10.4A: Riboflavin Functions

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

Citric Acid Cycle
Electron Transport Chain
Fatty Acid Oxidation
Niacin Synthesis
Vitamin B₆ Activation
Neurotransmitter Catabolism
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.png

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 .png

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 .png

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.png

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 .png

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.png

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.

Web Link

The Glutathione Oxidation Reduction (Redox) Cycle

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

References & Links

Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
en.Wikipedia.org/wiki/File:Ci...conitate_2.svg
en.Wikipedia.org/wiki/File:Mi...%80%94Etc4.svg
https://commons.wikimedia.org/wiki/F...synthesis2.png
en.Wikipedia.org/wiki/File:Py..._structure.svg
en.Wikipedia.org/wiki/Pyrido...-phosphate.svg
en.Wikipedia.org/wiki/File:Do...egradation.svg
en.Wikipedia.org/wiki/File:Se...osynthesis.svg

Links

The Glutathione Oxidation Reduction (Redox) Cycle - lpi.oregonstate.edu/infocente...enium/gsh.html