Valin: the pathway in the body · Biomarker & Blood Analysis

Valin is part of the pathway “Branched-chain amino acids”. This page shows the whole pathway; the station of Valin is highlighted.

Where this laboratory value sits: Leu, Ile, Val — branched-chain amino acids. Digestive enzymes break protein down into its building blocks. Transporters carry the three amino acids into the cells of the gut wall and from there into the blood. They are building blocks of almost all proteins. Source 1

In brief

Leucine, isoleucine and valine are three essential amino acids with a branched side chain. They serve as building blocks of proteins, leucine also acts as a signal for the mTORC1 pathway, and their breakdown feeds the energy metabolism. Their breakdown begins in muscle.

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. Dietary protein → Leu, Ile, Val Digestive enzymes Digestive enzymes break protein down into its building blocks. Transporters carry the three amino acids into the cells of the gut wall and from there into the blood. They are building blocks of almost all proteins. Source 1↑ supplies The three are building blocks of almost all proteins. Leucine also acts as a signal through which the cell switches on the building of new proteins. established physiology Source 1, 3
   ⚖ When the balance tips

   too much — If many BCAAs are available, more of them go into breakdown; the released amino groups end up on glutamate and from there on glutamine.

   too little — If few BCAAs are available, less protein is built, and leucine gives the mTORC1 pathway a weaker signal.

   established physiology · Source 1, 3

2. Leu, Ile, Val → In the blood Amino acid carriers Unlike the other amino acids, these three are barely broken down in the liver, because the first enzyme of the pathway is missing there. They therefore reach muscle, fat tissue, kidney and brain. There they supply nitrogen and energy. Source 1, 2↑ supplies Because the liver lets them pass, the three are distributed to muscle, fat tissue, kidney and brain. There they supply nitrogen for glutamate and carbon for energy metabolism. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If many BCAAs are in the blood, they compete at the blood-brain barrier with tryptophan, tyrosine and phenylalanine for the same transporter; less of these then reaches the brain.

   too little — If few BCAAs are in the blood, fewer of them reach the brain; less nitrogen is then available there for forming glutamate.

   observed in studies · Source 2

3. Inside the cell → Branched keto acids BCAT · Vitamin B6 (PLP) The enzyme BCAT transfers the amino group to α-ketoglutarate. This produces glutamate and the matching keto acid. The step is reversible. The keto acids can return to the amino acid or be broken down for good. Source 1, 2↑ supplies The keto acids are a switch point: BCAT can turn them back into the amino acid with an amino group, or the BCKDH complex breaks them down for good. established physiology Source 1, 2
   ⚖ When the balance tips

   too much — If the keto acids build up, they inhibit BCKDH kinase; the BCKDH complex thus stays active and breaks down the excess.

   too little — If few keto acids are present, the BCKDH complex stays more phosphorylated and thus slowed; the amino acids are preserved.

   established physiology · Source 1

4. Branched keto acids → Acyl-CoA stages BCKDH complex · Thiamine (TPP), Lipoic acid The BCKDH complex removes a carboxyl group and attaches coenzyme A. It works with thiamine pyrophosphate, lipoic acid, FAD and NAD⁺ and is the slowest step of the pathway. From here the breakdown can no longer be reversed. Source 1, 4↑ supplies With this step the breakdown can no longer be reversed. The acyl-CoA stages are broken down further into building blocks for energy metabolism. established physiology Source 1, 4
   ⚖ When the balance tips

   too much — If the BCKDH complex runs at a high rate, the three amino acids are used up quickly; BCKDH kinase then slows it down by phosphorylation.

   too little — If little thiamine pyrophosphate, lipoic acid, FAD or NAD⁺ is available, the complex cannot keep up; the keto acids then build up before this step.

