.wikipedia.org/wiki/File:Mitochondrial_electron_transport_chain%E2%80%94Etc4.svg
4. https://commons.wikimedia.org/wiki/File:Nicotinic_acid_biosynthesis2.png
5. http://en.wikipedia.org/wiki/File:Pyridoxine_structure.svg
6. https://en.wikipedia.org/wiki/Pyridoxal_phosphate#/media/File:Pyridoxal-phosphate.svg
7. http://en.wikipedia.org/wiki/File:Dopamine_degradation.svg
8. http://en.wikipedia.org/wiki/File:Serotonin_biosynthesis.svg

Links

The Glutathione Oxidation Reduction (Redox) Cycle – http://lpi.oregonstate.edu/infocenter/minerals/selenium/gsh.html