Amino Acid Catabolism Flashcards
Nitrogen + Bacteria/fungi/plants
N deficient so don’t excrete it, store it
Nitrogen + animals
Have excess N (protein) so don’t store but excretes
Pepsin
cleaves between hydrophobic and aromatic residues optimum activity at pH 2 (stomach)
Chymotrypsin
cleaves aromatic carboxyl side residues (duodenum)
Trypsin
cleaves carboxyl side of lysine or arginine (duodenum)
Amino peptidases
breaks down oligopeptides to amino acids and tri/dipeptides
Proteolytic enzymes
breaks down proteins to a.a. and oligopeptides
What are the main sources for protein degradation?
~ mis-folding of proteins
~ errors in translation
~ regulation of pathways
~ damaged proteins
Ubiquitin
the signal for protein death
Ubiquitination
ATP + ubiquitin + target (lysine residue) –> AMP + ubiquitin-target
Enzyme = Ubiquitin ligase
~ more and more molecules get added in a chain
How does ubiquitin ligase choose its target?
N terminal rule:
the N-terminal amino acid of a protein determines its half-life
~ diff a.a. have different half lives
If amino acids are not needed then…
~ their amino group is removed by aminotransferase
~ carbon skeleton degraded
Aminotransferase
removes amino group from amino acid and produces a keto acid + glutamate
~ all specific for their substrate
Glucogenic amino acids
when an a.a.’s carbon skeleton can be feed into the citric acid cycle via gluconeogenesis
Which 3 main keto acids do glycogenic a.a. donate their carbon skeletons to?
~ pyruvate
~ oxaloacetate
~ alpha-ketoglutarate
Ketogenic amino acids
~ help synthesis of fatty acids or ketone bodies (not glucose)
~ metabolised to acetyl CoA or acetoacetyl CoA
~ a.a. that cannot be metabolised to citric acid intermediates
Which amino acid is both glycogenic and ketogenic?
Tryptophan
What is the fate of the amino group?
~ amino group loaded onto glutamate via transamination reaction
~ glutamate dehydrogenase breaks down glutamate to ammonia + alpha-ketoglutarate
What happens to the toxic ammonia produced?
aquatic animals = release it directly into he water
birds/reptiles = secrete as uric acid in faeces
land vertebrates = synthesise urea and excrete as urine (urea cycle)
How is aspartate regenerated?
in the krebs ‘bi’cycle
How do the nitrogens enter the urea cycle?
one as carbamoyl phosphate
one as aspartate
Name two amino acid metabolism disease.
Phenyketonuria
Argininosuccinase deficiency
Phenyketonuria
~ no phenylalanine hydroxylase
~ accumulation of phenylalanine in tissues
~ risk of severe retardation
Argininosuccinase deficiency
~ lack of enzyme argininosuccinase
~ ammonia buildup –> lead to death
~ arginine surplus required
