Metabolic Pathways for the MCAT

Introduction

• Focus: Eukaryotic metabolism for MCAT preparation.
• Objective: Identify where metabolic pathways occur, their precursors, and products.
• Coverage: 13 major metabolic pathways.

Glycolysis

• Location: Cytoplasm
• Precursor: Glucose
• Requirements: NAD+, 2 ATP
• Products: 2 Pyruvate, NADH, 4 ATP (Net: 2 ATP)
• Importance: Initial step in carbohydrate metabolism.

Pyruvate Dehydrogenase Complex (PDH)

• Location: Mitochondria
• Function: Converts Pyruvate to Acetyl CoA
• Products: Acetyl CoA, NADH, CO2
• Condition: High oxygen and low ATP levels.

Citric Acid Cycle (Krebs Cycle)

• Location: Mitochondria
• Precursor: Acetyl CoA
• Requirements: NAD+, FAD
• Products: NADH, FADH2, CO2
• Cycle: Regenerates Acetyl CoA.

Electron Transport Chain

• Location: Inner mitochondrial membrane
• Inputs: NADH, Oxygen
• Products: Water, ATP
• Relation: Requires oxygen for aerobic respiration.

Aerobic Respiration

• Path: Glycolysis → PDH → Citric Acid Cycle → Electron Transport Chain
• Requirement: Oxygen for ATP production.

Anaerobic Conditions

• Response: Convert Pyruvate to Lactic Acid
• Enzyme: Lactate Dehydrogenase
• Goal: Regenerate NAD+ for Glycolysis.

Gluconeogenesis

• Precursor: Pyruvate (via Citric Acid Cycle)
• Product: Glucose
• Location: Liver cells
• Purpose: Produce glucose when energy is sufficient.

Glycogenesis and Glycogenolysis

• Glycogenesis: Formation of glycogen from glucose.
• Glycogenolysis: Breakdown of glycogen to glucose.
• Location: Liver cells

Pentose Phosphate Pathway

• Relation: Branches from Glycolysis
• Products: Nucleotides, NADPH
• Use: Lipid synthesis, DNA synthesis.

Lipid Metabolism

Beta Oxidation

• Precursor: Fatty acids
• Location: Mitochondria
• Requirement: Oxygen
• Product: Acetyl CoA

Fatty Acid Synthesis

• Location: Cytosol (via Citrate Shuttle)
• Products: Fatty acids, Triacylglycerides
• Requirement: NADPH from Pentose Phosphate Pathway

Protein Metabolism

Ketogenesis

• Location: Mitochondria
• Precursor: Acetyl CoA
• Product: Ketone bodies
• Use: Energy source for the brain when glucose is low.

Ketolysis

• Function: Breaks down Ketone bodies
• Location: Brain
• Purpose: Produce energy when glucose is unavailable.

Summary

• Pathways Covered: 13 metabolic pathways
• Advice: Understand regulation and memorize key intermediates and enzymes.
• Preparation Tip: Take practice exams to identify strengths and weaknesses.

Additional Resources

• Free Course: On practice exams and study plans.