Vitamin B2: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Vitamin B2 (riboflavin): which stations follow one another, which enzymes carry out each step and which cofactors they use. Every statement has a source. The page describes general textbook knowledge and says nothing about any individual person.

In brief

Vitamin B2 (riboflavin) is a water-soluble, yellow vitamin. The cell converts it to FMN and FAD, the cofactors of flavoenzymes that transfer electrons in the respiratory chain, fatty-acid breakdown, glutathione recovery and the activation of vitamin B6.

The pathway step by step

Each station states what the compound does there. Three signs: ↑ supplies — builds up or makes available · ↓ depletes — inhibits, consumes or withholds · ↕ both, depending on amount. Behind it stands what the statement rests on: established physiology, observed in studies, or contested. The signs do not grade; they name the direction.

1. Riboflavin (food) → Free riboflavin phosphatases Phosphatases in the small intestine split FAD and FMN until free riboflavin remains. Only in this form is it absorbed. Source 1↑ supplies Splitting turns the bound forms in food into the small riboflavin molecule that the transporters of the gut cell recognise. established physiology Source 1
   ⚖ When the balance tips

   too much — If a lot of free riboflavin is present, the transporters are soon saturated and only a small extra part enters by passive diffusion.

   too little — If little is released from FAD and FMN, less riboflavin is available for the transporters to absorb.

   established physiology · Source 1

2. Free riboflavin → Riboflavin in the blood RFVT3, RFVT1 The transporter RFVT3 brings riboflavin into the gut cell and RFVT1 releases it on the blood side. In the blood it is partly bound to albumin; the kidneys excrete any excess. Source 4, 1↑ supplies The blood distributes riboflavin to all tissues; every cell needs it because flavoenzymes work in all cells. established physiology Source 4
   ⚖ When the balance tips

   too much — If riboflavin in the blood rises, the kidneys quickly excrete the free fraction in the urine, which turns bright yellow.

   too little — If little follows from the gut, riboflavin in the blood falls and the tissues receive less for making FMN and FAD.

   established physiology · Source 1

3. Riboflavin in the cell → FMN riboflavin kinase · ATP Riboflavin kinase attaches a phosphate from ATP to riboflavin, producing FMN. It is a cofactor in complex I of the respiratory chain, for example, and in PNPO, which activates vitamin B6. Source 3, 2↑ supplies In its enzymes FMN accepts electrons and passes them on; in complex I it receives them from NADH, in PNPO from pyridoxine 5-phosphate. established physiology Source 2, 5
   ⚖ When the balance tips

   too much — If a lot of FMN forms, FAD synthetase converts part of it to FAD; FMN enzymes do not work beyond their own amount.

   too little — If little FMN is available, FMN enzymes such as PNPO work more slowly, and less FAD forms as well.

   established physiology · Source 2, 3

4. FMN → FAD FAD synthetase · ATP FAD synthetase joins FMN to the AMP part of an ATP. FAD is a cofactor of complex II, glutathione reductase, MTHFR and the enzymes of fatty-acid breakdown. Source 3, 2↑ supplies FAD transfers electrons in many enzymes: it accepts them during the breakdown of succinate and fatty acids and passes them to their target in glutathione reductase and MTHFR. established physiology Source 2, 6
   ⚖ When the balance tips

   too much — Once the flavoenzymes are loaded with FAD, further FAD brings no extra activity; some enzymes, however, are broken down more slowly when FAD is bound.

   too little — If little FAD is available, glutathione reductase, MTHFR and the enzymes of fatty-acid breakdown work more slowly; some flavoenzymes are broken down faster without FAD.

   established physiology · Source 2, 3

5. Pyridoxine 5-phosphate → PLP PNPO · FMN PNPO converts pyridoxine 5-phosphate to pyridoxal 5-phosphate (PLP); FMN accepts the electrons in the process. PLP is the cofactor of many enzymes of amino-acid metabolism. Source 5↑ supplies PLP sits as a cofactor in enzymes that convert amino acids, form messengers such as serotonin and GABA, and break down homocysteine. established physiology Source 5
   ⚖ When the balance tips

   too much — If a lot of PLP forms, it slows PNPO itself and is bound to proteins in the cell or dephosphorylated again.

   too little — If little FMN is available, PNPO forms less PLP; the B6-dependent enzymes then work more slowly too.

   established physiology · Source 5

6. GSSG → GSH glutathione reductase · FAD Glutathione reductase turns GSSG back into two GSH. FAD passes electrons from NADPH on to glutathione in the process. Source 2↑ supplies GSH donates electrons to glutathione peroxidases, which convert peroxides to water; FAD-dependent recovery lets the same molecule work many times. established physiology Source 2
   ⚖ When the balance tips

   too much — If a lot of GSH is present, the reductase is slowed by its product and more NADPH remains for other pathways.

