Metabolism of Nitrogenous Compounds

Nitrogen Acquisition and Metabolism

Nitrogen Fixation

  • Nitrogen Fixation: Reduction of dinitrogen to ammonia by soil bacteria, cyanobacteria, and symbiotic nodules (Rhizobium) in leguminous plants.
  • Haber Process: Industrial nitrogen fixation, converting nitrogen and hydrogen to ammonia (N2 + 3H2 ")]), using iron catalyst at 400-450°C and 200 atm.

Nitrogen Cycle

  • Conversion of nitrogen gas into usable compounds.
  • Involves reduction of nitrate to nitrite by nitrate reductase, then to ammonia by nitrite reductase, utilizing NADH or NADPH and ferredoxin.
  • N2 \rightarrow NH3 \rightarrow Amino acids/Nucleotides

Utilization of Ammonia

  • Ammonia is converted into organic nitrogen compounds by:
    • Glutamate dehydrogenase: Reductive amination of α-ketoglutarate to glutamate.
      • \alpha−Ketoglutarate+-Ketoglutarate +NH_3 → Glutamate
    • Glutamine synthetase: Converts glutamate to glutamine.
      • Glutamate + NH_3$$ + ATP → Glutamine + ADP + Pi
    • Asparagine synthetase: Converts aspartate to asparagine using ammonia or glutamine.
    • Carbamoyl phosphate synthetase: Generates carbamoyl phosphate for arginine and pyrimidine synthesis.

Amino Acid Metabolism

  • Essential Amino Acids: Must be obtained from the diet.
  • Transamination: Redistribution of amino acid nitrogen.
  • Protein Turnover: Proteins are constantly synthesized and degraded. Approximately 400 g of protein are synthesized and 400 g are broken down daily.

Amino Acid Degradation

  • Excess nitrogen is degraded, and the carbon skeleton is metabolized in the citric acid cycle.
  • Involves transamination or oxidative deamination.

Detoxification and Excretion of Ammonia

  • Urea Cycle (Krebs-Henseleit Cycle): Converts toxic ammonia to urea in the liver, excreted by the kidneys.
  • Urea Cycle Intermediates: Ornithine, Carbamoyl Phosphate, Citrulline, Aspartate, Arginosuccinate, Fumarate, Arginine, Urea.
  • Transport of Ammonia:
    • Glutamine synthetase converts ammonia to glutamine for transport. Glutamine is then cleaved by glutaminase in the liver.
    • Glucose-alanine cycle transports ammonia from muscle to liver.
      • Muscle: NH3 -> Glutamate -> Pyruvate -> Alanine
      • Liver: Alanine -> Pyruvate -> Glucose

Health Considerations

  • Urea Cycle Disorders: Genetic deficiencies in urea cycle enzymes cause hyperammonemia, leading to neurological damage.
  • Vegetarian Diets: Require careful planning to ensure adequate intake of essential amino acids and calories.
  • Amino Acid Supplements: Not scientifically supported for muscle building or hair strength and can be dangerous in large doses.