Amino Acid Metabolism Part II

Overview

• Digestion of amino acids
• Protein turnover
• Urea cycle
• Amino acid catabolism
• Errors in amino acid metabolism

Protein Synthesis Requirements

• All essential amino acids must be available in the cell
• Limiting amino acids slow or halt protein synthesis
• Inadequate energy consumption limits protein synthesis

Types of Proteins

• Incomplete Proteins: Lack some essential amino acids
• Complete Proteins: Contain all nine essential amino acids
  • Common sources: Animal proteins, soy, quinoa, chia seeds

Achieving Adequate Protein Intake

• Mutual Supplementation: Combining incomplete proteins to make a complete protein
• Complementary Proteins: Specific combinations of foods that provide all essential amino acids
  • Examples:
    • Legumes + Rice (e.g., red beans and rice)
    • Peanut butter + Bread
    • Hummus (chickpeas + sesame seeds)
    • Vegetables + Nuts/Seeds (e.g., tofu and broccoli with almonds)

Protein Digestion

• Stomach:
  • Proteases like pepsin break peptide bonds
• Small Intestine:
  • Proteases from pancreas (e.g. trypsin, chymotrypsin, elastase, carboxypeptidase)
  • Amino acids absorbed into the bloodstream
• Important Enzymes:
  • Enteropeptidase from the duodenum activates trypsin
• Nitrogen Balance:
  • Goal: Nitrogen intake equals nitrogen excretion
  • Positive Balance: Needed during pregnancy, growth
  • Negative Balance: Seen in fasting, starvation

Protein-Energy Malnutrition

• Marasmus:
  • Inadequate intake of protein and energy
  • Symptoms: Wasting, muscle weakening, heart failure
• Kwashiorkor:
  • Sufficient calories, insufficient protein
  • Symptoms: Edema (distended belly), muscle wasting, irritability

Amino Acids for Energy

• Breakdown: Cellular proteins and dietary proteins broken down into amino acids
• Conversion of Ammonia:
  • Ammonia to carbamoyl phosphate, enters urea cycle
• Carbon Skeletons:
  • Can be used in the citric acid cycle, produce ATP, glucose, fatty acids

Urea Cycle

• Purpose: Remove excess nitrogen from the body
• Process:
  • Ammonium, bicarbonate, aspartate, and 3 ATP --> urea + fumarate
  • Key Enzymes: Carbamoyl phosphate synthetase I
  • Energy Cost: 3 ATP equivalents per urea synthesized
• Fumarate Use:
  • Converted to malate, then to oxaloacetate
  • Oxaloacetate can be used in citric acid cycle or converted to aspartate

Urea Cycle Enzyme Deficiencies

• Example:
  • Argininosuccinate excreted in urine
  • High arginine dietary intake to compensate

Glucogenic vs. Ketogenic Amino Acids

• Glucogenic: Can form glucose precursors
• Ketogenic: Can form ketone bodies

Errors in Amino Acid Metabolism

• Phenylketonuria (PKU):
  • Deficiency in phenylalanine hydroxylase
  • Accumulation of phenylalanine leads to brain development issues
  • Treatment: Avoid phenylalanine, ensure tyrosine intake
• Albinism:
  • Deficiency in enzyme converting tyrosine to dopaquinone
  • Inability to produce melanin, leading to lack of pigmentation

Porphyrias

• Congenital Erythropoietic Porphyria:
  • Deficiency in enzymes for heme biosynthesis
  • Symptoms: Red urine, fluorescent teeth, sensitivity to sunlight, anemia
• Historical Beliefs:
  • King George III, Vincent Van Gogh, Vlad the Impaler possibly affected
  • Vampirism myths possibly linked to congenital erythropoietic porphyria.