Amino acid metabolism Flashcards
Glutamine (GLN) synthesis
- Glutamine synthetase responsible for 2 steps:
1) L-Glutamate + ATP –> gamma-glutamyl phosphate
2) gamma-glutaryl phosphate + NH4 –> L-Glutamine + PO4
Transamination
Transaminases:
alphaketoglutarate _ L-amino acid –> L-Glutamate + alpha-ketoacid
Transaminases contain PLP coenzymes.
- PLP coenzyme (prosthetic group of transaminase):
1) C4’ Aldehyde groupL forms covalent link with a-amino group of the substrate. Forms a schiff base (aldimine bond)
2) PO4 group: fits in enzyme
3) Pyridine ring: planar structure. e- sink
4) OH group: H-bond with substrate. Improve catalytic efficiency
Oxidative deamination
-Glutamate dehydrogenase
Glutamate + NAD(P) intermediate + H20 a-KGA + NH4
-GDH missense mutations: in the allosteric GTP binding site, cannot be shut down => hyperammonemia, hyperinsulinism
“Funneling” in AA catabolism
1) transamination (reversible):
a-KGA+ AAs –> Glu + a-ketoacids
2) Oxidative deamination:
Glu + NAD(P) + H2O–> NH4 + a-KGA
3) 1st N-assimilation reaction: NH4 + Glu + ATP –> Gln + ADP + Pi
4) Gln –> N-compounds + Glu
PLP reactions
1) PLP is covalently linked to the enzyme
2) Formation of an external aldimine intermediate
3) Formation of a carbanion intermediate (chelate ring (internal aldimine) + pyridine ring confers planar geometry)
4) Formation of pyrodoxamine phosphte (PMP)
Type 2 tyrosinemia
- deficiency in tyrosine aminotransferase in liver
- due to non sense mutation (protein unexpressed)
- defect in tyrosine catabolism
Glucose-alanine cycle
- Alanine transports NH4 from muscles to the liver
- Alanine Transaminase (ALT)
-Glutamate + Pyruvate a-KGA + Alanine => Blood circulation
In the liver : Alanine –> Glutamate –> NH4 –> Urea
The Urea Cycle
-Pre-cycle
1st (Glutaminase) : Glutamine –> Glutamate
2nd (GDH: regulated by cellular energy levels, GTP decrease and ADP increase activity): Glutamate –> NH4 + a-KGA
3rd (carbamoyl phosphate synthetase1 CPS1: activated allosterically by NAG), 1st step irreversible:
NH4 + HCO3 + 2ATP –> Carbamoyl phosphate + 2ADP + Pii
*NAG activates CPS1:
Acetyl CoA + Glutamate + NAG synthase (activated by arginine and requires glu)–> NAG + CoA-SH
CPS1 and NAGS deficiency (caused by nonsense or missense mutations): HyperAmmonemia, liver failure, death
-Urea Cycle:
-1st step (ornithine transcarbamoylase OTC): importation of ornithine into the mitochondria
Ornithine + Carbamoyl phosphate –> Citruline (exported to the cytoplasm)
*mutation in these transporters cause hyperAmmonemia.
-2nd Step (Arginosuccinate synthetase ASS):
Citrulline + ATP –> Citrullyl-AMP + PPi
Citrullyl-AMP + Aspartate (OAA + glutamate + Aspartate transaminase –> aspartate + a-KGA) –> Arginosuccinate + AMP
-3rd Step (Arginosuccinase):
Arginosuccinate –> Arginine + Fumarate (Fumarate goes back to CAC or gets converted to malate which then gets into the CAC)
- 4th step (Arginase)
Arginine + H2O –> Urea + Ornithine
Urea cycle, overall reaction
2NH4 + HCO3 + 3ATP + H2O –> Urea + 2ADP + 4Pi + AMP + 2H
-Synthesis of urea implies hydrolysis of 4 high-energy phosphate molecules
-The NADPH in the GDH step gets recuperated at the malate dehydrogenase
step
-REgulated rate of the synthesis of the urea cycle enzymes: coordinated transcription control, which increases with high load of AA. Transient increases in early stages of starvation
Ureal cycle Disorders
Mutations found in all the enzymes involved in the pathway + in transporter ORNT1. They can result in loss or lower activity of the enzymes.
- HyperAmmonemia (>240uM)
- 1/30 000 live births)
Treatments for deficiencies in Urea cycle enzymes
- Reducing protein intake
- Stimulating alternative pathways of NH4 excretion (can only stabilize the patient. There is still a risk of Sudden onset hyperammonemic coma):
- Administration of phenylbutyrate. It combines with glutamine to form phenylacetylglutamine which is then excreted in the urine. This allows to take up NH4 to regenarate the consumed glutamine
-Administration of Benzoate:
It combines with glycine to form Hippurate (benzoglycine) which is then excreted by the urine. This allows to take up excess NH4 to regenerate consumed glycine
- Ammonul = phenylacetate + benzoate
3. Gene Therapy: providing the w-t enzyme to the liver cell by administering the gene. (still is on clinical trial) - Treatment for GDH missense mutation: identify a chemical that can change the enzyme’s conformation so that the enzyme is downregulated
Amino acid catabolism/degradation
- N-free carbon skeletons
- Branched-chain amino acids are not degraded in the liver
- Converted to intermediates that feed into the CAC:
a) metabolized to CO2 and H2O
b) used for gluconeogenesis
-Funneling of AA carbon skeletons into the CAC can occur directly by deamination and transamination
- Asparagine readily enters CAC as OAA
- Glutamine readily enters CAC as a-KGA
- Aspartate –> arginosuccinate–> Fumarate (feeds to CAC)
Glucogenic a.a
Their catabolism end up to pyruvate or the 4 CAC intermediate.
-CAC intermediates can be used for gluconeogenesis
Ketogenic a.a.
Broken to acetyl-CoA or/and AcetoacetylCoA (partly or entirely) converted into ketone bodies (acetone, acetoacetate, D-ß-hydroxybutyrate –> energy for heart, skeletal muscle, kidney and brain.
Leucine and lysine
-Plants can convert Acetyl-CoA to glucogenic precursors but in mammal, there’s no net conversion of acetyl-CoA or acetoacetyl-CoA to glucogenic precursors.
No net synthesis of glucose is possible from leucine and lysine.
Glucogenic/ketogenic AAs
broken down to both types of Amphibolic intermediates
E.g threonine
