Source / episode info

• **Episode:**51
• **Title:**Divine Intervention Episode 51 – Comprehensive USMLE Step 1 Biochemistry Review (Session 2 of 2)
• **Published:**2018-09-25
• Source:Episode page

One-liner

This episode provides a comprehensive review of key metabolic pathways including iron/heme metabolism, glucose transport kinetics, glycolysis regulation, pyruvate fate (PDH complex), lipid synthesis and breakdown, cholesterol synthesis, and the spectrum of glycogen storage diseases.

High-yield summary

• Iron Metabolism: Iron is absorbed in the duodenum only in the Fe^{2+} form; Vitamin C enhances this absorption. Hemochromatosis results from HFE mutations causing excessive iron reabsorption, treated with phlebotomy.
• Glucose Transport: GLUT1 operates under zero-order kinetics (low K_M 5 mM), tracking normal blood glucose. GLUT2 has a high K_M, ensuring proportional uptake relative to plasma glucose levels.
• Glycolysis Regulation: Glucokinase (liver) is induced by insulin and regulated by the GKRP, which sequesters it in the nucleus when bound by Fructose-6-Phosphate (F6P).
• Pyruvate Fate: Pyruvate can enter mitochondria via the PDH complex (requiring 5 cofactors: Thiamine, Lipoic acid, CoA, {FAD}, {NAD}^{+}) or be converted to OAA by Pyruvate Carboxylase (requires Biotin).
• Glycogen Storage Diseases (GSDs): Must differentiate the specific enzyme deficiency and clinical presentation: Pompe's (acid maltase/lysosomal, severe heart failure in infancy); Von Gierke’s (glucose-6-phosphatase/liver, hypoglycemia, lactic acidosis); McArdle’s (muscle glycogen phosphorylase/muscle, exercise myopathy).

Learning objectives

• Describe the absorption mechanisms of iron, folate, and Vitamin B12.
• Differentiate between various types of hyperbilirubinemia based on urine findings.
• Explain the regulatory roles of insulin and glucagon on key metabolic enzymes (e.g., PFK2, Glucokinase).
• Outline the cofactors required for Pyruvate Dehydrogenase Complex function.
• Compare and contrast the clinical presentations and enzymatic defects of various Glycogen Storage Diseases (GSDs).

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