Q. 18.41 - An Introduction to General, Organic, and Biological Chemistry

Question

Refer to the diagram of the citric acid cycle in Figure $18.13$ answer each of the following:

a. What are the six-carbon compounds?

b. How is the number of carbon atoms decreased?

c. What is the five-carbon compound? 

d. What are the decarboxylation reactions?

Step-by-Step Solution

Answer

(a) The six-carbon compounds are Citrate and isocitrate 

(b) The number of carbon atoms decreased by the decarboxylation process.

(c) The five-carbon compound is $\mathrm{\alpha }$-ketoglutarate

(d) The decarboxylation reaction is the oxidation event at different stages of citric acid cycle

Part (a) Step 1: Given information

We need to find out the six-carbon compounds

Part (a) Step 2: Explanation

Citrate and isocitrate are two six-carbon molecules that participate in the citric acid cycle (also known as the $\mathrm{TCA}$ cycle). Citrate is made when the enzyme citrate synthase catalyzes the condensation reaction of acetyl $\mathrm{COA}$ and oxaloacetate. Isocitrate, on the other hand, is generated when citrate is isomerized by the enzyme aconitase.

Part (b) Step 1: Given information

We need to find out the number of carbon atoms decreased 

Part (b) Step 2: Explanation

During the decarboxylation reaction, carbon atoms are reduced. Carbon dioxide is emitted from the substrate during this process, lowering the number of carbon atoms. The citric acid cycle's stages $3$ and $4$ involve decarboxylation.

Part (c) Step 1: Given information

We need to find out the five-carbon compound 

Part (c) Step 2: Explanation

In the citric acid cycle, $\mathrm{\alpha }$-ketoglutarate is a five-carbon molecule. Isocitrate dehydrogenase participates in the oxidation-decarboxylation of isocitrate to produce it. Isocitrate dehydrogenase converts the secondary alcohol of isocitrate to a ketone, which is followed by the decarboxylation of one carbon from isocitrate, which produces carbon dioxide as a by-product of the reaction.

Part (d) Step 1: Given information

We need to find out the meaning of decarboxylation reactions 

Part (d) Step 2: Explanation

In the citric acid cycle, oxidation events occur at stages $3$ and $8$. Step $3$ entails the oxidation of secondary alcohol in isocitrate, resulting in the formation of ketoglutarate with the help of isocitrate dehydrogenase. Malate dehydrogenase catalyzes the oxidation of secondary alcohol in malate, resulting in the formation of oxaloacetate.