Leucin: the pathway in the body

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

The pathway step by step

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. Source 1
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. Source 1, 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. Source 1, 2
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. 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. 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. Source 1, 5
7. Acetyl-CoA → Citric acid cycle Acetyl-CoA and succinyl-CoA enter the citric acid cycle, the central hub of energy metabolism. Source 1
8. Succinyl-CoA → Citric acid cycle Acetyl-CoA and succinyl-CoA enter the citric acid cycle, the central hub of energy metabolism. Source 1

Cofactors in this pathway

• Vitamin B6 — As pyridoxal phosphate, cofactor of BCAT, which transfers the amino group to α-ketoglutarate 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 Source 1, 4
• Lipoic acid — Carries the acyl group within the BCKDH complex for transfer to coenzyme A Source 4
• Riboflavin (vitamin B2) — As FAD, part of the E3 subunit of the BCKDH complex Source 1, 4
• Niacin (NAD⁺) — NAD⁺ accepts the electrons in the BCKDH complex 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 Source 1
• Vitamin B12 — As adenosylcobalamin, cofactor of methylmalonyl-CoA mutase on the way to succinyl-CoA Source 5In the ORY catalogue as a laboratory value: Vitamin B12
• α-Ketoglutarate — Accepts the amino group in the BCAT step and becomes glutamate 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 B12 — 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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