In the presence of the enzyme aconitase, the double bond of aconitic acid undergoes hydration. The reaction is reversible, and the following equilibrium is established:

(a) The major tricarboxylic acid present is citric acid, the substance responsible for the tart taste of citrus fruits. Citric acid is achiral. What is its structure?
(b) What must be the constitution of isocitric acid? (Assume that no rearrangements accompany hydration.) How many stereoisomers are possible for isocitric acid?

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HO2C CO H H-0 Isocitric acid с-с Citric acid сн.co2H CGHgO7 6% at equilibrium) Aconitic acid (4% at equilibrium) (90% at equilibrium)