   established physiology · Source 1, 4

5. Acyl-CoA stages → Acetyl-CoA Further breakdown of leucine ends in acetyl-CoA and acetoacetate. The body turns these into ketone bodies or feeds them into the citric acid cycle. Ketone bodies serve the heart, muscle and brain as fuel. Source 1↑ supplies Acetyl-CoA and acetoacetate supply energy in the citric acid cycle or as ketone bodies. By this route leucine supplies only building blocks from which no glucose can form. established physiology Source 1
   ⚖ When the balance tips

   too much — If more acetyl-CoA forms than the citric acid cycle absorbs, the liver makes ketone bodies from it, which other tissues use as fuel.

   too little — If little acetyl-CoA forms from leucine and isoleucine, the cell's acetyl-CoA comes from glucose and fatty acids.

   established physiology · Source 1

6. Acyl-CoA stages → Succinyl-CoA · Biotin, Vitamin B12 Succinyl-CoA is formed via propionyl-CoA. The enzymes involved need biotin and adenosylcobalamin, the active form of vitamin B12. Succinyl-CoA replenishes the citric acid cycle; glucose can also form from it. Source 1, 5↑ supplies Succinyl-CoA replenishes the citric acid cycle with new carbon; glucose can also form from it. Valine and part of isoleucine thus serve the building of glucose. established physiology Source 1, 5
   ⚖ When the balance tips

   too much — If much propionyl-CoA flows in, methylmalonyl-CoA mutase needs more adenosylcobalamin to convert it via methylmalonyl-CoA into succinyl-CoA.

   too little — If little adenosylcobalamin is available, the mutase cannot keep up; methylmalonyl-CoA builds up and becomes methylmalonic acid.

   established physiology · Source 5

7. Acetyl-CoA → Citric acid cycle Acetyl-CoA and succinyl-CoA enter the citric acid cycle, the central hub of energy metabolism. There their carbon is broken down to CO₂, and the cell gains energy. Source 1↑ supplies In the citric acid cycle the carbon of the three amino acids is broken down to CO₂; in the process the cell gains energy, above all in muscle. established physiology Source 1
   ⚖ When the balance tips

   too much — If more building blocks arrive than the cycle can process, acetyl-CoA in the liver is diverted into ketone bodies.

   too little — If the three amino acids supply little, the cycle is fed from glucose and fatty acids; their route is one of several inflows.

   established physiology · Source 1

8. Succinyl-CoA → Citric acid cycle Acetyl-CoA and succinyl-CoA enter the citric acid cycle, the central hub of energy metabolism. There their carbon is broken down to CO₂, and the cell gains energy. Source 1↑ supplies In the citric acid cycle the carbon of the three amino acids is broken down to CO₂; in the process the cell gains energy, above all in muscle. established physiology Source 1
   ⚖ When the balance tips

   too much — If more building blocks arrive than the cycle can process, acetyl-CoA in the liver is diverted into ketone bodies.

   too little — If the three amino acids supply little, the cycle is fed from glucose and fatty acids; their route is one of several inflows.

   established physiology · Source 1

Further stations

• Dietary protein — Source of the amino acids Leucine, isoleucine and valine carry a branched side chain. The body cannot make them itself; they come from the protein in food. It supplies the three amino acids for protein building and energy. Source 1↑ supplies Dietary protein is the only outside supply of the three amino acids. Further amounts are released when the body turns over its own protein. established physiology Source 1
  ⚖ When the balance tips

  too much — If much protein arrives, the three rise in the blood; their keto acids then inhibit BCKDH kinase, and the BCKDH complex breaks down more of them.

  too little — If little protein arrives, BCKDH kinase switches the complex off by phosphorylation; the three amino acids are spared and remain for protein building.

  established physiology · Source 1

• Leucine as a signal — Sensor Sestrin2 Inside the cell, leucine binds to the protein Sestrin2. This switches on the mTORC1 signalling pathway, through which the cell controls the building of new proteins. When little leucine is present, this pathway stays off. Source 3↑ supplies Leucine detaches Sestrin2 from its partner GATOR2. This releases mTORC1, which switches on cell growth and protein building. This has been shown in cell culture. observed in studies Source 3
  ⚖ When the balance tips

  too much — If much leucine binds, Sestrin2 stays detached from GATOR2 and mTORC1 stays switched on; in cell culture the activity follows the amount of leucine.

