Which of the following correctly describes the stereospecific formation of a citric acid cycle intermediate?

Question

Quizplus · Accepted Answer

11ecba2d_0e3a_7731_a3bf_6d2a05b88eaf_MD0008_00 Stereoisomers are molecules that contain atoms bonded in the same order but differ in how the atoms are oriented in space. One or more atoms are bonded to four distinct chemical groups, and these atoms are referred to as chiral centers or stereocenters, which can have different arrangements. Stereospecific reactions produce chiral molecules in a specific arrangement. The citric acid cycle produces only two intermediates with chiral centers: isocitrate and malate. Therefore, only the reactions that form these molecules are classified as stereospecific. The first reaction in the cycle is the conversion of citrate to isocitrate by the aconitase enzyme through an intermediate called cis-aconitate. Aconitase catalyzes consecutive dehydration and hydration reactions that exchange the hydrogen atom and hydroxyl group of the second and third carbon. This isomerase activity results in the transformation of citrate, which is achiral (no chiral centers), into isocitrate, which contains two new chiral centers. Similarly, fumarate is converted to malate by fumarase in a stereospecific reaction. The enzyme referred to as fumarase splits a molecule of water and adds the hydroxyl group and hydrogen atom across the C=C double bond in fumarate. This reaction forms a new chiral at the second carbon, and generates L-malate only (Choice B). Furthermore, this is a hydration reaction, not a condensation reaction. (Choice A) Synthases such as citrate synthase do not require energy to break or form new bonds, and do not consume ATP. (Choice C) The energy from cleavage of the thioester bond in succinyl-CoA is used by succinyl-CoA synthetase to produce GTP. Educational objective: Chiral centers, also referred to as stereocenters, are atoms bonded to four distinct chemical groups with different possible arrangements in space. Stereospecific reactions produce chiral molecules, which have chiral centers in a specific conformation. The conversion of citrate into isocitrate by aconitase and the transformation of fumarate into malate by fumarase are stereospecifc reactions.

Which of the Following Correctly Describes the Stereospecific Formation of a Citric