U01.01.069 Vitamin B7

Vitamin B7, also known as biotin, is a water-soluble B vitamin essential for metabolic processes. It serves as a cofactor for several key carboxylation reactions.

Functions of Vitamin B7

Biotin acts as a cofactor for carboxylation enzymes (enzymes that add a 1-carbon group). These enzymes and their reactions include:

Enzyme	Reaction	Metabolic Pathway
Pyruvate carboxylase	Pyruvate (3C) → Oxaloacetate (4C)	Gluconeogenesis
Acetyl-CoA carboxylase	Acetyl-CoA (2C) → Malonyl-CoA (3C)	Fatty acid synthesis
Propionyl-CoA carboxylase	Propionyl-CoA (3C) → Methylmalonyl-CoA (4C)	Fatty acid metabolism (odd-chain)

Deficiency of Vitamin B7

Biotin deficiency is relatively rare, but when present, it can cause:

Symptoms:
- Dermatitis (skin inflammation)
- Enteritis (intestinal inflammation)
- Alopecia (hair loss)
Causes:
- Long-term antibiotic use: Reduces gut bacteria that produce biotin.
- Excessive raw egg white ingestion: Raw egg whites contain avidin, a protein that “avidly” binds biotin, preventing its absorption.

Mnemonic: “Avidin in egg whites avidly binds biotin.”

Key Points to Remember

Vitamin B7 deficiency is rare, thanks to its production by gut microbiota.
Biotin is crucial for carboxylation reactions in metabolism.
Watch for dermatitis, alopecia, and enteritis in clinical scenarios suggesting deficiency.
Dietary advice: Avoid excessive raw egg whites to prevent binding of biotin by avidin.

USMLE Step 1 Review Series

Curriculum

U01.01.069 Vitamin B7

Functions of Vitamin B7

Deficiency of Vitamin B7

Key Points to Remember

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