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10.4: Riboflavin

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    A student once asked this question:

    "I started taking the Mega Man Sport Multi-vitamin from GNC and about an hour or two after consumption, with a meal, my pee is bright, practically neon yellow. What does that mean?"

    Since this question is leading off the riboflavin section, you have probably surmised that riboflavin is somehow involved. Indeed, flavin means yellow in Latin, and riboflavin is bright yellow as shown in Figure \(\PageIndex{1}\).

    A hand holding a small, clear tube filled with yellow liquid, topped with a white cap.
    Figure \(\PageIndex{1}\): Riboflavin in solution1
    Chemical structure of a compound with functional groups including amines and hydroxyls, illustrated with line and molecular formulas.
    Figure \(\PageIndex{2}\): Structure of riboflavin2

    Riboflavin is a water-soluble B vitamin, so the student was excreting large amounts of riboflavin in his urine, leading it to become "bright, practically neon yellow." The structure of riboflavin is shown in Figure \(\PageIndex{2}\).

    Riboflavin is important for the production of two cofactors: flavin adenine dinucleotide (\(\ce{FAD}\)) & flavin mononucleotide (\(\ce{FMN}\)).

    \(\ce{FAD}\) has been introduced before, but structurally you can see where riboflavin is within the compound below.

    Chemical structure diagram of a molecule with highlighted nitrogen atoms (N) and various functional groups.
    Figure \(\PageIndex{3}\): Structure of \(\ce{FAD}\)3

    The 2 circled nitrogens are the sites that accept hydrogen to become \(\ce{FADH2}\) as illustrated below.

    Chemical structure diagram showing the transformation of one molecule to another, with highlighted functional groups.
    Figure \(\PageIndex{4}\): Addition of two hydrogens to the rings of FAD to form \(\ce{FADH2}\)4

    The structure of \(\ce{FMN}\) as shown below, is similar to \(\ce{FAD}\), except that it only contains one phosphate group (versus 2) and doesn't have the ring structures off the phosphate groups that are found in \(\ce{FAD}\).

    Chemical structure of a nucleotide, featuring nitrogenous bases and phosphate groups.
    Figure \(\PageIndex{5}\): The structure of \(\ce{FMN}\)5

    Riboflavin is photosensitive, meaning that it can be destroyed by light. This was a problem in the old days when the milkman delivered milk in clear glass bottles. These have now been replaced by cartons or opaque plastic containers to help protect the riboflavin content of the milk.

    A milk carton and a delivery person in a blue uniform carrying a basket of groceries.
    Figure \(\PageIndex{6}\): Milk is no longer packaged in clear glass bottles to help protect its riboflavin from light destruction

    Riboflavin in foods is free, protein-bound, or in \(\ce{FAD}\) or \(\ce{FMN}\). Only free riboflavin is taken up so it must be cleaved, or converted before absorption6. Riboflavin is highly absorbed through an unresolved process, though it is believed that a carrier is involved7. As you would guess from the description above, riboflavin is primarily excreted in the urine.

    ADAPT \(\PageIndex{1}\)

    Riboflavin Functions

    Riboflavin is required for the production of \(\ce{FAD}\) and \(\ce{FMN}\). Below are some of the functions of \(\ce{FAD}\) and \(\ce{FMN}\)6:

    Citric Acid Cycle

    \(\ce{FAD}\) is reduced to \(\ce{FADH2}\) in the citric acid cycle when succinate is converted to fumarate by succinic dehydrogenase as circled below.

    Diagram of the citric acid cycle, showing key metabolites, enzymes, and reactions involved in cellular respiration.
    Figure \(\PageIndex{7}\): The citric acid cycle requires \(\ce{FAD}\)8

    Electron Transport Chain

    Under aerobic conditions, the electron transport chain is where the \(\ce{FADH2}\) is used to produce ATP. Complex I of the electron transport chain includes an \(\ce{FMN}\) molecule. The electron transport chain is shown below.

    Diagram illustrating cellular respiration, showing the mitochondrion's inner and outer membranes, matrix, and reactions involved.
    Figure \(\PageIndex{8}\): Complex I in the electron transport chain contains \(\ce{FMN}\)9

    Fatty Acid oxidation

    During fatty acid oxidation \(\ce{FAD}\) is converted to \(\ce{FADH2}\) as shown below.

    Diagram illustrating biochemical pathways, showing ATP/AMP conversion and entry into the electron transport chain (ETC) and TCA cycle.
    Figure \(\PageIndex{9}\): Fatty acid oxidation requires \(\ce{FAD}\)

    Niacin synthesis

    As you will learn 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 \(\ce{FAD}\).

    Chemical pathway illustrating the conversion of Tryptophan to Kynurenine and then to Niacin, with steps highlighted.
    Figure \(\PageIndex{10}\): Niacin synthesis from tryptophan requires \(\ce{FAD}\)10

    Vitamin \(B_6\) Activation

    The enzyme that creates the active form of vitamin \(B_6\) (pyridoxal phosphate) requires \(\ce{FMN}\).

