Question

When oxaloacetic acid and acetyl-coenzyme A (acetyl-CoA) labeled with radioactive carbon-14 in position 2 are incubated with citrate synthase, an enzyme of the tricarboxylic acid cycle, only the following enantiomer of \(\left[2{ }^{14} \mathrm{C}\right]\) citric acid is formed stereoselectively. Note that citric acid containing only \({ }^{12} \mathrm{C}\) is achiral. Assign an \(R\) or \(S\) configuration to this enantiomer of \(\left[2{ }^{-14} \mathrm{C}\right]\) citric acid. (Note: Carbon-14 has a higher priority than carbon-12.)

Short answer

The enantiomer of [2^14C] citric acid formed stereoselectively is of S configuration.

Step-by-step solution

Understand the structure of citric acid

First, we need to know the structure of citric acid to determine the configuration of the enantiomer. Citric acid structure is as follows: ``` OH-COOH | HO-C-COOH | C-H (C-14) | OH ``` In this structure, the chiral center is the C atom (C-14) in position 2.

Assign priority to the groups attached to the chiral center

Based on the Cahn-Ingold-Prelog rules, we will assign the priority to the groups attached to the chiral center. Since the carbon-14 has a higher priority than the carbon-12, the priorities are: 1. COOH group (with the \(\left[2{ }^{12} \mathrm{C}\right]\)) 2. COOH group (with the \(\left[2{ }^{14} \mathrm{C}\right]\)) 3. OH group 4. H atom

Determine R or S configuration

To determine the configuration, we need to place the chiral center in the center of an imaginary 3D space and arrange the groups according to their priority: The lowest-priority group (H atom) should be pointing away from you (towards the back). Now, considering the positions of the other three groups: 1. COOH group (with the \(\left[2{ }^{12} \mathrm{C}\right]\)): Highest priority (1) 2. COOH group (with the \(\left[2{ }^{14} \mathrm{C}\right]\)): Second priority (2) 3. OH group: Third priority (3) Like reading a book, check if the highest priorities are arranged clockwise (like "d" or R) or counterclockwise (like "l" or S). When looking at the 3D structure, the arrangement of priorities 1, 2, and 3 counterclockwise, which assigns it an S configuration.

Answer

The enantiomer of \(\left[2{ }^{14} \mathrm{C}\right]\) citric acid formed stereoselectively is of S configuration.