Metabolic Regulation Overview

Overview of Cellular Energy and Metabolic Pathways

• Digestion

  • Involves stomach acid and enzymes.
  • Absorption occurs in the gut.

• Glycolysis

  • Takes place in the cytosol.
  • Converts glucose to pyruvate.

• Citric Acid Cycle & Oxidative Phosphorylation (OXPHOS)

  • Takes place in the mitochondria.
  • Employs an electron transport chain to produce ATP.

Key Topics Covered in Chapter 13

1. Cellular Energy
2. Glycolysis
3. The Citric Acid Cycle
4. Metabolic Regulation
5. Mitochondria and OXPHOS

Learning Objectives

• Evaluate the catabolic versatility of acetyl-CoA.
• Relate catabolic intermediates to biosynthesis.
• Describe mechanisms of energy storage in animals and plants.

Regulation of Metabolism

• Cells function as intricate chemical machines requiring ATP for various processes.
• Glucose is the main energy source for most cells.
• Many biosynthetic pathways begin with glycolysis or the citric acid cycle.

Metabolic Control Mechanisms

• Feedback Regulation
• Compartmentalization
• Temporal Regulation
• Unique Pathway Enzymes
• Hormonal Regulation

Sources of Acetyl-CoA

1. Sugars
2. Fats
3. Amino Acids

• Fatty acids are oxidized to produce acetyl-CoA in the mitochondria via beta-oxidation.

Catabolism of Fats

• Fats are highly reduced compounds, yielding more energy per gram than carbohydrates.
• Stored as triacylglycerols in adipocytes.
• Triacylglycerols are hydrolyzed to glycerol and fatty acids, where glycerol can enter glycolysis.
• Beta-oxidation in mitochondria yields:
  • Acetyl-CoA
  • NADH
  • FADH2

Catabolic Fates of Amino Acids

• Proteins require proteolysis for catabolism and amino acid transport to mitochondria.
• Some amino acids can convert to pyruvate (glucogenic); others convert to acetyl-CoA or citric acid cycle intermediates (ketogenic).

Energy Storage in Organisms

• Plants store glucose as starch.
• Animals store glycogen in muscle and liver cells:
  • Glycogen: branched glucose polymer; hydrolyzable energy reserve that provides energy during low blood glucose.

Glycogenolysis

• Process converting glycogen to glucose 6-phosphate for energy production.

Gluconeogenesis

• De novo glucose synthesis from non-carbohydrate substrates (e.g., pyruvate, lactate, glycerol).
• An expensive process, consuming ATP.
• Occurs primarily in kidneys and liver to maintain blood glucose levels.

Comparison: Glycolysis vs. Gluconeogenesis

• Glycolysis: Glucose → Pyruvate (Catabolic)
• Gluconeogenesis: Pyruvate → Glucose (Anabolic)
  • Both processes have unique enzymes for irreversible steps and are highly regulated.

The Cori Cycle

• Supports gluconeogenesis during intense workouts when oxygen is limited (anaerobic gluconeogenesis).

Summary

• Energy can be sourced from sugars, fats, or proteins.
• Glycogen reserves facilitate quick energy access (glycogenolysis).
• Fat serves as a long-term energy reserve (lipolysis, beta-oxidation).
• Proteins provide a last resort energy source (via gluconeogenesis).
• Metabolic intermediates are crucial for biosynthetic reactions, requiring strict regulation.