A 23-year-old bodybuilder visits his physician with complaints of fatigue, depression, insomnia, hair loss, and dry skin. He tells the physician he has been “bulking up” for an upcoming competition, and his meals consist mostly of eight raw eggs along with low-fat milk. The physician suspects biotin deficiency given the patients’ diet and symptoms. Which of the following is biotin required for?
- The formation of cis-retinal
- Transfer of 1-carbon units
- Hydroxylation reactions
- Carboxylation of pyruvate
- Decarboxylation of α-Keto acids
Answer- D- Carboxylation of pyruvate is the right answer. Biotin is required for carboxylation reactions. The common examples are-
a) Conversion of pyruvate to oxaloacetate- The first step of gluconeogenesis. The reaction is catalyzed by pyruvate carboxylase (figure).
Figure- Pyruvate to Oxaloacetate conversion
b) Conversion of Acetyl co A to Malonyl co A- The first step of the fatty acid synthesis. The reaction is catalyzed by Acetyl co A carboxylase.
c) Conversion of Propionyl co A to D-methyl malonyl co A. The reaction is catalyzed by propionyl co A carboxylase. Propionyl co A is utilized in the TCA cycle through the formation of succinyl co A. D and L- methyl malonyl co A are formed during the process of conversion of propionyl co A to succinyl co A.
In the given situation, signs and symptoms of the patient are highly suggestive of egg white injury that occurs upon consumption of raw eggs. Biotin present in egg yolk binds with Avidin of egg white due to its great affinity for Avidin making a nonfunctional Biotin- Avidin complex. A deficiency of biotin is precipitated that is manifested by hair loss; premature graying of hair, brittle nails, and dry skin. Biotin is considered a “beauty vitamin” due to its role in the maintenance of skin, hair, and nails.
As regards other options-
- The formation of cis retinal is mediated by Vitamin A.
- Transfer of 1-carbon units is carried out by folic acid
- Hydroxylation reactions require vitamin C as a coenzyme
- Decarboxylation of α-Keto acids is carried out by Thiamine. Multienzyme complexes are involved that also requires pantothenic acid, lipoic acid, riboflavin, and niacin. The examples are PDH complex, α- Keto glutarate dehydrogenase complex, α- Keto acid dehydrogenase complex, etc.
