Four metabolic routes replenish TCA cycle intermediates. Hover, click, and trace each reaction from substrate to the cycle entry point.
Each anaplerotic pathway feeds a specific intermediate. The dashed arrows show where substrates enter.
The primary anaplerotic enzyme — carboxylates pyruvate to oxaloacetate (OAA) using biotin and ATP.
ATP-dependent carboxylation of the biotin prosthetic group. CO2 is activated and tethered to biotin's ureido nitrogen in the BC domain.
BC domain Biotin Mg²⁺The carboxyl group swings on the biotin arm (~14 Å tether) from the BC domain to the CT domain, where it's transferred to pyruvate's C-3 position, yielding oxaloacetate.
CT domainAcetyl-CoA is an obligate allosteric activator. When acetyl-CoA accumulates (signaling cataplerotic drain), PC activity increases to replenish OAA — a classic feedback-forward mechanism.
Acetyl-CoA ⊕PC deficiency (OMIM 266150) causes lactic acidosis, hypoglycemia, and neurological damage. Type B (French) is the most severe with neonatal onset. Triheptanoin (C7 oil) provides odd-chain anaplerotic substrate that bypasses the PC block.
View disorder details →Two-step conversion of glutamine to α-ketoglutarate — the dominant anaplerotic route in rapidly dividing cells and tumors.
Mitochondrial glutaminase hydrolyzes glutamine's amide bond, releasing NH4+ and generating glutamate. Two isoforms: GLS1 (kidney-type, dominant in tumors) and GLS2 (liver-type).
GLS1 / GLS2 phosphate ⊕Oxidative deamination of glutamate produces α-KG plus a second NH4+. Uses NAD+ or NADP+. Alternative: transaminases (ALT, AST) can convert glutamate to α-KG without releasing free ammonia.
GLUD1 NAD⁺Rapidly dividing cells are "glutamine-addicted" — they siphon TCA intermediates for biosynthesis (cataplerosis) and rely on glutaminolysis to refill the cycle. c-MYC transcriptionally upregulates GLS1. This is why CB-839 (telaglenastat), a GLS inhibitor, is in clinical trials.
Glutaric Acidemia Type I (GCDH deficiency) disrupts lysine/tryptophan catabolism that feeds α-KG. Hyperinsulinism-hyperammonemia syndrome (GLUD1 gain-of-function) causes excessive GDH activity with hypoglycemia and hyperammonemia.
View disorder details →Odd-chain fatty acids and branched-chain amino acids generate propionyl-CoA, which enters the TCA cycle as succinyl-CoA via a biotin- and B12-dependent route.
Biotin-dependent carboxylation of propionyl-CoA, consuming ATP and CO2. The heterododecamer (α6β6) is encoded by PCCA and PCCB. Deficiency causes propionic acidemia.
PCCA/PCCB Biotin ATPMCEE converts the D-stereoisomer to the L-form — the only substrate recognized by the downstream mutase. Rarely deficient clinically but essential for flux.
MCEEAdenosylcobalamin (vitamin B12)-dependent isomerase catalyzes a remarkable carbon-skeleton rearrangement. Deficiency causes methylmalonic acidemia. Complementation groups: mut0, mut−, cblA, cblB.
MUT AdoCbl (B₁₂)Triheptanoin (C7:0 triglyceride, Dojolvi) provides odd-chain fatty acids that are β-oxidized to propionyl-CoA — deliberately feeding this pathway. It bypasses upstream enzymatic blocks in FAO disorders and refills the succinyl-CoA pool.
This pathway harbors two of the most common organic acidemias: propionic acidemia (PCC deficiency) and methylmalonic acidemia (MUT / cbl deficiency). Both present with metabolic crisis, hyperammonemia, and progressive neurological damage.
View PA details → View MMA details →Phosphoenolpyruvate carboxykinase bridges the TCA cycle and gluconeogenesis. In certain tissues, the reverse reaction replenishes OAA.
PEPCK is unique among anaplerotic enzymes because it operates in both directions depending on tissue context. In liver and kidney, it primarily drives gluconeogenesis (OAA → PEP). In muscle and tumors, the reverse reaction can replenish OAA.
PCK1 / PCK2 GTP Mn²⁺PCK1 (cytosolic) is the classic gluconeogenic enzyme, induced by glucagon and cortisol. PCK2 (mitochondrial) is constitutively expressed and increasingly recognized as an anaplerotic enzyme in cancer cells and brown adipose tissue.
PEPCK sits at the crossroads of gluconeogenesis, the TCA cycle, and amino acid catabolism. It enables PEP recycling — the TCA cycle converts amino acid carbons through OAA → PEP → glucose, or reverses to refill OAA when cataplerosis outpaces supply.
PEPCK deficiency is exceedingly rare. Cytosolic form loss causes hypoglycemia and hepatic failure. Mitochondrial form loss is associated with lactic acidosis. Overexpression of PCK2 is emerging as a metabolic hallmark of certain cancers, where it supports biosynthesis and proliferation.