Nitrogen Metabolism Notes

Nitrogen Metabolism

Learning Objectives

  • Discuss nitrogen homeostasis and nitrogen balance.
  • Describe the role of urea as the excretory form of surplus nitrogen in humans.
  • Outline the metabolic fates of amino acids.
  • Describe the metabolic classification of amino acids.
  • Discuss, with examples, the metabolic significance of glutamate, glutamine, and α-ketoglutarate.

Topics Covered

  • Nitrogen homeostasis / balance
  • Metabolic fate of amino acids
  • Metabolic classification of amino acids
  • Ketogenic and glucogenic amino acids
  • Importance of glutamate and related metabolites
  • Excretion of surplus nitrogen (Urea cycle)

Nitrogen Homeostasis / Balance

  • Where does nitrogen come from?
  • How is nitrogen stored?
  • What is nitrogen balance?

Amino Acids

  • Why are there 20 amino acids? Amino acids have different properties depending on their side chains.
  • Some are essential amino acids.
  • Some are conditionally essential amino acids (e.g., Arginine).

Uses of Amino Acids

  • Most common function is building proteins.
  • Many amino acids have a specific function of their own.
  • As intermediates in other metabolic cycles.
  • As building blocks of other chemicals or used as signaling molecules.

Glucogenic and Ketogenic Amino Acids

  • Glucogenic: Can be converted into glucose by gluconeogenesis and can feed into the TCA cycle as pyruvate or one of its intermediates.
  • Ketogenic: Can be converted to ketone bodies and feed TCA cycle mostly by being converted to acetyl-CoA or acetoacetate.
  • Some amino acids are both glucogenic and ketogenic.
  • Glucogenic Amino Acids: Alanine, Serine, Cysteine, Glycine, Tryptophan, Threonine, Tyrosine, Arginine, Asparagine, Aspartate, Glutamate, Glutamine, Histidine, Methionine, Valine, Proline
  • Ketogenic Amino Acids: Leucine, Lysine
  • Glucogenic and Ketogenic Amino Acids: Isoleucine, Phenylalanine, Tryptophan, Tyrosine

Glucose-Alanine Cycle (Cahill Cycle)

  • Alanine is a branched-chain amino acid (BCAA).
  • Most amino acids are broken down in the liver.
  • Muscle uses branched-chain amino acids as a fuel source.
  • Amino groups in muscle form glutamate.
  • Glutamate transfers amino group to pyruvate to produce alanine.
  • Alanine is transported to the liver, producing pyruvate and ultimately urea.

Reactions of Amino Acids

  • Transamination: Switching of an amine group from one keto-acid to another. Generates the amino acid version of the keto-acid and converts one amino acid to another.
  • Deamination: Removal of amine group, which results in the release of ammonium (NH4+NH_4^+).

Transamination

  • Process by which new amino acids can be made by using the carbon skeleton of other keto-acids and transferring the amino group (NH2NH_2) on it.
  • Catalyzed by transaminase enzymes (important in the liver).
  • Requires an intermediary called Pyridoxal phosphate (derived from vitamin B6).
  • The reaction is completely reversible.
  • Vitamin B6 Deficiency Symptoms: Skin Inflammation, Cardiovascular Problems, Depression, Anemia, Neurological Degeneration, Dementia, Fatigue.

Deamination

  • During reactions that produce TCA intermediaries (deamination), the amino group is no longer required and is given off in the form of ammonia.
  • Ammonia is toxic, highly reactive, and alkaline, so it can alter pH.
  • Excess ammonia is removed from the body to prevent its buildup and is excreted in the form of UREA via the UREA cycle.

Glutamine and Its Metabolites

  • Glutamine is by far the most abundant amino acid in the body.
  • Glutamate, an amino acid produced from glutamine, is an important neurotransmitter present in more than half of nervous tissues.
  • The conversion of glutamine to glutamate and then to α-ketoglutarate generates free ammonia that needs to be removed via glutaminase.

Functions of Glutamine and Its Metabolites

  • Important source of fuel during fasting, particularly in muscles and immune cells.
  • For gluconeogenesis, particularly in the kidney.
  • Produces ammonia that can act as a buffer for protons (removed in urea).
  • Anti-inflammatory properties – in gut

Glutamine – Important Renal Acid-Base Regulator

  • Glutamine from the blood is deaminated to glutamate and then to α-ketoglutarate, which can be converted to glucose (gluconeogenesis).
  • Ammonia generated can be used to produce protons to exchange with sodium in the filtrate.

Disposal of Excess Nitrogen - UREA Cycle

  • Sources of ammonia:
    • Microflora in the gut release ammonia when breaking down food.
    • Deamination of amino acids (glutamine).
    • Breakdown of DNA/RNA.
    • Metabolism of amino acids (glutamate recycling in the liver – dehydrogenase).
    • Ketogenesis/gluconeogenesis from amino acids releases ammonia.

UREA Cycle

  • Primarily in the liver.
  • Requires two amino groups:
    • One comes from aspartate.
    • One comes from ammonia (carbamoyl phosphate).
  • Key regulating enzyme: Carbamoyl phosphate synthetase (CPS1).
  • Requires 2 ATP molecules.
  • Controlled allosterically by a metabolite of glutamate, N-acetyl glutamate, formed in an excess of glutamate, so it drives the urea cycle: Glutamate + acetyl-CoA → N-acetyl glutamate.

Defects of Urea Formation

  • Regulated by 6 enzymes; defects of any of which will cause a reduction in urine formation.
  • Symptoms related to the accumulation of ammonia can be fatal in newborns, including vomiting, nausea, neurological disorders, lethargy, and coma.
  • Two types of onset: neonatal and late-onset (post-natal; 70%).
  • Treatment includes dietary management and phenylbutyrate (ammonia scavengers).
  • Thought to be undiagnosed causes of SIDS.
  • Type 1:
    • Autosomal recessive.
    • 1:200,000.
    • Cerebral edema, coma, death.
  • Type 2:
    • X-linked.
    • Cerebral edema, coma, death.

Summary

  • Nitrogen must be kept in balance (intake/production vs. loss/metabolism).
  • Amino acids have numerous functions besides building proteins.
  • Two important reactions:
    • Transamination: Keto acid 1 + amino acid 2 -> amino acid 1 + keto acid 2 – vitamin B6
    • Deamination: Amino acid to keto acid – release of NH4NH_4
  • Reactions that generate ammonia and its disposal (UREA CYCLE).
  • Importance of glutamine -> glutamate -> α-ketoglutarate for gluconeogenesis in the kidney, urea formation & overall protein balance.