Biotin (Vitamin B7): the pathway in the body

Biotin (Vitamin B7) is part of the pathway “Biotin”. This page shows the whole pathway; the station of Biotin (Vitamin B7) is highlighted.

Where this laboratory value sits: Biotin in the blood — absorbed via SMVT. The transporter SMVT carries biotin together with sodium into the gut cell, also in the large intestine, where bacterial biotin arrives. In the blood it is mostly dissolved freely. Source 2, 1

In brief

Biotin (vitamin B7) is a water-soluble vitamin containing sulphur. Bound firmly to five carboxylases, it transfers carbon dioxide and so links the building of fatty acids, the refilling of the citric acid cycle and the breakdown of certain amino acids.

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. Biotin in food → Free biotin biotinidase Digestive enzymes break down the protein to which biotin is attached, leaving biocytin. The enzyme biotinidase releases free biotin from it. Source 1, 4↑ supplies Only free biotin is recognised by the transporter of the gut cell; biotinidase is what makes the bound biotin in food usable. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If a lot of free biotin is present, it competes with pantothenic acid for the same transporter.

   too little — If biotinidase works slowly, more biotin stays bound as biocytin and passes on unused.

   established physiology · Source 1, 4

2. Free biotin → Biotin in the blood SMVT · sodium The transporter SMVT carries biotin together with sodium into the gut cell, also in the large intestine, where bacterial biotin arrives. In the blood it is mostly dissolved freely. Source 2, 1↑ supplies The blood distributes biotin to the liver, muscle, adipose tissue and all other tissues in which carboxylases work. established physiology Source 1
   ⚖ When the balance tips

   too much — If biotin in the blood rises, the kidneys excrete the excess in the urine. Very high biotin in the blood can interfere with laboratory tests that use biotin as a linker.

   too little — If little follows, biotin in the blood falls and the tissues receive less for new carboxylases.

   established physiology · Source 1

3. Biotin in the cell → Biotin on carboxylases HLCS · ATP Holocarboxylase synthetase (HLCS) activates biotin with ATP and attaches it firmly to a lysine of the carboxylase. Five carboxylases carry biotin in this way. Source 5, 3↑ supplies Bound biotin picks up carbon dioxide from bicarbonate and passes it on to another molecule. In this way the carboxylases build fatty acids, refill the citric acid cycle and break down amino acids. established physiology Source 3, 5
   ⚖ When the balance tips

   too much — Once all carboxylases are loaded, further biotin brings no extra activity. When carboxylases are broken down, biotinidase recovers the biotin.

   too little — If little biotin is available, the carboxylases work more slowly; their substrates build up and breakdown products such as 3-hydroxyisovaleric acid appear in the urine.

   established physiology · Source 5, 1

4. Acetyl-CoA → Malonyl-CoA ACC · bicarbonate, ATP Acetyl-CoA carboxylase uses biotin to attach carbon dioxide to acetyl-CoA, producing malonyl-CoA, the first committed step in building fatty acids. Source 3↕ both, depending on amount Malonyl-CoA supplies the building blocks to fatty acid synthase and at the same time slows the entry of fatty acids into the mitochondrion, so that synthesis and breakdown do not run at once. established physiology Source 3
   ⚖ When the balance tips

   too much — If malonyl-CoA builds up, fatty-acid breakdown in the mitochondrion stays slowed and the cell tends to build fats.

   too little — If little malonyl-CoA forms, fewer fatty acids are built and the brake on fatty-acid breakdown is released.

   established physiology · Source 3

5. Pyruvate → Oxaloacetate pyruvate carboxylase · bicarbonate, ATP Pyruvate carboxylase uses biotin to attach carbon dioxide to pyruvate, forming oxaloacetate. It refills the citric acid cycle and in the liver is the starting point for making new glucose. Source 3↑ supplies Oxaloacetate brings acetyl-CoA into the citric acid cycle; in the liver and kidney it is the first building block from which new glucose is made. established physiology Source 3
   ⚖ When the balance tips

   too much — If a lot of oxaloacetate forms, the citric acid cycle absorbs more acetyl-CoA, and the liver makes more new glucose.

   too little — If little oxaloacetate forms, the citric acid cycle absorbs less acetyl-CoA; the liver then forms more ketone bodies.

   established physiology · Source 3

6. Propionyl-CoA → Methylmalonyl-CoA PCC · bicarbonate, ATP Propionyl-CoA carboxylase uses biotin to attach carbon dioxide, forming methylmalonyl-CoA. A vitamin B12-dependent mutase converts it to succinyl-CoA. Source 3, 6↑ supplies Methylmalonyl-CoA is rearranged to succinyl-CoA and so refills the citric acid cycle; this route therefore needs two vitamins in sequence, biotin and B12. established physiology Source 6
   ⚖ When the balance tips

   too much — If methylmalonyl-CoA builds up, for example because the B12-dependent mutase works slowly, it is split into methylmalonic acid.

   too little — If little methylmalonyl-CoA forms because the carboxylase has little biotin, propionyl-CoA builds up in front of it instead.

