MCAT Glycolysis

Simple glycolysis pathway reaction steps.

Step 1

Glucose ---> Glucose-6-Phosphate (G6P). Enzyme: Hexokinase (all tissues) or Glucokinase (liver). Cofactor: ATP -> ADP + H+. Importance: Irreversible & trapping step. Phosphorylates glucose to place a negative charge on it, trapping it inside the cell so it cannot exit via GLUT transporters.

Step 2

Glucose-6-Phosphate (G6P) ---> Fructose-6-Phosphate (F6P). Enzyme: Phosphoglucose Isomerase.

Step 3

Fructose-6-Phosphate (F6P) ---> Fructose-1,6-Bisphosphate (F-1,6-BP). Enzyme: Phosphofructokinase-1 (PFK-1). Cofactor: ATP -> ADP + H+. Importance: Committed & rate-limiting step. Main control hub of glycolysis; inhibited by high ATP and citrate; activated by high AMP and Fructose-2,6-bisphosphate (F-2,6-BP).

Step 4

Fructose-1,6-Bisphosphate (F-1,6-BP) ---> Dihydroxyacetone phosphate (DHAP) + Glyceraldehyde-3-phosphate (GLAP/GAP). Enzyme: Aldolase.

Step 5

Dihydroxyacetone phosphate (DHAP) ---> Glyceraldehyde-3-phosphate (GLAP/GAP). Enzyme: Triose Phosphate Isomerase. Importance: Unifies the pathway so 2 identical 3-carbon GLAP molecules proceed into the payoff phase (doubling all downstream yields).

Step 6

Glyceraldehyde-3-phosphate (GLAP/GAP) ---> 1,3-Bisphosphoglycerate (1,3-BPG). Enzyme: Glyceraldehyde-3-Phosphate Dehydrogenase (GAPDH). Cofactor: NAD+ + Pi -> NADH + H+. Importance: Oxidation & NADH yield step. The only step in glycolysis that generates reduced NADH coenzymes for the electron transport chain.

Step 7

1,3-Bisphosphoglycerate (1,3-BPG) ---> 3-Phosphoglycerate (3-PG). Enzyme: Phosphoglycerate Kinase. Cofactor: ADP -> ATP (Substrate-level phosphorylation).

Step 8

3-Phosphoglycerate (3-PG) ---> 2-Phosphoglycerate (2-PG). Enzyme: Phosphoglycerate Mutase.

Step 9

2-Phosphoglycerate (2-PG) ---> Phosphoenolpyruvate (PEP). Enzyme: Enolase. Cofactor: Releases H2O.

Step 10

Phosphoenolpyruvate (PEP) ---> Pyruvate. Enzyme: Pyruvate Kinase. Cofactor: ADP + H+ -> ATP (Substrate-level phosphorylation). Importance: Irreversible final payoff & feed-forward step. Generates ATP and pyruvate; allosterically activated via feed-forward regulation by F-1,6-BP from Step 3.

Products

2 ATP, 2 NADH, 2 Pyruvate