   too little — If the reductase works slowly, for instance because little FAD is available, less GSH is recovered and peroxides are broken down more slowly.

   established physiology · Source 2

Further stations

• Riboflavin (food) — mostly as FAD and FMN Riboflavin, vitamin B2, is a yellow, light-sensitive vitamin from dairy products, eggs, meat and wholegrains. In food it is mostly bound to proteins as FAD and FMN. Source 1↑ supplies The body does not make riboflavin itself and stores little; what it needs for its flavoenzymes comes continuously from food and to a small extent from gut bacteria. established physiology Source 1, 3
  ⚖ When the balance tips

  too much — If a lot of riboflavin arrives at once, the transporters absorb only a limited part; the rest passes on through the gut.

  too little — If little arrives, the small store is soon used up, because the body hardly stores riboflavin.

  established physiology · Source 1

• Riboflavin in the cell — absorbed via RFVT Tissue cells bring riboflavin in through transporters of the RFVT family and convert it straight away to FMN and FAD. Source 4, 3↑ supplies Riboflavin hardly stays free in the cell: conversion to FMN traps it and makes it usable for the flavoenzymes. established physiology Source 3
  ⚖ When the balance tips

  too much — If more riboflavin enters the cell than the kinase converts, the free remainder leaves the cell again.

  too little — If little riboflavin enters the cell, less FMN and FAD form; flavoenzymes then partly remain without their cofactor.

  established physiology · Source 3

• Pyridoxine 5-phosphate — from vitamin B6 Vitamin B6 from food is first given a phosphate in the cell. Pyridoxine 5-phosphate is not yet the active form. Source 5↑ supplies Pyridoxine 5-phosphate is the precursor from which the FMN-dependent PNPO forms active PLP. established physiology Source 5
  ⚖ When the balance tips

  too much — If pyridoxine 5-phosphate builds up because PNPO is working slowly, less PLP forms even though enough vitamin B6 arrives.

  too little — If little vitamin B6 arrives, little pyridoxine 5-phosphate forms and PNPO has less to convert.

  established physiology · Source 5

• GSSG — oxidised glutathione When glutathione defuses peroxides, two molecules are joined to form GSSG. In this form it can no longer donate electrons. Source 2↓ depletes GSSG is spent glutathione; until it is recovered, the cell lacks it as an electron donor for breaking down peroxides. established physiology Source 2
  ⚖ When the balance tips

  too much — If GSSG builds up because the reductase cannot keep up, the ratio of GSH to GSSG in the cell shifts.

  too little — If little GSSG forms, little peroxide is currently being broken down and the reductase has little to do.

  established physiology · Source 2

Cofactors in this pathway

• Vitamin B6 — Is converted into its active form PLP by the FMN-dependent PNPO Source 5In the ORY catalogue as a laboratory value: Vitamin B6
• Glutathione — Is recovered from GSSG by the FAD-dependent glutathione reductase Source 2In the ORY catalogue as a laboratory value: Glutathion (GSH)
• Folate — MTHFR needs FAD to form 5-methyltetrahydrofolate Source 6In the ORY catalogue as a laboratory value: Folsäure
• Magnesium — Accompanies ATP in the formation of FMN and FAD Source 3In the ORY catalogue as a laboratory value: Magnesium
• ATP — Supplies the phosphate for FMN and the AMP part for FAD Source 3
• NADPH — Supplies the electrons that FAD passes to GSSG in glutathione reductase Source 2

Sources

1. Said HM, Nexo E. Gastrointestinal Handling of Water-Soluble Vitamins. Compr Physiol 2018 · PubMed 30215865
2. Lienhart WD, Gudipati V, Macheroux P. The human flavoproteome. Arch Biochem Biophys 2013 · PubMed 23500531
3. Balasubramaniam S, Christodoulou J, Rahman S. Disorders of riboflavin metabolism. J Inherit Metab Dis 2019 · PubMed 30680745
4. Yonezawa A, Inui K. Novel riboflavin transporter family RFVT/SLC52: identification, nomenclature, functional characterization and genetic diseases of RFVT/SLC52. Mol Aspects Med 2013 · PubMed 23506902
5. di Salvo ML, Contestabile R, Safo MK. Vitamin B(6) salvage enzymes: mechanism, structure and regulation. Biochim Biophys Acta 2011 · PubMed 21182989
6. Froese DS, Fowler B, Baumgartner MR. Vitamin B12, folate, and the methionine remethylation cycle-biochemistry, pathways, and regulation. J Inherit Metab Dis 2019 · PubMed 30693532

Related pathways

• Glutathione — gssg
• Glycine and serine — Vitamin B6, Folsäure
• Liver enzymes — Vitamin B6, Glutathion (GSH)
• Methylation: methionine and homocysteine — Vitamin B6, Folsäure
• Alanine — Vitamin B6, Magnesium

As of 2026-10-05. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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