Control of Enzyme Activity & Energy Production

Control of Enzyme Activity

• Enzyme activity is highly sensitive to several factors:
  • Temperature
  • pH
  • Concentration of substrate
  • Stimulatory effects
  • Inhibitory effects

Energy for Life: Metabolic Pathways

• The primary goal is to trap energy for life.
• Photosynthesis: Produces glucose.
• Glucose Metabolism: Glucose is processed through glycolysis to yield pyruvate.
• Aerobic Respiration (Cellular Respiration):
  • Involves complete oxidation of glucose.
  • Waste products are $H<em>2O$ and $CO</em>2$.
  • Net energy trapped: $32$ ATP.
• Anaerobic Respiration (Fermentation):
  • Involves incomplete oxidation.
  • Can produce lactate or alcohol.
  • Waste products include organic compounds and $CO_2$.
  • Net energy trapped: $2$ ATP.

Oxidation and Reduction in Metabolic Pathways

• Oxidation and reduction are coupled processes, meaning they occur together.
  • A reduced compound has gained electrons.
  • An oxidized compound has lost electrons (is missing electrons).
• NAD+ acts as an electron transporter; it is reduced to NADH when it gains electrons.

Glucose Metabolism Flow

• Glucose is stored in the liver as glycogen.
• Blood Plasma: Glucose is transported in the blood plasma.
• Glycogen Breakdown: Glycogen from the liver is broken down into glucose.
• Cellular Uptake: Glucose travels through the blood to the cytoplasm of cells.
• Glycolysis: Glucose then undergoes glycolysis, breaking down into pyruvic acid.
• Pathways from Pyruvic Acid:
  • Anaerobic: Pyruvic acid is converted into lactic acid.
  • Aerobic Respiration: Pyruvic acid enters the mitochondrion, proceeds through the citric acid cycle, and is then used in the electron transport chain.

The Cori Cycle (Skeletal Muscles and Liver)

• This cycle describes the inter-organ cooperation in lactate metabolism during and after exercise.
  1. Exercise (Skeletal Muscles): During exercise, glucose-6-phosphate is metabolized, leading to the production of lactic acid.
  2. Lactic Acid Release: Lactic acid enters the bloodstream.
  3. Transport to Liver: The blood carries lactic acid to the liver.
  4. Conversion in Liver: In the liver, lactic acid is converted to pyruvic acid.
  5. Gluconeogenesis: Pyruvic acid in the liver is then converted into glucose-6-phosphate, which can be further converted to glucose.
  6. Glucose Release: Glucose is released from the liver into the blood.
  7. Uptake by Muscles: Glucose returns to resting skeletal muscles for storage or energy use.

Glycolysis

• Definition: Glycolysis is the metabolic pathway that converts glucose into pyruvate.
• Key Enzymes in Energy-Investing Reactions:
  • Hexokinase
  • Phosphofructokinase
  • Phosphohexose isomerase
  • Aldolase
• Hexokinase Function: Hexokinase acts on glucose, converting it into glucose-6-phosphate (an initial step in glycolysis).

Citric Acid Cycle (Krebs Cycle)

• The citric acid cycle is a central metabolic pathway that generates ATP, NADH, and $FADH_2$ as part of aerobic respiration.

Role of Macronutrients in Metabolism

• Carbohydrates: Serve as a crucial source of glucose, vital for overall metabolism.
• Proteins: Provide amino acids, which can be used for protein synthesis or energy production.
• Fats: Provide free fatty acids and glycerol, important for energy and other metabolic processes.

Glucose and Lipid Metabolism Pathways

• Gluconeogenesis: The process of synthesizing glucose from non-carbohydrate precursors, such as amino acids or free fatty acids.
• Lipogenesis: The process by which excess glucose is converted into fat for storage.

Organ-Specific Energy Metabolism

• Brain:
  • Primarily uses glucose for energy production.
  • Can also utilize ketone bodies as an alternative energy source during prolonged fasting or carbohydrate restriction.
• Muscle:
  • Utilizes stored glycogen for energy.
  • Breaks down fatty acids for energy.
  • Can break down its own proteins into amino acids, especially during prolonged stress or fasting.
• Liver:
  • Converts stored glycogen into glucose (glycogenolysis).
  • Converts glycogen into pyruvate.
  • Converts free fatty acids into ketone bodies, particularly during periods of low glucose availability, which can then be used by other tissues like the brain and muscles for energy.