The citric acid cycle is a series of biological reactions that plays a central role in cell metabolism. The cycle includes dehydration reactions of both malic and citric acids, yielding fumaric and aconitic acids, respectively (all common names). Both proceed strictly by en/me-catalyzed (/////-elimination mechanisms.

(a) In dehydration only the hydrogen identified by an asterisk is removed, together with the OH group on the carbon below. Write the structures for fumaric and aconitic acids as they are formed in these reactions. Make sure that the stereochemistry of each product is clearly indicated,
(b) Specify the stereochemistry of each of these products, using either cis-trans or E.Z notation, as appropriate,
(c) Isocitric acid (shown in the margin) also is dehydrated by aconitase. How many stereo-isomers can exist for isocitric acid? Remembering that this reaction proceeds through anti elimination, write the structure of a stereoisomer of isocitric acid that will give on dehydration the same isomer of aconitic acid that is formed from citric acid, label the chiral carbons in this isomer of isocitric acid, using R.S notation.

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CO,H CO,H H* Fumarase fumaric acid HO CH Co,H Aconitase -H,O 1,CO-H → aconitic acid CO,H Malic acid CO H Citric acid