Citrate is a "prochiral" molecule, meaning that it is an achiral molecule (i.e. lacks a stereoce

Step 1: The first carboxyl group lost during the citric acid cycle is the one attached to C3 in

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Citrate is a "prochiral" molecule, meaning that it is an achiral molecule (i.e. lacks a stereocenter) that could become chiral through a change of one of its identical arms. Indeed, this is what occurs upon conversion of citrate to isocitrate, upon which both C2 and C3 become stereocenters in isocitrate. This concept is illustrated in the figure below: citrate (2R,3S)-isocitrate Considering that enzymes are stereospecific, aconitase, the enzyme that converts to citrate to isocitrate, can only catalyze the reaction that produces the molecule shown above, rather than transferring the OH group to C4. This has consequences on the downstream steps of the citric acid cycle. Answer the following questions about the steps that follow isocitrate formation to understand these consequences. a. Citrate and isocitrate are tricarboxylic acids. During the citric acid cycle, two oxidative decarboxylation events occur and two of these carboxylic acid groups are eliminated from metabolites, resulting in formation of CO2. Which carboxyl group is lost first? b. Which carboxyl group is lost second? b. The carbon of the carboxyl group attached to C4 in the isocitrate structure above originated from the acetyl group that most recently entered the citric acid cycle. At which step will this carbon be fully oxidized through an oxidative decarboxylation step (choose one of the statements below)? 1. During the first oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters 2. During the second oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters 3. During the first oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters 4. During the second oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters 5. During either oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters 6. During either oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters

Citrate is a "prochiral" molecule, meaning that it is an achiral molecule (i.e. lacks a stereocenter) that could become chiral through a change of one of its identical arms. Indeed, this is what occurs upon conversion of citrate to isocitrate, upon which both C2 and C3 become stereocenters in isocitrate. This concept is illustrated in the figure below:

citrate
(2R,3S)-isocitrate

Considering that enzymes are stereospecific, aconitase, the enzyme that converts to citrate to isocitrate, can only catalyze the reaction that produces the molecule shown above, rather than transferring the OH group to C4. This has consequences on the downstream steps of the citric acid cycle. Answer the following questions about the steps that follow isocitrate formation to understand these consequences.

a. Citrate and isocitrate are tricarboxylic acids. During the citric acid cycle, two oxidative decarboxylation events occur and two of these carboxylic acid groups are eliminated from metabolites, resulting in formation of CO2. Which carboxyl group is lost first?

b. Which carboxyl group is lost second?

b. The carbon of the carboxyl group attached to C4 in the isocitrate structure above originated from the acetyl group that most recently entered the citric acid cycle. At which step will this carbon be fully oxidized through an oxidative decarboxylation step (choose one of the statements below)?

1. During the first oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters
2. During the second oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters
3. During the first oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters
4. During the second oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters
5. During either oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters
6. During either oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters

Citrate is a "prochiral" molecule, meaning that it is an achiral molecule (i.e. lacks a stereocenter) that could become chiral through a change of one of its identical arms. Indeed, this is what occurs upon conversion of citrate to isocitrate, upon which both C2 and C3 become stereocenters in isocitrate. This concept is illustrated in the figure below:

citrate
(2R,3S)-isocitrate

Considering that enzymes are stereospecific, aconitase, the enzyme that converts to citrate to isocitrate, can only catalyze the reaction that produces the molecule shown above, rather than transferring the OH group to C4. This has consequences on the downstream steps of the citric acid cycle. Answer the following questions about the steps that follow isocitrate formation to understand these consequences.

a. Citrate and isocitrate are tricarboxylic acids. During the citric acid cycle, two oxidative decarboxylation events occur and two of these carboxylic acid groups are eliminated from metabolites, resulting in formation of CO2. Which carboxyl group is lost first?

b. Which carboxyl group is lost second?

b. The carbon of the carboxyl group attached to C4 in the isocitrate structure above originated from the acetyl group that most recently entered the citric acid cycle. At which step will this carbon be fully oxidized through an oxidative decarboxylation step (choose one of the statements below)?

1. During the first oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters
2. During the second oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters
3. During the first oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters
4. During the second oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters
5. During either oxidative decarboxylation step on the first turn of the citric acid cycle after the acetyl group enters
6. During either oxidative decarboxylation step on the second turn of the citric acid cycle after the acetyl group enters

Added by Michael O.

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