Protein Synthesis and Metabolism

Vitamins and Minerals

• Fat-soluble vitamins: A, D, E, and K
  • Absorbed with lipid digestion products
  • Stored in the body, except for vitamin K
  • Vitamins A, C, and E act as antioxidants
• Minerals:
  • Seven required in moderate amounts: calcium, phosphorus, potassium, sulfur, sodium, chloride, and magnesium
  • Others required in trace amounts
  • Work with nutrients to ensure proper body functioning
  • Uptake and excretion must be balanced to prevent toxic overload
  • Examples: calcium, phosphorus, and magnesium salts harden bone; iron is essential for oxygen binding to hemoglobin; iodine is necessary for thyroid hormone synthesis; sodium and chloride are major electrolytes in the blood

Metabolism

• Metabolism: biochemical reactions inside cells involving nutrients
• Two types of reactions:
  • Anabolism: synthesis of large molecules from small ones
  • Catabolism: hydrolysis of complex structures to simpler ones
• Cellular respiration: catabolism of food fuels and capture of energy to form ATP in cells
• Stages of Metabolism:
  1. Digestion, absorption, and transport to tissues
  2. Cellular processing (in cytoplasm) synthesis of lipids, proteins, and glycogen, or catabolism (glycolysis) into intermediates
  3. Oxidative (mitochondrial) breakdown of intermediates into CO2, water, and ATP

Carbohydrate Metabolism

• Oxidation of glucose: C6H12O6 + 6O2 → 6H2O + 6CO2 + 36 ATP + heat
• Glucose is catabolized in three pathways:
  1. Glycolysis: catabolic reaction based on the conversion of glucose into two molecules of pyruvic acid
     • Anaerobic, occurs in the cytoplasm
     • Final products: 2 pyruvic acid, converted to lactic acid if O2 not readily available
     • Net gain: 2 ATP
  2. Krebs Cycle: occurs in the mitochondrial matrix
     • Does not directly use O2
     • Utilizes the 2 pyruvic acids from glycolysis
     • Products: 3NADH + H+, FADH2, CO2, ATP
  3. Electron Transport Chain and Oxidative Phosphorylation: occurs on the inner mitochondrial membrane
     • Directly uses oxygen
     • Utilizes the hydrogen atoms from NADH + H+ and FADH2 from glycolysis and Krebs Cycle
     • Products: about 28 ATP

Protein Metabolism

• Three aspects of protein metabolism:
  1. All-or-none rule: all amino acids needed must be present for protein synthesis to occur
  2. Adequacy of caloric intake: protein will be used as fuel if there is insufficient carbohydrate or fat available
  3. Nitrogen balance: in a healthy body, the rate of protein synthesis equals the rate of protein breakdown
     • Positive nitrogen balance: protein synthesis > protein breakdown (normal in children and tissue repair)
     • Negative nitrogen balance: protein breakdown > protein synthesis (e.g., stress, burns, infection, injury, or starvation)

Lipid Metabolism

• Lipid metabolism:
  • Lipogenesis: formation of triglycerides (fat) when ATP and glucose levels are high
  • Lipolysis: breakdown of triglycerides into glycerol and fatty acids
  • Triglycerides are routinely oxidized for energy (ATP)
  • Beta Oxidation: breakdown of fatty acids for the production of ATP
    • Occurs in the mitochondria
    • Two-carbon fragments at a time are used from the fatty acid chains and enter Krebs cycle
    • Reduced coenzymes, which enter the electron transport chain

Energy Balance

• Energy balance: heat energy cannot be used to do work
  • Warms the tissues and blood
  • Helps maintain the homeostatic body temperature
  • Allows metabolic reactions to occur efficiently
• Obesity:
  • Body mass index (BMI) = wt (lb) × 705/ht (inches)2
  • Considered overweight if BMI is 25-30
  • Considered obese if BMI is >30
  • Higher incidence of atherosclerosis, diabetes mellitus, hypertension, heart disease, and osteoarthritis

Regulation of Body Temperature

• Body temperature reflects the balance between heat production and heat loss
• At rest, the liver, heart, brain, kidneys, and endocrine organs generate most heat
• During exercise, heat production from skeletal muscles increases dramatically
• Normal body temperature = 37°C (98.6°F)
• Optimal enzyme activity occurs at this temperature
• Increased temperature denatures proteins and depresses neurons
• Core and shell temperature:
  • Organs in the core have the highest temperature
  • Blood is the major agent of heat exchange between the core and the shell
  • Core temperature is regulated
  • Core temperature remains relatively constant, while shell temperature fluctuates substantially (20°C-40°C)