   established physiology · Source 6

Further stations

• Biotin in food — mostly bound to protein Biotin, vitamin B7, is found in egg yolk, offal, nuts and pulses, for example. In food it is mostly bound to protein; gut bacteria also make biotin. Source 1, 2↑ supplies The body does not make biotin itself and keeps only small stores; it does, however, recover it from broken-down enzymes, so the same molecule is used several times. established physiology Source 1
  ⚖ When the balance tips

  too much — If a lot of biotin arrives, the transporter SMVT is soon saturated; extra biotin then enters mainly by passive diffusion and is excreted by the kidneys.

  too little — If little arrives, supply depends more on recovery from broken-down carboxylases and on the small store.

  established physiology · Source 1, 2

• Biotin in the cell — absorbed via SMVT Cells bring in biotin via SMVT. There it waits for the enzyme that binds it firmly to the carboxylases. Source 1, 5↑ supplies Biotin works only when bound: only once attached to a carboxylase can it carry carbon dioxide. established physiology Source 5
  ⚖ When the balance tips

  too much — If more biotin enters the cell than there are carboxylases to load, the remainder stays free and leaves the cell again.

  too little — If little biotin enters the cell, newly made carboxylases partly remain without biotin and therefore inactive.

  established physiology · Source 5

• Acetyl-CoA — activated acetic acid Acetyl-CoA comes from sugar, fat and amino acids. In the cytoplasm it is the starting material for building fatty acids. Source 3↑ supplies To build fatty acids, acetyl-CoA must first be activated: the biotin-dependent acetyl-CoA carboxylase attaches carbon dioxide to it. established physiology Source 3
  ⚖ When the balance tips

  too much — If a lot of acetyl-CoA is present in the cytoplasm and the carboxylase is active, more malonyl-CoA and so more fatty acids form.

  too little — If little acetyl-CoA is available, little malonyl-CoA forms and fatty-acid synthesis rests.

  established physiology · Source 3

• Pyruvate — from glucose breakdown Pyruvate is the end product of glycolysis. In the mitochondrion it can become acetyl-CoA or, with biotin, oxaloacetate. Source 3↕ both, depending on amount Pyruvate stands at a junction: the biotin-dependent pyruvate carboxylase directs it to oxaloacetate, pyruvate dehydrogenase to acetyl-CoA. established physiology Source 3
  ⚖ When the balance tips

  too much — If pyruvate builds up because the carboxylase is working slowly, more of it is converted to lactate.

  too little — If little pyruvate is available, the carboxylase receives less substrate and the citric acid cycle is replenished less from this side.

  established physiology · Source 3

• Propionyl-CoA — from amino and fatty acids Propionyl-CoA forms during the breakdown of the amino acids valine, isoleucine, methionine and threonine and of odd-chain fatty acids. Source 3, 6↑ supplies Via propionyl-CoA, carbon skeletons from these amino acids and fatty acids reach the citric acid cycle once the carboxylase passes them on. established physiology Source 3, 6
  ⚖ When the balance tips

  too much — If propionyl-CoA builds up because the carboxylase works slowly, by-products such as 3-hydroxypropionic acid form and appear in the urine.

  too little — If little propionyl-CoA forms, less material reaches the citric acid cycle by this route.

  established physiology · Source 1, 3

Cofactors in this pathway

• Magnesium — Accompanies the ATP that the carboxylases use to attach carbon dioxide Source 3In the ORY catalogue as a laboratory value: Magnesium
• Vitamin B12 — Cofactor of the mutase that converts methylmalonyl-CoA from propionyl-CoA carboxylase to succinyl-CoA Source 6In the ORY catalogue as a laboratory value: Vitamin B12
• Bicarbonate — Supplies the carbon dioxide that biotin picks up and passes on Source 3
• ATP — Activates biotin for binding to the carboxylase and drives each carboxylation Source 3, 5
• Coenzyme A (vitamin B5) — Carries the acetyl and propionyl groups to which the carboxylases attach carbon dioxide Source 3
• Sodium — Flows into the cell together with biotin through the transporter SMVT Source 2

Sources

1. Mock DM. Biotin: From Nutrition to Therapeutics. J Nutr 2017 · PubMed 28701385
2. Said HM, Nexo E. Gastrointestinal Handling of Water-Soluble Vitamins. Compr Physiol 2018 · PubMed 30215865
3. Tong L. Structure and function of biotin-dependent carboxylases. Cell Mol Life Sci 2013 · PubMed 22869039
4. Zempleni J, Wijeratne SS, Hassan YI. Biotin. Biofactors 2009 · PubMed 19319844
5. León-Del-Río A. Biotin in metabolism, gene expression, and human disease. J Inherit Metab Dis 2019 · PubMed 30746739
6. Takahashi-Iñiguez T, García-Hernandez E, Arreguín-Espinosa R et al. Role of vitamin B12 on methylmalonyl-CoA mutase activity. J Zhejiang Univ Sci B 2012 · PubMed 22661206

Whole pathway: Biotin

Related pathways

• Threonine — propionyl-coa
• Citric acid cycle — oxaloacetate
• Lactate and pyruvate — oxaloacetate
• Liver enzymes — oxaloacetate
• Vitamin B12 — methylmalonyl-coa

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