Chapter 15: Nitrogen Metabolism II: Degradation Notes

Overview of Nitrogen Metabolism II: Degradation

  • Living cells undergo continuous renovation through the turnover of proteins and nucleic acids.
  • This process facilitates nitrogen recycling within organisms.

Nitrogen Excretion Methods

  • Ammonotelic organisms (e.g., crustaceans)**: Excrete ammonia directly into water.
  • Uricotelic organisms (e.g., birds, insects)**: Excrete uric acid in a solid form to conserve water.
  • Ureotelic organisms (e.g., mammals)**: Convert ammonia to urea for excretion with minimal water loss.
  • Mammals also excrete uric acid from purine catabolism.

Amino Acid Catabolism

  • Begins with the removal of the amino group, which is essential for urea synthesis.
  • Carbon skeletons lead to seven metabolic products:
    • (Acetyl−CoA,Acetoacetyl−CoA,Pyruvate,α−ketoglutarate,Succinyl−CoA,Fumarate,Oxaloacetate)(Acetyl-CoA, Acetoacetyl-CoA, Pyruvate, α-ketoglutarate, Succinyl-CoA, Fumarate, Oxaloacetate)
  • These products can be utilized to synthesize fatty acids, glucose, or generate energy.
Types of Amino Acids
  • Ketogenic Amino Acids: Converted into fatty acids or ketone bodies.
  • Glucogenic Amino Acids: Degraded into pyruvate or intermediates of the citric acid cycle, used in gluconeogenesis (most amino acids are glucogenic).

Processes of Deamination

  • Involves two reactions: Transamination and Oxidative Deamination.
  • Reversible reactions allow for amino group transfers among amino acids.
    • Excess amino groups contribute to urea synthesis, especially during high protein diets or protein breakdown (e.g., starvation).

Muscle Metabolism of Amino Acids

  • Excess amino groups from muscle are transferred to α-ketoglutarate to form glutamate.
  • Glutamate is then transported to the liver via the glucose-alanine cycle where it undergoes oxidative deamination, producing α-ketoglutarate and ammonia (NH₄⁺).

Glutamine and Ammonia Transport

  • Most ammonia produced outside the liver is carried as glutamine (amide group).
  • In the liver, glutamine is hydrolyzed to form glutamate and NH₄⁺.
  • Additional NH₄⁺ is generated by the action of glutamate dehydrogenase, converting glutamate back to α-ketoglutarate.

Urea Cycle

  • Urea is synthesized from ammonia, CO₂, and aspartate in the Urea Cycle (also known as the Krebs urea cycle).
  • Urea synthesis formula:
    extCO<em>2+extNH</em>4++extAspartate+3extATP+2extH2extO<br/>ightarrowextUrea+extFumarate+2extADP+2extPi+extAMP+extPPi+5extH+ext{CO}<em>2 + ext{NH}</em>4^+ + ext{Aspartate} + 3 ext{ATP} + 2 ext{H}_2 ext{O} <br /> ightarrow ext{Urea} + ext{Fumarate} + 2 ext{ADP} + 2 ext{Pi} + ext{AMP} + ext{PPi} + 5 ext{H}^+
  • The urea cycle converts toxic ammonia into urea, which is then excreted via the kidneys.
  • It is regulated tightly to prevent toxicity from urea accumulation.

Urea Cycle Regulation

  • Controlled by substrate concentrations (e.g., dietary protein increases activity).
  • Long-term regulation occurs through dietary changes affecting enzyme levels.
    • Glucagon and glucocorticoids stimulate urea cycle enzymes' transcription, whereas insulin represses it.
  • The cycle is activated by N-acetylglutamate (NAG), formed from glutamate and acetyl-CoA.

Catabolism of Amino Acid Carbon Skeletons

  • Important metabolic intermediates formed include:
    • Acetyl-CoA, Acetoacetyl-CoA, Pyruvate, α-ketoglutarate, Succinyl-CoA, and Oxaloacetate.
  • Carbon skeletons are converted to these intermediates, further linking amino acid catabolism to energy production and glucose synthesis.

Nucleotide Degradation

  • Purines and pyrimidines continuously catabolized.
  • Nucleic acids are hydrolyzed into oligonucleotides by nucleases, then further into bases and sugars.
  • Dietary purines limitedly used for nucleic acid synthesis; purines broken down into uric acid and pyrimidines to β-alanine and β-aminoisobutyric acid.

Purine Catabolism Overview

  • AMP is hydrolyzed to adenosine, which is deaminated to form inosine.
  • Subsequent pathways lead to xanthine and uric acid, with enzymes such as nucleotidase, purine nucleoside phosphorylase, and xanthine oxidase actively participating in these transformations.
  • Gout results from excess uric acid production due to purine over-catabolism or under-excretion.

Pyrimidine Catabolism Overview

  • Cytidine and deoxycytidine undergo conversions to uridine, yielding β-alanine and acetyl-CoA next, via degenerative pathways involving ammonia release.

Related Disorders

  • Diseases like gout arise from dysfunctional purine catabolism.
  • Adenosine deaminase deficiency leads to increased deoxyadenosine, causing immune disorders due to toxicity.