Chapter 22: Metabolic Pathways for Carbohydrates

Metabolism and Energy

• Metabolism refers to processes that convert food into energy and smaller molecules.
  • Catabolic reactions: Break down large molecules to release energy.
  • Anabolic reactions: Use ATP to synthesize larger molecules.

Stages of Catabolism

• Stage 1: Digestion and hydrolysis. Large molecules are broken down into smaller ones for absorption into the bloodstream.
• Stage 2: Degradation in cells to two- or three-carbon compounds.
• Stage 3: Oxidation of these smaller molecules via the citric acid cycle and electron transport chain to produce ATP.

Cell Structure for Metabolism

• Eukaryotic cells (plants, animals) have a nucleus that contains DNA.
• Key Components:
  • Cell Membrane: Separates interior of the cell from the external environment.
  • Nucleus: Contains genetic information necessary for replication and protein synthesis.
  • Cytosol: Fluid part of the cytoplasm with electrolytes and enzymes.
  • Mitochondria: Site for ATP synthesis, involved in energy-releasing reactions.

ATP (Adenosine Triphosphate)

• Structure: Composed of adenine, ribose, and three phosphate groups.
• Hydrolysis: ATP → ADP + Pi + Energy (7.3 kcal/mol).
• Role: Energy from ATP hydrolysis is harnessed to drive metabolic reactions.

Important Coenzymes

• Coenzyme A (CoA): Comprised of pantothenic acid, phosphorylated ADP, and aminoethanethiol. It prepares small acyl groups for reactions with enzymes.
• NAD+ (Nicotinamide Adenine Dinucleotide): Involved in oxidation reactions, gains electrons and is reduced to NADH during these reactions.
• FAD (Flavin Adenine Dinucleotide): Similar to NAD; reduced during specific metabolic reactions to FADH2.

Metabolic Reactions

• Oxidation: Loss of hydrogen or electrons; increases bonds to oxygen.
• Reduction: Gain of hydrogen ions/electrons; decreases bonds to oxygen.
• Coenzymes enable the transfer of hydrogen ions/electrons between substrates.

Digestion of Carbohydrates

• Begins in the mouth with salivary amylase, breaking down polysaccharides.
• In the stomach, carbohydrate digestion halts due to acidic conditions.
• In the small intestine: Pancreatic enzymes convert dextrins to glucose and monosaccharides are absorbed into the bloodstream.

Glycolysis: Oxidation of Glucose

• Glucose is metabolized through glycolysis in the cytosol of cells, degrading glucose to pyruvate.
• Energy Investment Phase (Reactions 1-5): ATP is consumed to add phosphates to glucose.
• Energy Generation Phase (Reactions 6-10): Net gain of 2 ATP and 2 NADH from one glucose molecule conversion.

Specific Reactions in Glycolysis

• Reaction 1 (Phosphorylation): ATP phosphorylates glucose to form glucose-6-phosphate.
• Reaction 3 (Phosphorylation): Another ATP is used to convert fructose-6-phosphate to fructose-1,6-bisphosphate.
• Reaction 10 (Phosphate Transfer): Two phosphoenolpyruvate molecules generate two ATP and two pyruvates.

Regulation of Glycolysis

• Hexokinase: Inhibited by high levels of glucose-6-phosphate.
• Phosphofructokinase: Allosterically regulated by ATP (inhibitor) and ADP (activator).
• Pyruvate Kinase: Inhibited by ATP and acetyl CoA.

Gluconeogenesis

• Synthesis of glucose from non-carbohydrate sources, mainly in the liver and kidneys.
• Energy Cost: Requires 4 ATP, 2 GTP, and 2 NADH.
• Key steps include the conversion of pyruvate to phosphoenolpyruvate and to glucose-6-phosphate.

Glycogen Synthesis and Degradation

• Glycogenesis: Formation of glycogen from glucose, occurs when glucose levels are high.
• Glycogenolysis: Breakdown of glycogen to glucose, initiated by low glucose levels.
• Regulation involves opposing actions of hormones like insulin and glucagon.

Pathways for Pyruvate

• Under aerobic conditions, pyruvate is converted to acetyl CoA for energy production.
• Under anaerobic conditions, pyruvate is reduced to lactate, which leads to temporary energy production during strenuous exercise.
• Cori Cycle: Lactate produced in muscles can be converted back to glucose in the liver.

Glycogen Storage Diseases

• Result from defective enzymes in glycogen metabolism, leading to symptoms like hypoglycemia, muscle weakness, and fatigue.
• Common types include:
  • Von Gierke's Disease: Lack of glucose-6-phosphatase.
  • Pompe's Disease: Deficiency in lysosomal alpha-glucosidase.
  • Cori's Disease: Difficulty breaking down glycogen due to a defective debranching enzyme.