DHAP

Meaning: DHAP is a three-carbon ketose phosphate. In glycolysis, aldolase cleaves fructose 1,6-bisphosphate into DHAP and glyceraldehyde 3-phosphate (G3P). Only G3P is oxidized by the next enzyme, G3P dehydrogenase. Triose phosphate isomerase therefore converts DHAP into G3P (ketone to aldehyde), so one glucose gives two G3P. This is why the payoff phase is counted twice per glucose.

DHAP has a second fate. It can be reduced by NADH to glycerol 3-phosphate, the backbone for triglycerides and phospholipids. In the liver, glycerol from fat breakdown can run the other way (glycerol → glycerol 3-phosphate → DHAP) and enter gluconeogenesis.

On the MCAT: Passages ask what happens if aldolase or triose phosphate isomerase is blocked. Without the isomerase, only half the carbons of glucose reach pyruvate, so net ATP falls from 2 to 0 per glucose, because the investment of 2 ATP is made before the split. Questions may also ask you to trace a labeled carbon through the split, or to link glycolysis to fat synthesis.

Common confusion: DHAP is not a dead-end byproduct, and it is not oxidized directly by G3P dehydrogenase. It must first be isomerized to G3P. The isomerase reaction is freely reversible, and it proceeds toward G3P because G3P is consumed downstream.