Metabolism of Nitrogenous Compounds

Metabolism of Nitrogenous Compounds

• Learning objectives include describing protein digestion, reactions in separating amino groups of amino acids, the fate of carbon skeletons, glucogenic and ketogenic amino acids, biogenic amines, and examples of nitrogenous compounds.
• Nitrogenous compounds include amino acids (proteins), biogenic amines, porphyrins, and nucleobases.
• Protein digestion begins in the stomach, with proteolytic enzymes breaking peptide bonds. Protein digestion continues in the pancreas and small intestine.
• Amino acid metabolism involves the breakdown of dietary protein to create amino acids. Some amino acids can be used for energy or to create other compounds.
• If energy is not available, proteins are used to create new compounds.
• Another important aspect of amino acid degradation is the separation of the amino group and the metabolism of the carbon skeleton. This is done through transamination and oxidative deamination.
• Transamination transfers the amino group to another molecule, often creating glutamate.
• Oxidative deamination removes the amino group, creating ammonia and a-keto acid.
• The carbon skeleton can be converted into glucose or fat.
• Oxidative deamination involves free ammonia and creation of NAD+ and NADP.

Amino Acid Degradation

• The separation of the amino group and metabolism of the carbon skeleton are crucial aspects of amino acid degradation.
• Transamination is a key reaction that transfers amino groups, creating new amino acids or necessary intermediates.
• Oxidative deamination removes the amino group, generating ammonia, a critical waste product.
• The metabolic fate of the resulting carbon skeleton (following the removal of the amino group) is either converted into glucose or fatty acids.
• Decarboxylation is a reaction that removes a carboxyl group from an amino acid, producing a biogenic amine.

Urea Cycle

• Ammonia, a toxic byproduct of amino acid metabolism, is converted to urea for excretion. This process is crucial for the liver to remove toxic ammonia.
• The urea cycle is a series of enzymatic reactions in the liver.
• Ammonia or NH3 is converted to urea through a series of reactions.
• Ammonia is transferred in two steps:
  • Forming carbamoyl phosphate.
  • Forming urea.
• The urea cycle links to the TCA cycle via aspartate and fumarate.

Biogenic Amines

• Biogenic amines are nitrogen-containing organic compounds derived from amino acids through decarboxylation.
• Many biogenic amines function as neurotransmitters or hormones in the brain.
• Examples include histamine, serotonin, and catecholamines (such as dopamine, norepinephrine, and epinephrine).
• They are important for various physiological processes including allergic and inflammatory reactions, gastric acid secretion, and neural transduction.

Porphyrin Degradation

• Porphyrin degradation disorders can lead to jaundice due to problems in breaking down porphyrins.
• Defective enzyme activity can cause accumulation which interferes with physiological pathways, leading to toxic intermediates.

Nucleotide Metabolism

• Nucleotides are composed of three parts: a nitrogenous base, a pentose sugar, and phosphate group.
• Purines and pyrimidines are two types of nitrogenous bases.
• Key compounds in nucleotide metabolism are adenine, guanine, cytosine, thymine, and uracil.
• Nucleotide synthesis utilizes intermediates of the pentose phosphate pathway.
• Nucleotide synthesis involves various amino acids, including aspartic acid, glycine, and glutamine.