Threonine: the pathway in the body

This page shows the biochemical pathway behind the laboratory value Threonine: 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

Threonine is an amino acid that the body cannot make itself, so it comes from dietary protein. It is built into proteins, notably the gut mucus protein MUC2, and is broken down in the liver via propionyl-CoA to succinyl-CoA.

The pathway step by step

1. Dietary protein → Threonine Digestive enzymes The body cannot make threonine itself; all of it comes from food. Its side chain carries an OH group. Source 1, 11
2. Threonine → Intestinal mucosa B⁰AT transporter The transporter B⁰AT in the brush-border membrane of the small intestine takes up threonine. Much of it is used directly in the intestinal mucosa; the rest enters the blood. Source 1, 2
3. Intestinal mucosa → Mucin MUC2 The backbone of the mucus protein MUC2 is especially rich in threonine. Dense sugar chains are attached to its OH group (O-glycosylation), forming the mucus layer on the gut wall. Source 1, 3
4. Threonine in the liver → 2-Oxobutyrate Dehydratase · Vitamin B6 (PLP) Liver serine/threonine dehydratase removes the amino group as ammonia, forming 2-oxobutyrate. It works with pyridoxal phosphate, the active form of vitamin B6. Source 5, 1
5. 2-Oxobutyrate → Propionyl-CoA Keto acid DH complex · Thiamine (TPP), NAD⁺ A keto acid dehydrogenase complex in the mitochondrion removes carbon dioxide and attaches coenzyme A. Such complexes work with thiamine pyrophosphate, lipoic acid, FAD and NAD⁺. Source 6, 7
6. Propionyl-CoA → Methylmalonyl-CoA PCC · Biotin Propionyl-CoA carboxylase adds a carboxyl group, using ATP. The enzyme carries biotin as a bound cofactor. Source 8, 9, 6
7. Methylmalonyl-CoA → Succinyl-CoA MCM · Vitamin B12 Methylmalonyl-CoA mutase rearranges the molecule into succinyl-CoA. It requires adenosylcobalamin, an active form of vitamin B12. Succinyl-CoA is an intermediate of the citric acid cycle. Source 10, 6

Cofactors in this pathway

• Vitamin B6 (PLP) — Cofactor of serine/threonine dehydratase, which converts threonine to 2-oxobutyrate Source 5In the ORY catalogue as a laboratory value: Vitamin B6
• Biotin — Bound cofactor of propionyl-CoA carboxylase, which converts propionyl-CoA to methylmalonyl-CoA Source 8, 9
• Vitamin B12 — As adenosylcobalamin, cofactor of methylmalonyl-CoA mutase on the way to succinyl-CoA Source 10In the ORY catalogue as a laboratory value: Vitamin B12
• Thiamine (vitamin B1) — As thiamine pyrophosphate, cofactor of the keto acid dehydrogenase complex that removes CO₂ Source 7
• NAD⁺ — Accepts the electrons in the keto acid dehydrogenase complex Source 7In the ORY catalogue as a laboratory value: NAD⁺ (Nicotinamidadenindinukleotid)
• Lipoic acid — Carries the acyl group to coenzyme A in the keto acid dehydrogenase complex Source 7

Sources

1. Tang Q, Tan P, Ma N et al. Physiological Functions of Threonine in Animals: Beyond Nutrition Metabolism. Nutrients 2021 · PubMed 34444752
2. Bröer S. Amino acid transport across mammalian intestinal and renal epithelia. Physiol Rev 2008 · PubMed 18195088
3. Arike L, Hansson GC. The Densely O-Glycosylated MUC2 Mucin Protects the Intestine and Provides Food for the Commensal Bacteria. J Mol Biol 2016 · PubMed 26880333
4. Ubersax JA, Ferrell JE Jr. Mechanisms of specificity in protein phosphorylation. Nat Rev Mol Cell Biol 2007 · PubMed 17585314
5. Sun L, Bartlam M, Liu Y et al. Crystal structure of the pyridoxal-5'-phosphate-dependent serine dehydratase from human liver. Protein Sci 2005 · PubMed 15689518
6. Bui D, Ravasz D, Chinopoulos C. The Effect of 2-Ketobutyrate on Mitochondrial Substrate-Level Phosphorylation. Neurochem Res 2019 · PubMed 30810978
7. 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
8. Wongkittichote P, Ah Mew N, Chapman KA. Propionyl-CoA carboxylase - A review. Mol Genet Metab 2017 · PubMed 29033250
9. Tong L. Striking Diversity in Holoenzyme Architecture and Extensive Conformational Variability in Biotin-Dependent Carboxylases. Adv Protein Chem Struct Biol 2017 · PubMed 28683917
10. 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
11. Edgar AJ. The human L-threonine 3-dehydrogenase gene is an expressed pseudogene. BMC Genet 2002 · PubMed 12361482

Related pathways

• Branched-chain amino acids — dietary protein
• Vitamin B12 — methylmalonyl-coa
• Citric acid cycle — succinyl-coa
• IgG antibodies to food — dietary protein
• Pancreatic elastase — dietary protein

As of 2026-09-17. Draft written by Claude to schema v2; sources checked in PubMed; expert review pending
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