MCAT Biochemistry: Carbohydrate Metabolism Part 1

Introduction

• Importance of maintaining constant blood glucose concentration:
  • Typical concentration: 5.6 millimolar
  • High blood glucose causes damage to retina, kidneys, blood vessels, and nerves
• Interplay between endocrine, digestive, and neurological systems in glucose regulation

Key Topics

1. Glycolysis
2. Galactose and Fructose Metabolism
3. Pyruvate Dehydrogenase
4. Glycogenesis and Glycogenolysis
5. Gluconeogenesis
6. Pentose Phosphate Pathway

Important Concepts

• Blood Glucose Entry: Concentration-driven, independent of sodium
• Glucose Transporters: GLUT-1, GLUT-2, GLUT-3, GLUT-4
  • GLUT-2: Liver and pancreatic cells
  • GLUT-4: Adipose tissue and muscle; stimulated by insulin

Glycolysis

• Part of cellular respiration along with citric acid cycle and oxidative phosphorylation
• Occurs in cytosol; breaks glucose into two pyruvate molecules
• Two Phases:
  1. Energy Investment Phase: ATP is used
  2. Energy Payoff Phase: ATP and NADH are produced

Key Enzymes in Glycolysis

• Hexokinase: Converts glucose to glucose-6-phosphate
• Phosphofructokinase: Produces fructose 1,6-bisphosphate
• Glyceraldehyde 3-phosphate Dehydrogenase: Produces NADH
• Phosphoglycerate Kinase & Pyruvate Kinase: Involved in substrate-level phosphorylation

Galactose and Fructose Metabolism

• Galactose: Derived from lactose, converted to glucose 1-phosphate
• Fructose: Derived from honey, fruit; converted by fructokinase and cleaved by aldolase B

Pyruvate Dehydrogenase

• Converts pyruvate to acetyl-CoA for citric acid cycle or fatty acid synthesis
• Stimulated by insulin, inhibited by acetyl-CoA

Glycogenesis and Glycogenolysis

• Glycogenesis: Synthesis of glycogen from glucose
  • Key enzyme: Glycogen synthase
  • Regulated by insulin and glucagon
• Glycogenolysis: Breakdown of glycogen
  • Key enzyme: Glycogen phosphorylase
  • Regulated by glucagon

Gluconeogenesis

• Synthesis of glucose from non-carbohydrate sources
• Occurs mainly in the liver
• Key regulatory enzyme: Fructose 1,6-bisphosphatase

Pentose Phosphate Pathway (PPP)

• Produces NADPH and ribose 5-phosphate for nucleotide synthesis
• Key enzyme: Glucose 6-phosphate dehydrogenase

Important Enzymes to Remember for MCAT

• Glycolysis: Phosphofructokinase-1
• Fermentation: Lactate dehydrogenase
• Glycogenesis: Glycogen synthase
• Glycogenolysis: Glycogen phosphorylase
• Gluconeogenesis: Fructose 1,6-bisphosphatase
• PPP: Glucose 6-phosphate dehydrogenase

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Conclusion

• Understanding of these pathways is crucial for the MCAT
• Focus on regulation, key enzymes, and steps yielding high-energy molecules
• Practice problems will aid in memorization and application of these concepts

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Good luck and happy studying! 🎓