  too little — If little leucine is available, Sestrin2 holds on to GATOR2; mTORC1 stays off, and the cell makes fewer new proteins. Shown in cell culture.

  observed in studies · Source 3

• Inside the cell — Muscle, fat tissue, kidney Breakdown of the three amino acids begins in these tissues. Muscle holds particularly large amounts of the first enzyme. Their amino groups pass to glutamate, and from there to glutamine and alanine. Source 1, 2↑ supplies In muscle, BCAT transfers the amino groups and so supplies nitrogen for glutamate, glutamine and alanine. Muscle releases part of the keto acids into the blood, and the liver breaks them down further. established physiology Source 1, 2
  ⚖ When the balance tips

  too much — If muscle takes up many BCAAs, more keto acids form than its BCKDH complex can break down; the excess enters the blood and is broken down in the liver.

  too little — If few BCAAs arrive, muscle forms less glutamate from their amino groups and releases less nitrogen as glutamine and alanine.

  established physiology · Source 1

Cofactors in this pathway

• Vitamin B6 — As pyridoxal phosphate, cofactor of BCAT, which transfers the amino group to α-ketoglutarate; without PLP, BCAT stalls Source 1In the ORY catalogue as a laboratory value: Vitamin B6
• Thiamine (vitamin B1) — As thiamine pyrophosphate, cofactor of the BCKDH complex, which removes the carboxyl group; without TPP it stalls Source 1, 4
• Lipoic acid — Carries the acyl group within the BCKDH complex for transfer to coenzyme A; without it no acyl-CoA Source 4
• Riboflavin (vitamin B2) — As FAD, part of the E3 subunit of the BCKDH complex, which reoxidises lipoic acid Source 1, 4
• Niacin (NAD⁺) — NAD⁺ accepts the electrons in the BCKDH complex and becomes NADH Source 1, 4In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Biotin — Cofactor of propionyl-CoA carboxylase in the breakdown of valine and isoleucine; without biotin this route stalls Source 1
• Vitamin B12 — As adenosylcobalamin, cofactor of methylmalonyl-CoA mutase on the way to succinyl-CoA; without it the route backs up Source 5In the ORY catalogue as a laboratory value: Vitamin B12
• α-Ketoglutarate — Accepts the amino group in the BCAT step and becomes glutamate; glutamate thus collects the nitrogen Source 1In the ORY catalogue as a laboratory value: Alpha-Ketoglutarat (2-Oxoglutarat)

Sources

1. Harris RA, Joshi M, Jeoung NH. Overview of the molecular and biochemical basis of branched-chain amino acid catabolism. J Nutr 2005 · PubMed 15930464
2. Sperringer JE, Addington A, Hutson SM. Branched-Chain Amino Acids and Brain Metabolism. Neurochem Res 2017 · PubMed 28417264
3. Wolfson RL, Chantranupong L, Saxton RA et al. Sestrin2 is a leucine sensor for the mTORC1 pathway. Science 2016 · PubMed 26449471
4. Szabo E, Nagy B, Czajlik A et al. Mitochondrial Alpha-Keto Acid Dehydrogenase Complexes: Recent Developments on Structure and Function in Health and Disease. Subcell Biochem 2024 · PubMed 38963492
5. Takahashi-Iñiguez T, García-Hernandez E et al. Role of vitamin B12 on methylmalonyl-CoA mutase activity. J Zhejiang Univ Sci B 2012 · PubMed 22661206

Whole pathway: Branched-chain amino acids

Related pathways

• Threonine — dietary protein
• Citric acid cycle — succinyl-coa
• IgG antibodies to food — dietary protein
• Pancreatic elastase — dietary protein
• Vitamin B1 — succinyl-coa

As of 2026-09-16. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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