Nitrogen cycle_ELC

Chapter 18: Amino Acid Oxidation

This chapter continues the exploration of how amino acids are oxidized for energy in animals. Key concepts include nitrogen fixation, ammonia incorporation into biomolecules, and the biosynthesis and degradation of nucleotides.

I. Pathways for Nitrogen Introduction into Biological Systems

  • Importance of Nitrogen: Nitrogen is essential for various biological processes, particularly in the synthesis of amino acids (AAs) and nucleotides (NAs).
    • Biologically useful forms of nitrogen are typically through NH3NH_3 (ammonia).
  • Natural Environments: Soluble, biologically useful nitrogen compounds are scarce; amino acids and nucleotides often salvaged and reused, necessitating a strict economy on nitrogen usage.

II. The Use of Amino Acids as Fuel in Mammals

  • Amino acids are exported to tissues requiring glucose (Glc) as a fuel.
  • Energy Production: Excess amino acid carbon skeletons can be utilized for energy.
  • Toxic Waste Management: The removal or transamination of amino groups is crucial due to the production of toxic waste NH4NH_4 from amino acid catabolism.
  • Comparison of Toxicity: Dissolved urea is significantly less toxic than ammonia, forming a primary means of dealing with NH4NH_4 accumulation.

III. Amino Acid Catabolism

  • Classification of Amino Acids:
    • Ketogenic Amino Acids: Can be converted to ketone bodies.
    • Glucogenic Amino Acids: Can be converted to glucose.
    • Some amino acids yield multiple intermediates in catabolism.
  • Grouped by Degradative End Products: Amino acids are categorized based on their primary degradation products:
    • Exclusively ketogenic amino acids: Leucine and lysine.
    • Five amino acids are both glucogenic and ketogenic but lack parts of their carbon skeletons capable of glucose synthesis through citric acid cycle (CAC) intermediates.

IV. Specific Amino Acid Catabolic Pathways

  • Catabolism of Amino Acids: Focus on tryptophan, lysine, phenylalanine, tyrosine, leucine, and isoleucine.
    • Carbon Donation: Amino acids donate carbons (in red) to acetyl-CoA and (in blue) to pyruvate or citric acid