Glycolysis is the first major pathway of cellular glucose metabolism. It occurs in the cytoplasm of virtually all living cells and converts one molecule of glucose into two molecules of pyruvate, producing a net gain of ATP and NADH.

The Phases of Glycolysis

1. Energy Investment Phase

The first half of glycolysis uses two ATP molecules to phosphorylate glucose, trapping it inside the cell. Glucose is converted to glucose-6-phosphate, then to fructose-6-phosphate, and finally to fructose-1,6-bisphosphate. This six-carbon sugar is then split into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).

2. Energy Payoff Phase

The second half of glycolysis generates energy. Each G3P molecule is oxidized and phosphorylated, producing 1,3-bisphosphoglycerate. The high-energy phosphate groups are then transferred to ADP to produce ATP. Through a series of steps, each G3P is converted to pyruvate, producing two ATP and one NADH per G3P.

3. Net Yield

Starting from one glucose molecule, glycolysis produces:

• 2 ATP (net, after subtracting the 2 used in the investment phase)
• 2 NADH
• 2 pyruvate molecules

4. Regulation

Glycolysis is regulated at three key irreversible steps. Phosphofructokinase-1 (PFK-1) is the most important regulatory enzyme. It is activated by AMP and ADP (low energy signals) and inhibited by ATP and citrate (high energy signals).

5. Fate of Pyruvate

Pyruvate can follow different paths depending on oxygen availability. Under aerobic conditions, it enters the mitochondria and is converted to acetyl-CoA for the citric acid cycle. Under anaerobic conditions, it is converted to lactate in animals or ethanol in yeast.

Practical Glycolytic Flux Assay

Glycolytic flux is most commonly measured by the extracellular acidification rate (ECAR) using a Seahorse XF Analyzer. Seed cells at 1–4 × 10⁴ cells/well in an XF96 cell culture plate and incubate overnight. One hour before the assay, replace the growth medium with Seahorse XF DMEM (pH 7.4) containing 10 mM glucose, 2 mM glutamine, and 1 mM pyruvate, and incubate at 37°C without CO2. Inject 10 mM glucose through port A to stimulate glycolysis — ECAR will rise as the cells produce lactate. Inject 1 µM oligomycin (ATP synthase inhibitor) through port B — this shifts energy production to glycolysis, causing a further ECAR increase that defines the maximal glycolytic capacity. Inject 50 mM 2-deoxyglucose (2-DG, a hexokinase inhibitor) through port C — ECAR drops to baseline, confirming that the measured signal is glycolysis-derived. The glycolytic rate is expressed as mpH/min per µg protein or per cell number. For a direct lactate dehydrogenase (LDH) assay, collect culture medium at intervals and measure lactate using an enzymatic kit coupled to NADH fluorescence (excitation 340 nm, emission 460 nm). LDH reaction: lactate + NAD⁺ → pyruvate + NADH + H⁺.

Real-World Application

The Warburg effect in cancer cells is demonstrated by comparing ECAR in A549 lung cancer cells vs. normal lung fibroblasts. Cancer cells show a 4-fold higher basal ECAR (65 vs. 16 mpH/min) and a blunted response to oligomycin, indicating that they are already relying heavily on glycolysis for ATP production. This glycolytic dependence is the basis for FDG-PET imaging in clinical oncology, where ¹⁸F-fluorodeoxyglucose accumulates in tumors.