Glycolysis

Definition

• Glycolysis: the ten-step cytosolic pathway that oxidizes one glucose to two pyruvate, netting 2 ATP and 2 NADH. It requires no oxygen and no mitochondria, which makes it the universal "first pass" of carbohydrate catabolism in every human cell.
• Fate of pyruvate defines the context: with adequate oxygen and mitochondria, pyruvate is decarboxylated to acetyl-CoA for the TCA cycle; without them, lactate dehydrogenase reduces it to lactate to regenerate NAD⁺ so flux can continue.
• Obligate glycolyzers: mature red blood cells (no mitochondria), the renal medulla, corneal and lens tissue, and white blood cells — the tissues that fail first when a glycolytic enzyme is defective.

Why it matters clinically

• Lactate as a bedside readout: when oxygen delivery falls short of demand, or in sepsis-associated metabolic derangement, glycolytic flux outstrips oxidative capacity and lactate rises. The Surviving Sepsis Campaign uses serum lactate measurement and remeasurement to identify tissue hypoperfusion and guide resuscitation.
• Inherited enzymopathies present as nonspherocytic hemolytic anemia (pyruvate kinase), exercise intolerance with myoglobinuria (PFK-1), or diet-triggered hypoglycemia and liver injury (aldolase B).
• Glucose sensing: glucokinase sets the threshold for β-cell insulin release, so its mutations produce a distinct monogenic diabetes phenotype that the ADA Standards of Care specifically warns is misclassified as type 1 or type 2 diabetes.
• Oncology and imaging: tumor reliance on aerobic glycolysis is the physiologic basis of FDG-PET staging used across NCCN pathways.

Epidemiology worth recalling

• Pyruvate kinase deficiency is the most common inherited defect of the glycolytic pathway itself and the leading enzymatic cause of hereditary nonspherocytic hemolytic anemia; it is nonetheless rare, and autosomal recessive.
• Hereditary fructose intolerance is rare and typically unmasked at weaning when fruit, sucrose, or sorbitol is introduced.
• Monogenic diabetes accounts for only a small fraction of all diabetes, with GCK-MODY among its most frequent forms — classically a lean, antibody-negative young patient with mild stable fasting hyperglycemia.

Preparatory (investment) phase — 2 ATP spent

• Hexokinase/glucokinase: glucose → glucose-6-phosphate, irreversible. Hexokinase is ubiquitous, low Km/low Vmax, feedback-inhibited by G6P; glucokinase (liver, pancreatic β cell) is high Km/high Vmax, insulin-inducible, sequestered by glucokinase regulatory protein and not inhibited by G6P.
• Phosphoglucose isomerase: G6P → fructose-6-phosphate (reversible aldose→ketose).
• PFK-1: F6P → fructose-1,6-bisphosphate. The committed, rate-limiting step. Inhibited by ATP and citrate (Krebs intermediate signaling fuel excess); activated by AMP and fructose-2,6-bisphosphate.
• Aldolase A: F-1,6-BP → DHAP + glyceraldehyde-3-phosphate; triose phosphate isomerase interconverts them so both flow forward.

Payoff phase — 4 ATP + 2 NADH generated

• Glyceraldehyde-3-phosphate dehydrogenase: uses inorganic phosphate and NAD⁺ to make 1,3-bisphosphoglycerate; this is where arsenate substitutes for Pi and uncouples ATP production.
• Phosphoglycerate kinase: substrate-level phosphorylation → 3-phosphoglycerate + ATP.
• Phosphoglycerate mutase, then enolase (→ phosphoenolpyruvate; inhibited by fluoride, the basis of gray-top sodium fluoride glucose tubes).
• Pyruvate kinase: PEP → pyruvate + ATP, irreversible. Feed-forward activated by F-1,6-BP; inhibited by ATP and alanine.

Hormonal control of the F-2,6-BP switch

• Insulin (fed): dephosphorylates the bifunctional PFK-2/FBPase-2 enzyme → PFK-2 active → ↑F-2,6-BP → glycolysis on, gluconeogenesis off.
• Glucagon (fasting): protein kinase A phosphorylation flips it to FBPase-2 → ↓F-2,6-BP; PKA also inactivates liver pyruvate kinase.

