Glycolysis & Gluconeogenesis
The reciprocal pathways, their rate-limiting gates, and why fat can't become glucose
Glycolysis: glucose to pyruvate
Where and why. Glycolysis runs in the cytoplasm of every cell, splitting one glucose into two pyruvate. It needs no oxygen, and in red blood cells, which lack mitochondria, it is the only source of ATP.
Energy balance. Two ATP are invested and four are made, for a net 2 ATP plus 2 NADH per glucose. The ATP comes from substrate-level phosphorylation at two steps — phosphoglycerate kinase (1,3-BPG → 3-PG) and pyruvate kinase (PEP → pyruvate) — which is why glycolysis can work anaerobically.
- Hexokinase / glucokinaseGlucose → glucose-6-phosphate; uses ATP and traps glucose in the cell
- PFK-1Fructose-6-phosphate → fructose-1,6-bisphosphate; uses ATP; rate-limiting, committed step
- Glyceraldehyde-3-phosphate dehydrogenaseMakes NADH and 1,3-BPG
- Phosphoglycerate kinase1,3-BPG → 3-PG; makes ATP
- Pyruvate kinasePEP → pyruvate; makes ATP
Regulation: the gates and the switch
PFK-1. The rate-limiting, committed step. It is activated by AMP and fructose-2,6-bisphosphate (its most potent activator) and inhibited by ATP and citrate — signals that energy is already plentiful.
The fructose-2,6-bisphosphate switch. A single bifunctional enzyme carries both PFK-2 and fructose-2,6-bisphosphatase (FBPase-2) activity and sets the F-2,6-BP level:
- Fed (insulin) — enzyme dephosphorylated, PFK-2 active, F-2,6-BP high, PFK-1 on: glycolysis runs.
- Fasting (glucagon, via cAMP and protein kinase A) — enzyme phosphorylated, FBPase-2 active, F-2,6-BP low, PFK-1 off: gluconeogenesis runs.
Because one signal controls both directions, the two pathways do not run at once in a wasteful futile cycle.
Pyruvate kinase. Activated in feed-forward fashion by fructose-1,6-bisphosphate; inhibited by ATP and alanine, and in the liver by glucagon-driven phosphorylation.
| Feature | Hexokinase | Glucokinase |
|---|---|---|
| Location | Most tissues | Liver, pancreatic β cells |
| Km / affinity | Low Km (high affinity) | High Km (lower affinity) |
| Vmax | Low | High |
| Regulation | Inhibited by glucose-6-phosphate | Induced by insulin; not inhibited by glucose-6-phosphate |
| Role | Traps glucose in every cell, even at low levels | Glucose sensor; handles the load when glucose is high |
Fates of pyruvate
| Product | Enzyme (cofactor) | Purpose |
|---|---|---|
| Alanine | ALT (B6) | Carries nitrogen from muscle to liver (Cahill cycle) |
| Oxaloacetate | Pyruvate carboxylase (biotin, ATP) | Gluconeogenesis; replenishes TCA intermediates (anaplerosis) |
| Acetyl-CoA | Pyruvate dehydrogenase (thiamine, lipoic acid, others) | Enters the TCA cycle; irreversible |
| Lactate | LDH (uses NADH) | Regenerates NAD+ so anaerobic glycolysis continues (Cori cycle) |
Cori and Cahill cycles. Lactate from red cells and exercising muscle travels to the liver and is rebuilt into glucose (Cori cycle). Alanine from muscle carries nitrogen to the liver for urea synthesis while its carbon skeleton feeds gluconeogenesis (Cahill cycle).
Gluconeogenesis
Purpose and site. Gluconeogenesis maintains blood glucose during fasting once liver glycogen is depleted (about 12–24 hours). It occurs mainly in the liver, with contributions from the kidney cortex and intestinal epithelium.
Bypass enzymes. Glycolysis has three irreversible steps (hexokinase/glucokinase, PFK-1, pyruvate kinase), so gluconeogenesis detours around them with four enzymes. Mnemonic: Pathway Produces Fresh Glucose.
- Pyruvate carboxylasePyruvate → oxaloacetate; mitochondria; needs biotin and ATP; activated by acetyl-CoA
- PEPCKOxaloacetate → PEP; uses GTP
- Fructose-1,6-bisphosphataseFructose-1,6-bisphosphate → fructose-6-phosphate; rate-limiting
- Glucose-6-phosphataseFinal step, in the ER; liver and kidney (muscle lacks it)
Substrates. Lactate (Cori cycle), glycerol from triglyceride breakdown, glucogenic amino acids (especially alanine), and propionyl-CoA from odd-chain fatty acids, which becomes succinyl-CoA.
Why fat cannot become glucose. Even-chain fatty acids yield only acetyl-CoA. Pyruvate dehydrogenase is irreversible, so acetyl-CoA cannot return to pyruvate, and in the TCA cycle its two carbons are lost as CO2 — no net carbon remains for glucose. The exceptions are odd-chain fatty acids (via propionyl-CoA) and the glycerol backbone of triglycerides.
| Feature | Glycolysis | Gluconeogenesis |
|---|---|---|
| Location | All cells, cytoplasm | Mainly liver; also kidney cortex, intestinal epithelium |
| Rate-limiting enzyme | PFK-1 | Fructose-1,6-bisphosphatase |
| State / hormone | Fed, insulin | Fasting, glucagon |
| F-2,6-BP | High (PFK-2 active) | Low (FBPase-2 active) |
| Energy | Net +2 ATP, 2 NADH | Costs about 6 ATP equivalents per glucose |
Clinical correlations
Pyruvate kinase deficiency. Autosomal recessive, and the second most common red-cell enzyme defect after G6PD deficiency. RBCs cannot make enough ATP, so their membranes stiffen and the spleen removes them (extravascular hemolysis); burr cells (echinocytes) may be seen. Upstream intermediates accumulate and 2,3-BPG rises, right-shifting the O2 dissociation curve. Classic presentation: hemolytic anemia in a newborn.
Arsenic. Arsenic inhibits lipoic acid–dependent enzymes, including pyruvate dehydrogenase. Arsenate also substitutes for phosphate at the step that forms 1,3-BPG, so the phosphoglycerate kinase ATP is never captured — glycolysis proceeds but yields no net ATP. Presentation: garlic breath, vomiting, rice-water stools, and QT prolongation, for example after drinking contaminated well water.
Alcohol and fasting hypoglycemia: ethanol metabolism raises the NADH/NAD+ ratio, diverting pyruvate to lactate and oxaloacetate to malate, which starves gluconeogenesis.