amino acid catabolism Flashcards
What are the two main methods of amino acid acquisition by cells?
1) digestion and absorption of dietary protein
2) turn over of out own cellular proteins (cellular protein degradation by ubiquitin mediated proteosomal degradation to produce amino acids)
What three enzymes are involved in the breakdown of dietary proteins into amino acids that can be absorbed? What are their targets within the protein sequence?
Pepsin - cleaves between hydrophobic and aromatic residues
Chymotrypsin - cleaves the C-terminal aromatic residues
Trypsin - cleaves on the carboxyl side of Lys or Arg residues
What enzyme group is involved in the break down of amino acids into their constituent parts when they are not required by the cell?
Aminotransferases
What is the most common keto acid involved in the transamination reactions to breakdown amino acids? what amino acid does this form?
- a-ketoglutarate
- the transfer of an amino group to a-ketoglutarate forms the amino acid glutamate.
What happens to the glutamate product of transamination (where a-ketoglutarate is used as the keto acid)?
oxidative deamination of glutamate by glutamate dehydrogenase to produce a-ketoglutarate and ammonium, also produces NADH
What are the three potential fates of the keto acid product formed from the removal of the broken down amino acid?
1) degraded
2) fed into CA cycle (if keto acid is pyruvate, oxaloacetate, or a-ketoglutarate) to form intermediates that can be used in gluconeogenesis = Glucogenic amino acids
3) metabolised to acetyl-CoA, which is then oxidised to carbon dioxide in CA cycle = Ketogenic amino acids
What is the name of the process that removes ammonium from the body?
Urea cycle
Describe the 5 stages of the urea cycle? and name the enzyme that catalyses each step and ATP expense
1) formation of carbamoyl phosphate from ammonium and carbon dioxide
- enzyme = carbamoyl phosphate synthetase
- 2 ATP consumed
2) ornithine unit combines with carbamoyl phosphate to form citrulline
- enzyme = ornithine transcarbamoylase
3) citrulline transported out of mitochondria and condensed with aspartate (second amino group added) to form arginosuccinate
- enzyme = ornithine transcarbamoylase
- 1 ATP consumed
4) fumarate removed and arginine formed
- enzyme = arginosuccinase
5) hydrolysis of arginine to release urea and reform ornithine
- enzyme = arginase
What is the fate of fumarate produced in the urea cycle?
goes into the CA cycle, converted to malate then oxaloacetate
- oxaloacetate (a keto acid) undergoes a transamination with an amino acid to produce a keto acid and Aspartate.
What are the two important reasons for passing fumarate through the CA cycle following production in the urea cycle?
1) reforms aspartate to allow the continuation of the urea cycle
2) oxidation of malate to oxaloacetate produces NADH which offsets the ATP usage of the urea cycle (since theoretically oxidation of NADH yields up to 3 ATP)
How many ATP are used by the urea cycle?
3
What is phenylketonuria?
reduced activity of phenylalanine hydroxylase enzyme (unable to metabolise phenylalanine so accumulates and cause mental retardation)
What are the effects of arginosuccinate deficiency?
build up of ammonium that can lead to death
The deamination of what amino acid produces pyruvate keto acid (3C)?
any 3 Carbon amino acid: alanine, serine, glycine, tryptophan etc…
The deamination of what amino acid produces oxaloacetate keto acid (4C)?
any 4 carbon amino acid: aspartate, asparagine
