11. Amino acid metabolism; the fate of the amino group

Last updated on January 11, 2020 at 14:34

Summary

  • During degradation of amino acids, the amino group and the carbon skeleton follow different pathways
  • The amino group (-NH3+) can be excreted as urea, or be used for biosynthesis of amino acids, nucleotides and amines.
  • Intracellular proteins are degraded into amino acids that follow the same pathway
  • Aspartate is involved in multiple metabolic processes
    • Pyrimidine synthesis
    • Purine synthesis
    • Urea cycle
    • Synthesis of non-essential amino acids
  • Ammonia is removed from the body mainly by glutamine synthetase
  • The only amino acids which are not broken down in the liver are the branched chained amino acids
    • Leucine
    • Isoleucine
    • Valine
    • They’re broken down in muscle instead

Intracellular protein degradation

Proteins in the cell are broken down constantly. To not waste resources, they should be recycled as much as possible. The protein to be broken down is first tagged with ubiquitin, a process that is catalysed by three enzymes, E1, E2 and E3. The ubiquitinated protein is then degraded by the proteasome, a protein complex that degrade proteins by proteolysis. Proteolysis is the chemical breaking of peptide bonds. Complete proteolysis of a protein results in the amino acids the protein was built up of. These amino acids can then be broken down for energy or reused.

Removing the amino group from the amino acid

This is performed by aminotransferases, enzymes that catalyse the switching of an amino group with a keto group between to molecules. Check the MRTs. Often, these aminotransferases move the amino group from the amino acid to α-ketoglutarate. The former becomes a keto acid, while the latter becomes glutamate. These reactions need PLP, a cofactor derived from vitamin B6, pyridoxine.

Two specific aminotransferases, alanine aminotransferase and aspartate aminotransferase are increased in case of liver damage, for example by alcoholism.

Other fates of the amino group

The amino group can also be used to synthesize glutamine from glutamate (an MRT).

The urea cycle is covered in the next chapter.

The two most common aminotransferases. Note that “transaminase” is an old name for aminotransferase.

Removal of ammonia

Most ammonia in the body is removed by glutamine synthethase, which incorporates it into glutamine.


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10. Regulation and disorders of lipid metabolism

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12. Urea cycle

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