Question

In the degradation pathway for isoleucine (Fig. 21-21), draw the reactions that convert tigyl CoAto acetyl CoA andpropionyl CoA.

Short answer

Isoleucine is a branched and non-polar, uncharged, and essential amino acid. Degradation of such branched-chain amino acids occurs in three steps:

1. Transamination: This process involves transferring amino groups to a new amino acid and leaving behind the $\alpha$keto acid of the original amino acid.
2. Oxidative decarboxylation: This process involves oxidation reactions in which the carboxylate groups are removed and results in the formation of carbon dioxide.
3. Dehydrogenation by $FAD$: It is a process in which the hydrogen $\left(H\right)$is removed from the amino acid, resulting in the formation of a new compound.
   
   

Step-by-step solution

Degradation of branched-chain amino acid

Isoleucine is a branched and non-polar, uncharged, and essential amino acid. Degradation of such branched-chain amino acids occurs in three steps:

1. Transamination: This process involves transferring amino groups to a new amino acid and leaving behind the $\alpha$keto acid of the original amino acid.
2. Oxidative decarboxylation: This process involves oxidation reactions in which the carboxylate groups are removed and results in the formation of carbon dioxide.
3. Dehydrogenation by $FAD$: It is a process in which the hydrogen $\left(H\right)$is removed from the amino acid, resulting in the formation of a new compound.
   
   

Degradation pathway of isoleucine

After the transamination, oxidative decarboxylation, and dehydrogenation steps, isoleucine is converted to $tiglyl\mathit{}CoA$. This compound further undergoes degradation to $acetyl\mathit{}CoA$and $propionyl\mathit{}CoA$in 3 steps;

1. Addition of - $OH$group to double bond by an enzyme called hydrolase.
2. Oxidation of an intermediate is formed by moving the $H$from the intermediate to $NA{D}^{\mathit{+}}$
3. The final step breaks intermediate to localid="1649405842430" $acetyl\mathit{}CoA\mathrm{and}propionyl\mathit{}CoA$. This reaction is catalyzed by an enzyme called thiolase.

Degradation reactions of isoleucine

Thus, $tiglyl\mathit{}CoA$is converted to $acetyl\mathit{}CoA\mathit{}and\mathit{}propionyl\mathit{}CoA$in three steps- the addition of water, oxidation by $NA{D}^{\mathit{+}}$, and degradation by $CoASH$.