• Pyruvate kinase deficiency: autosomal recessive, the classic glycolytic hemolytic anemia. Mature RBCs lack mitochondria, so glycolysis is their only ATP source; ATP failure collapses Na⁺/K⁺-ATPase, cells dehydrate and become rigid echinocytes cleared by splenic macrophages (extravascular hemolysis with jaundice, splenomegaly, pigment gallstones, neonatal hyperbilirubinemia). Upstream 2,3-BPG accumulates, right-shifting the oxygen dissociation curve, so patients are often less symptomatic than the hemoglobin predicts. Care is supportive (transfusion, folate, splenectomy in severe cases); the PK activator mitapivat is FDA-approved for adults with PK deficiency.
• **PFK-1 (muscle) deficiency — Tarui disease, glycogen storage disease VII: exercise intolerance, cramps, myoglobinuria, and no rise in venous lactate** with ischemic forearm exercise; unlike McArdle disease (myophosphorylase), there is no "second wind" because even blood-borne glucose cannot be used.
• Aldolase B deficiency (hereditary fructose intolerance): vomiting, hypoglycemia, and hepatic dysfunction after sucrose/fructose/sorbitol is introduced at weaning. Trapped fructose-1-phosphate sequesters intracellular phosphate, crippling glycogenolysis and gluconeogenesis. Treatment is lifelong elimination of fructose, sucrose, and sorbitol.
• Triose phosphate isomerase deficiency: hemolytic anemia plus progressive neurodegeneration — the most severe glycolytic enzymopathy. Phosphoglycerate kinase deficiency is X-linked and pairs hemolysis with myopathy and CNS disease.
• Glucokinase mutations: heterozygous inactivating mutations cause GCK-MODY (MODY 2) — mild, stable, nonprogressive fasting hyperglycemia in a lean young patient with a strong family history; the ADA Standards of Care note that monogenic diabetes is frequently misclassified as type 1 or type 2 and that GCK-MODY generally requires no pharmacotherapy. Activating mutations cause congenital hyperinsulinism.
• Tumor metabolism: the Warburg effect (aerobic glycolysis) underlies FDG-PET — hexokinase phosphorylates FDG, and FDG-6-phosphate cannot leave the cell.

• PFK-1 is the answer to "rate-limiting step of glycolysis"; F-2,6-BP is the answer to "most potent activator." The stem hint for fed state is insulin → dephosphorylated bifunctional enzyme → PFK-2 active.
• Net yield per glucose: 2 ATP, 2 NADH, 2 pyruvate. Gross ATP is 4; two are consumed in the preparatory phase. Substrate-level phosphorylation occurs at phosphoglycerate kinase and pyruvate kinase — no oxygen required, which is why glycolysis is the only ATP source for the mature RBC.
• Irreversible steps = the three regulated enzymes: hexokinase/glucokinase, PFK-1, pyruvate kinase. These are the steps gluconeogenesis must bypass.
• **Hemolytic anemia + echinocytes + elevated 2,3-BPG + normal osmotic fragility** → pyruvate kinase deficiency. The distractor is spherocytes/positive osmotic fragility, which points to hereditary spherocytosis, and hemolysis after oxidant stress with bite cells and Heinz bodies, which points to G6PD deficiency (HMP shunt, not glycolysis).
• Exercise intolerance with no lactate rise: PFK-1 deficiency (Tarui) shows no second wind; McArdle disease does. Both give myoglobinuria, so use the second-wind detail to separate them.
• Hypoglycemia timed to weaning/fruit juice → aldolase B; the single best immediate step is IV dextrose, followed by permanent dietary fructose/sucrose/sorbitol restriction. Contrast with essential fructosuria (fructokinase deficiency): benign, asymptomatic, reducing sugar in urine, no treatment.
• Lean adolescent, mild stable hyperglycemia, autosomal dominant family history, negative islet autoantibodies → GCK-MODY; per the ADA Standards of Care, recognizing monogenic diabetes matters because these patients typically need no insulin or sulfonylurea.
• Test artifacts examiners love: fluoride inhibits enolase (why glucose tubes preserve the specimen) and arsenate uncouples the GAPDH step, yielding NADH but no ATP.

• 10-step cytoplasmic pathway converting glucose → pyruvate, producing 2 ATP net + 2 NADH per glucose
• Rate-limiting enzyme: Phosphofructokinase (PFK) — inhibited by ATP/citrate, activated by AMP/ADP and F-2,6-BP
• Hexokinase traps glucose in cell via glucose-6-phosphate (not freely diffusible)
• Pyruvate kinase deficiency → hemolytic anemia (loss of ATP-dependent RBC membrane pump)
• Aldolase deficiency (type I fructose intolerance) → fructose accumulation, severe hypoglycemia

• Hypoglycemic neonate after fructose feeding → aldolase B deficiency (fructose intolerance)
• Hemolytic anemia with jaundice, normal reticulocyte response → pyruvate kinase deficiency
• Lactic acidosis + high pyruvate/lactate ratio → hypoxic state or mitochondrial dysfunction (glycolysis upregulated)
• Hexokinase deficiency → pseudohyperglycemia (glucose accumulates, but cannot enter cells for metabolism)

Mnemonic (PFK Regulation): PFK ↑ with "AMP it up" (↑AMP, ↓ATP); inhibited by "Citrate/ATP = energy full"

1. 1.Confusing hexokinase vs. glucokinase: Hexokinase is inhibited by G6P (product inhibition) and found in all tissues; glucokinase (liver/pancreas) is NOT inhibited by G6P and acts as a glucose sensor.
2. 2.Misremembering PFK regulation: Students forget F-2,6-BP is the most potent PFK activator—produced when insulin is high (fed state). Citrate/ATP inhibit PFK (energy abundant).
3. 3.Equating pyruvate kinase deficiency with metabolic acidosis: PK deficiency causes hemolysis from ATP loss, not lactic acidosis; lactate actually may be LOWER (less pyruvate available).

• Glycolytic enzyme deficiencies: Supportive care (transfusion for hemolysis, avoid fructose/galactose in aldolase deficiency)
• Acute hypoglycemia (aldolase B deficiency): IV dextrose immediately
• Lactic acidosis (glycolytic overload): Treat underlying cause (restore oxygenation, thiamine if alcohol-related); sodium bicarbonate may be used if pH <7.1