Thiamine (B1) and Pyruvate Dehydrogenase

Thiamine (B1): Structure & Activation - Nerve Nourisher

• Structure: Heterocyclic; substituted pyrimidine ring linked to a thiazole ring by a methylene bridge.
  • Water-soluble vitamin.
• Activation to Thiamine Pyrophosphate (TPP):
  • Thiamine + ATP $\xrightarrow{\text{Thiamine Pyrophosphokinase, Mg}^{2+}}$ TPP + AMP
  • TPP is the biologically active coenzyme form.
  • Site: Cytosol (mainly liver, brain, kidney).
• Crucial for nerve cell integrity and function.

⭐ Thiamine is absorbed in the jejunum and ileum, and its active form, Thiamine Pyrophosphate (TPP), is synthesized by thiamine pyrophosphokinase-1 (TPK1).

Thiamine (TPP): Metabolic Roles - Coenzyme Power

Active form of Vitamin B1. Essential coenzyme for:

• Oxidative Decarboxylation (α-keto acids):
  • Pyruvate Dehydrogenase Complex (PDC): $Pyruvate \rightarrow Acetyl-CoA$ (Glycolysis to TCA link).
  • α-Ketoglutarate Dehydrogenase (KGDH): $\alpha-Ketoglutarate \rightarrow Succinyl-CoA$ (TCA cycle).
  • Branched-Chain α-Keto Acid Dehydrogenase (BCKDH): Metabolizes BCAAs (Leu, Ile, Val).
• Transketolase Reactions:
  • Transketolase: Pentose Phosphate Pathway (PPP); transfers 2-C units.

⭐ Transketolase (PPP enzyme) activity, measured by TPP effect, is a diagnostic marker for thiamine status.

📌 Mnemonic (PDC/KGDH cofactors): Tender Loving Care For Nancy (Thiamine, Lipoate, CoA, FAD, NAD+).

Pyruvate Dehydrogenase Complex (PDH) - Gatekeeper Enzyme

• Location: Mitochondrial matrix.
• Function: Irreversibly converts pyruvate to acetyl-CoA via oxidative decarboxylation. Links glycolysis to TCA cycle & FA synthesis.
  • Overall Reaction: $Pyruvate + CoA + NAD^+ \rightarrow Acetyl-CoA + CO_2 + NADH + H^+$
• Components & 5 Cofactors: 📌 "Tender Loving Care For Nancy"
  • E1 (Pyruvate Dehydrogenase): Uses TPP (B1). Action: Decarboxylates pyruvate.
  • E2 (Dihydrolipoyl Transacetylase): Uses Lipoamide & CoA (B5). Action: Transfers acetyl group to CoA.
  • E3 (Dihydrolipoyl Dehydrogenase): Uses FAD (B2) & NAD+ (B3). Action: Regenerates oxidized lipoamide.
• Regulation: Tightly controlled.
  • Product Inhibition: ATP, Acetyl-CoA, NADH (↑ inhibit PDH).
  • Substrate Activation: Pyruvate, ADP (↑ activate PDH by inhibiting PDH Kinase).
  • Covalent Modification:
    • PDH Kinase: Inactivates PDH (phosphorylates). Activated by ↑ATP, ↑Acetyl-CoA, ↑NADH.
    • PDH Phosphatase: Activates PDH (dephosphorylates). Activated by Ca²⁺, Mg²⁺, Insulin.

⭐ Arsenic poisoning inhibits the PDH complex by irreversibly binding to the sulfhydryl groups of lipoamide, a cofactor for the E2 (dihydrolipoyl transacetylase) component.

Thiamine Deficiency: Manifestations & Diagnosis - Energy Crisis Syndromes

• Pathophysiology: ↓ Thiamine → impaired PDH, α-KGDH, transketolase → ↓ ATP; affects brain, heart, nerves.
• Clinical Syndromes:
  • Wernicke's Encephalopathy (WE): Acute. Triad: Confusion, Ataxia, Ophthalmoplegia (nystagmus, lateral rectus palsy). 📌 CAN O' (Confusion, Ataxia, Nystagmus, Ophthalmoplegia).

    ⭐ Wernicke's encephalopathy (presenting with the triad of confusion, ataxia, and ophthalmoplegia) is a medical emergency due to acute thiamine deficiency, often seen in chronic alcoholics, and requires immediate parenteral thiamine administration BEFORE glucose.

  • Korsakoff Syndrome: Chronic. Amnesia, confabulation. Often irreversible.
  • Dry Beriberi (Peripheral Neuropathy): Symmetrical motor & sensory neuropathy, muscle weakness, areflexia.
  • Wet Beriberi (Cardiovascular): High-output heart failure, edema, tachycardia. Shoshin Beriberi: fulminant.
• Diagnosis:
  • Clinical; response to thiamine.
  • Labs: ↓ erythrocyte transketolase activity (↑ TPP effect), ↑ pyruvate/lactate.

High‑Yield Points - ⚡ Biggest Takeaways

• Thiamine (B1) is activated to Thiamine Pyrophosphate (TPP).
• TPP is a vital coenzyme for Pyruvate Dehydrogenase (PDH), α-ketoglutarate dehydrogenase, and transketolase.
• PDH complex links glycolysis to TCA cycle by converting pyruvate to acetyl-CoA.
• Thiamine deficiency leads to Beriberi (peripheral neuropathy, heart failure) and Wernicke-Korsakoff syndrome (CNS effects).
• Wernicke's encephalopathy presents with confusion, ophthalmoplegia, ataxia; Korsakoff's psychosis with memory loss and confabulation.
• Diagnosis: ↑ erythrocyte transketolase activity upon thiamine (B1) administration.