Q. 7.52 - An Introduction to General, Organic, and Biological Chemistry

Question

In one of the reactions in the citric acid cycle, which provides energy, succinic acid is converted to fumaric acid. 

$C_4{H}_6{O}_4 \to C_4{H}_4{O}_4 + 2H$

Succinic acid    Fumaric acid 

The reaction is accompanied by a coenzyme, flavin adenine dinucleotide 

$\mathrm{FAD} + 2\mathrm{H} \to {\mathrm{FADH}}_2$

a. Is succinic acid oxidized or reduced? 

b. Is FAD oxidized or reduced? 

c. Why would the two reactions occur together?

Step-by-Step Solution

Answer

(a) Succinic acid is oxidized.

(b) FAD is reduced.

(c) When succinic acid is oxidized two hydrogen are released which gets add into FAD and reduced it. So, in order to utilize the hydrogen it is necessary to perform both reaction together.

Part (a) Step 1: Given Information

We are given the equations and we have to find out whether succinic acid is oxidized or a reduced

Part (a) Step 2: Explanation

In citric cycle the reaction is,

$C_4{H}_6{O}_4 \to C_4{H}_4{O}_4 + 2H$

and in this number of atoms of carbon and oxygen does not change during the reaction but number of hydrogen reduced by two and the reaction in which the number of hydrogen atoms decreases on the product side is termed as oxidation reaction.

Hence, the succinic acid is oxidized.

Part (b) Step 1: Given Information

We are given the equations and we have to find out whether FAD is oxidized or a reduced.

Part (b) Step 2: Explanation

The coenzyme reaction is,

$\mathrm{FAD} + 2\mathrm{H} \to {\mathrm{FADH}}_2$

and in this whole molecular formula but two hydrogen get add in it and the reaction in which the number of hydrogen atoms increases on the product side is termed as reduced reaction. 

Hence, FAD is reduced in this reaction.

Part (c) Step 1: Given Information

We are given with two equations and we have to find out the reason that why they always occur together.

Part (c) Step 2: Explanation

In the reaction succinic acid is oxidized, which forms four-carbon molecule two hydrogen atoms are released and that hydrogen atoms are taken by the enzyme called $FAD$and gets reduced So, in order to utilize the hydrogen it is necessary that these reactions occur together otherwise hydrogen will be an obstacle in further reactions.