    Chemical structures of Pyridoxine and Pyridoxal Phosphate, connected by an arrow labeled "FMN."
    Figure \(\PageIndex{11}\): Vitamin \(B_6\) activation requires \(\ce{FMN}\)11,12

    Neurotransmitter Catabolism

    The enzyme monoamine oxidase (\(\ce{MAO}\)) requires \(\ce{FAD}\). This enzyme shown below is important in the catabolism of neurotransmitters such as dopamine and serotonin.

    Chemical pathway diagram showing the synthesis of dopamine, 3-Methoxytyramine, homovanillic acid, and related compounds with enzymatic reactions.
    Figure \(\PageIndex{12}\): Catabolism of dopamine involves monoamine oxidase, an enzyme that requires \(\ce{FAD}\)13
    A simple black silhouette of a person sitting cross-legged in meditation.
    Figure \(\PageIndex{13}\): Catabolism of serotonin involves monoamine oxidase, an enzyme that requires \(\ce{FAD}\)14

    Antioxidant Enzymes

    The antioxidant enzymes glutathione reductase and thioredoxin reductase both require \(\ce{FAD}\) 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, \(\ce{FAD}\) is also used in folate activation, choline catabolism, and purine metabolism6.

    ADAPT \(\PageIndex{2}\)

    Riboflavin Deficiency & Toxicity

    Ariboflavinosis, riboflavin deficiency, is a rare condition that often occurs with other nutrient deficiencies. The symptoms of this condition are shown in the figure below.

    Diagram of a stick figure with labels indicating symptoms: sore red eyes and lips, angular stomatitis, glossitis, and cheilosis of lips.
    Figure \(\PageIndex{14}\): The symptoms of riboflavin deficiency15

    The most notable symptoms include angular stomatitis (aka angular cheilitis, cheilosis), which is a lesion or cracking that forms at the corners of the mouth as shown below.

    Close-up of a person's lips, highlighting a small, raised area on the skin near the lip line.
    Figure \(\PageIndex{15}\): Angular Cheilitis16

    Glossitis is the inflammation of the tongue, which can be accompanied by redness or inflammation of the oral cavity. Dermatitis (skin inflammation) is also frequently a symptom6,17.

    Rare reports of riboflavin toxicity has resulted in optic problems18.​​​​​​

    ADAPT \(\PageIndex{3}\)

    References

    1. http://www.chm.bris.ac.uk/motm/vitam...n-solution.jpg
    2. en.Wikipedia.org/wiki/File:Riboflavin.svg
    3. https://courses.lumenlearning.com/suny-nutrition/chapter/6-11-cofactors/
    4. https://courses.lumenlearning.com/suny-nutrition/chapter/6-11-cofactors/
    5. https://en.Wikipedia.org/wiki/Flavin_mononucleotide#/media/File:Flavin_mononucleotide.png
    6. Gropper SS, Smith JL, Groff JL. (2008) Advanced nutrition and human metabolism. Belmont, CA: Wadsworth Publishing.
    7. Said H, Mohammed Z. (2006) Intestinal absorption of water-soluble vitamins: An update. Curr Opin Gastroenterol 22(2): 140-146.
    8. https://en.Wikipedia.org/wiki/Citric_acid_cycle#/media/File:Citric_acid_cycle_with_aconitate_2.svg
    9. https://en.Wikipedia.org/wiki/Electron_transport_chain#/media/File:Mitochondrial_electron_transport_chain%E2%80%94Etc4.svg
    10. https://courses.lumenlearning.com/suny-nutrition/chapter/6-32-fatty-acid-oxidation-beta-oxidation/
    11. https://courses.lumenlearning.com/suny-nutrition/chapter/10-41-riboflavin-functions/
    12. https://courses.lumenlearning.com/suny-nutrition/chapter/10-7-vitamin-b6/
    13. https://courses.lumenlearning.com/suny-nutrition/chapter/10-41-riboflavin-functions/
    14. https://courses.lumenlearning.com/suny-nutrition/chapter/10-41-riboflavin-functions/
    15. https://courses.lumenlearning.com/suny-nutrition/chapter/10-42-riboflavin-deficiency-toxicity/
    16. https://www.medicalnewstoday.com/articles/320053#what-is-angular-cheilitis
    17. Byrd-Bredbenner C, Moe G, Beshgetoor D, Berning J. (2009) Wardlaw's perspectives in nutrition. New York, NY: McGraw-Hill.
    18. Pinto JT, Zempleni J. (2016) Riboflavin. Advances in Nutrition 7(5): 973–975.

    This page titled 10.4: Riboflavin is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Brian Lindshield via source content that was edited to the style and standards of the LibreTexts platform.