In a series of reactions catalyzed by aconitase, citrate is dehydrated and then rehydrated, the net process resulting in the movement of the hydroxyl group by one position.

Using 14C labelling, it was shown that although citrate is a symmetrical, achiral molecule, aconitase is selective for the end of the molecule toward which the hydroxyl group is moved in the transformation to isocitrate, meaning the process is selective for the end decarboxylated. Although the central carbon of citrate is achiral, the two -CH2COO- groups of citrate are sterically nonequivalent. From the perspective of either group the rest of the molecule will look different. This means that they can be distinguished by the active site of aconitase. Citrate is a prochiral molecule. Its substituents are distinguishable and it can become chiral in one step. This is exactly the kind of material that becomes an MCAT passage.