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

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. This yields energy.

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 → Threonine Digestive enzymes The body cannot make threonine itself; all of it comes from food. Its side chain carries an OH group. This OH group is the attachment site for sugars and phosphate. Source 1, 11↑ supplies With its OH group threonine becomes an attachment point: for sugar chains in mucus and for phosphate in signalling proteins. Since the body does not make it, the pool depends entirely on intake. established physiology Source 1
   ⚖ When the balance tips

   too much — If more threonine is present than protein synthesis and mucus formation need, the excess is broken down in the liver; the body has no store for free threonine.

   too little — If little threonine is available, the body cannot switch to another source, because it does not make it itself; protein synthesis and mucus formation then receive less building material.

   established physiology · 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. The mucosa uses it mainly for its mucus. Source 1, 2↑ supplies The intestinal mucosa keeps a large share of the absorbed threonine for itself, mainly for its mucus; in animal studies only the rest reaches the liver and other tissues. observed in studies Source 1, 2
   ⚖ When the balance tips

   too much — If a lot of threonine arrives, the mucosa’s demand is met, and a larger share passes on into the blood.

   too little — If little threonine is available, the mucosa first keeps what it needs for its mucus; in animal models correspondingly less remains for other tissues.

   observed in studies · Source 1

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. The sugar chains bind water. Source 1, 3↑ supplies The dense sugar chains on threonine bind water and turn MUC2 into a gel. The inner layer keeps bacteria away from the cells of the gut wall; the outer layer serves gut bacteria as food. established physiology Source 3
   ⚖ When the balance tips

   too much — If a lot of MUC2 is made, the mucus layer grows; bacteria in the outer layer break down the sugar chains and use them as food.

   too little — If little threonine is available, animal models show less MUC2 being made and a thinner mucus layer; bacteria then get closer to the cells of the gut wall.

   observed in studies · Source 3, 1

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. Its carbon moves on into energy metabolism. Source 5, 1↑ supplies 2-Oxobutyrate is the transition into energy metabolism: via propionyl-CoA its carbon enters the citric acid cycle. In isolated mitochondria it is used there as fuel. observed in studies Source 6
   ⚖ When the balance tips

   too much — If 2-oxobutyrate builds up, it occupies a dehydrogenase complex in the mitochondrion that other keto acids also use; in isolated mitochondria it alters substrate-level phosphorylation.

   too little — If little 2-oxobutyrate forms, correspondingly little propionyl-CoA comes from threonine; the pathway then receives it from other sources such as isoleucine, valine or odd-chain fatty acids.

   observed in studies · Source 6, 7, 8

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⁺. This produces NADH. Source 6, 7↑ supplies The complex turns the keto acid into an activated acid and gains NADH for the respiratory chain in the process. Propionyl-CoA is then ready for its route into the citric acid cycle. established physiology Source 7, 8
   ⚖ When the balance tips

   too much — If propionyl-CoA builds up because the carboxylase cannot keep up, by-products such as propionic acid and propionylcarnitine form from it.

   too little — If little thiamine or NAD⁺ is available, the complex works more slowly, and 2-oxobutyrate is converted to propionyl-CoA more slowly.

   established physiology · Source 8, 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. With little biotin, propionyl-CoA builds up. Source 8, 9, 6↑ supplies Carboxylation adds one carbon and so prepares the conversion into a citric acid cycle molecule; for this it uses ATP and needs bound biotin. established physiology Source 8, 9
   ⚖ When the balance tips

   too much — If methylmalonyl-CoA builds up, part of it is split into methylmalonic acid, which appears in blood and urine.

   too little — If little biotin is available, the carboxylase works more slowly, and propionyl-CoA builds up before this step.

   established physiology · Source 10, 8

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. With little vitamin B12, methylmalonyl-CoA builds up. Source 10, 6↑ supplies Succinyl-CoA replenishes the citric acid cycle; the carbon from threonine thus becomes fuel. The step to succinate directly yields an energy-rich phosphate (substrate-level phosphorylation). established physiology Source 10, 6
   ⚖ When the balance tips

   too much — If plenty of succinyl-CoA arrives, it is converted further in the citric acid cycle; the cycle takes up the carbon from threonine as fuel.

   too little — If little vitamin B12 is available, the mutase cannot keep up; less succinyl-CoA forms, and methylmalonyl-CoA builds up before it.

   established physiology · Source 10

Further stations

• Dietary protein — source of threonine Threonine is a building block of dietary proteins. Digestive enzymes break these proteins down in the gut into single amino acids. They are the only source of threonine. Source 1↑ supplies Dietary protein is the only source of threonine: what digestion releases from it determines how much is available for protein synthesis, mucus formation and breakdown. established physiology Source 1
  ⚖ When the balance tips

  too much — If a lot of threonine arrives with food, the liver breaks down the part not built into proteins via 2-oxobutyrate.

  too little — If little protein arrives, less threonine is available; in animal models less mucus protein is then made in the gut wall.

  observed in studies · Source 1, 3

• Phosphate on threonine — serine/threonine kinases In proteins, the OH group of threonine is an attachment site for phosphate. Serine/threonine kinases add it and so switch the activity of many proteins. Phosphatases remove it again. Source 4↑ supplies A phosphate on threonine changes the charge and shape of the protein; depending on the protein, it switches an enzyme on or off. Phosphatases remove it again, so the signal stays reversible. established physiology Source 4
  ⚖ When the balance tips

  too much — If many phosphate groups stay attached because kinases work faster than phosphatases, the proteins concerned remain in the switched state.

  too little — If few phosphate groups are attached, the proteins stay in their initial state, and the kinase’s signal does not get through.

  established physiology · Source 4

• Threonine in the liver — start of breakdown Breakdown starts in the liver. The route via threonine dehydrogenase to glycine is shut down in humans, because the gene no longer yields a working enzyme. Here threonine becomes fuel. Source 1, 11↓ depletes Breakdown withdraws threonine from protein synthesis and turns it into fuel. In humans this runs mainly via the dehydratase, because threonine dehydrogenase does not yield a working enzyme. established physiology Source 1, 11
  ⚖ When the balance tips

  too much — If a lot of threonine reaches the liver, more runs through the dehydratase; the route to glycine is not open for this in humans.

  too little — If little threonine arrives, less is broken down, and more of it remains for protein synthesis.

  established physiology · Source 1, 11

Cofactors in this pathway

• Vitamin B6 (PLP) — Cofactor of serine/threonine dehydratase, which converts threonine to 2-oxobutyrate; with little PLP, breakdown stalls 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 and uses ATP Source 8, 9
• Vitamin B12 — As adenosylcobalamin, cofactor of methylmalonyl-CoA mutase on the way to succinyl-CoA; without B12 the step stalls 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; without TPP, it stalls Source 7
• NAD⁺ — Accepts the electrons in the keto acid dehydrogenase complex and becomes NADH 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; without it, breakdown stalls 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
• Biotin — propionyl-coa
• Vitamin B12 — methylmalonyl-coa
• Citric acid cycle — succinyl-coa
• IgG antibodies to food — 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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