glyceraldehyde

Meaning: Glyceraldehyde is the simplest aldose, a three-carbon sugar with an aldehyde at C1, so it is an aldotriose. Its C2 is the only stereocenter, so it has two enantiomers. In a Fischer projection with the aldehyde on top, D-glyceraldehyde has the C2 hydroxyl on the right and L-glyceraldehyde has it on the left. Other sugars are named D or L by comparing the chiral carbon farthest from the carbonyl to this reference. Its ketose isomer, dihydroxyacetone, has no stereocenter.

On the MCAT: Expect Fischer projections where you assign D or L, decide whether two sugars are enantiomers or diastereomers, or classify aldose versus ketose. In glycolysis, aldolase splits fructose 1,6-bisphosphate into dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (G3P). Triose phosphate isomerase converts DHAP to G3P, so each glucose yields two G3P. G3P dehydrogenase then oxidizes the aldehyde, adds inorganic phosphate, and reduces NAD⁺ to NADH, giving 1,3-bisphosphoglycerate.

Common confusion: Students treat D and L as the sign of optical rotation or as R and S. D/L is a structural label based on the hydroxyl position at the reference carbon. The lowercase d and l, or (+) and (−), describe measured rotation and must be determined by experiment. D-glyceraldehyde happens to be (+) and R, but that does not hold for every D sugar.