KN

KIN 343 - 11.28 Vitamin B7 Functions

Section 1128: Biotin (Vitamin B7) Functions

Overview of Biotin as a Cofactor

  • Biotin is a crucial coenzyme involved in several enzymatic reactions, now recognized to assist 6 different enzymes, previously thought to be 4.

  • All 6 enzymes are identified as carboxylases, which are responsible for adding an activated carbon dioxide as a carboxylic acid group to existing molecules.

Key Enzymes Supported by Biotin

1. Pyruvate Carboxylase

  • Catalyzes the formation of oxaloacetate from pyruvate.

    • Essential for replenishing oxaloacetate, which increases the capacity of the TCA (tricarboxylic acid) cycle, critical for energy generation.

    • Plays a role in integrating various metabolic sources (fat, carbohydrates, amino acids) into energy production.

  • Important Concept: "Fat burns in the flame of carbohydrate" which emphasizes the need for adequate carbohydrate supply to fully oxidize fats.

    • Excess acetyl CoA needs sufficient oxaloacetate to continue entering the TCA cycle. Without it, pyruvate cannot be processed effectively; it creates a bottleneck in metabolism until the cycle completes.

  • Significance: Increased oxidative demand (exercise or energy requirement) necessitates more oxaloacetate, which is produced by pyruvate carboxylase facilitated by biotin.

2. Acetyl CoA Carboxylase

  • Functions in the first step of fatty acid biosynthesis.

    • Biotin is used to convert acetyl CoA into malonyl CoA, initiating fatty acid chain growth.

    • Although energetically costly, synthesizing fatty acids is essential for energy storage and mobility, addressing problems related to water weight from glycogen storage.

  • Process:

    • Malonyl CoA provides a building block for incorporating carboxylic acid groups onto growing fatty acids; however, this process requires energy (electrons).

  • Produces a shunting of acetyl CoA away from the TCA cycle towards fatty acid synthesis during energy storage states.

Biotin in Gene Regulation

  • Beyond coenzyme roles, biotin acts through a process called biotinylation.

  • Biotinylation involves the covalent addition of biotin to transcription factors and histones, thereby impacting gene expression.

    • Impact on Gene Expression:

      • Over 2,000 human genes rely on biotin for proper expression.

      • Alters protein conformation, influencing the regulatory functions of transcription factors and modifying gene transcription potential.

  • This action highlights the vital dual role of biotin not just in enzyme activity but also in gene regulation and expression modulation.