U01.01.079 Vitamin B7

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Learning Objectives

  • Identify Biotin as the essential cofactor for carboxylation reactions.
  • Understand the three critical carboxylase enzymes and their metabolic pathways.
  • Recognize the clinical causes of deficiency, including avidin interference and antibiotic use.

1. Biochemical Function: The Carboxylation Cofactor

Vitamin B7 (Biotin) functions as a coenzyme for enzymes that add a 1-carbon group (in the form of CO_2) to various substrates. It is a “carrier” of activated carbon dioxide.

The “3 Carboxylases” to Know:

  1. Pyruvate Carboxylase: Converts Pyruvate (3C) to Oxaloacetate (4C). This is the first step of Gluconeogenesis.
  2. Acetyl-CoA Carboxylase: Converts Acetyl-CoA (2C) to Malonyl-CoA (3C). This is the rate-limiting step of Fatty Acid Synthesis.
  3. Propionyl-CoA Carboxylase: Converts Propionyl-CoA (3C) to Methylmalonyl-CoA (4C). Involved in odd-chain fatty acid oxidation and branched-chain amino acid breakdown.

2. Deficiency: Causes and Presentation

Biotin deficiency is relatively rare because it is synthesized by intestinal flora and found in many foods. However, specific scenarios can trigger it.

  • Raw Egg Whites: Contain Avidin, a protein that “avidly” binds biotin, preventing its absorption.
  • Antibiotic Use: Long-term use can deplete the gut bacteria that naturally produce biotin.
Clinical Finding Description
Dermatitis Scaly skin rash, often periorificial.
Alopecia Thinning or loss of hair.
Enteritis Inflammation of the small intestine.

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