L15: Ammonia assimilation Flashcards
Why is ammonia toxic to humans?
NH3 crosses the membrane, dissipating H+ gradients/ depletes alpha-KG, inhibiting TCA activity
What 4 enzymes catalyze NH3 assimilation?
GDH, Glutamine synthetase, carbamoyl-Phosphate synthetase 1, & Glycine synthase
Reaction for GDH?
NADPH + NH3 + alpha-KG -> Glutamate + H2O + NADP+
Reaction for Glutamine synthetase?
ATP + NH3 + Glutamate -> Glutamine + Pi + ADP + H+
Reaction for Carbamoyl-phosphate synthetase 1?
2ATP + NH3 + CO2 -> 2HN(C=O)-O-P + Pi + 2ADP + H+
reaction for glycine synthetase?
NH3 + CO2 + NADH + N5,N10-CH2-BH4 -> Glycine + NAD+ + BH4
What is the main way to trap NH3?
Glutamine synthetase; Gln is a major nitrogen source in many biosynthetic rxns; it is a major inter-organ nitrogen shuttle that avoids direct transfer of NH3 from distant organs to the liver
What kind of rxn is Glutamine synthetase?
Double-displacement mechanism- two sn2 rxns
What is the essential intermediate in glutamine synthetase?
gamma-Glutamyl P
What drives ammonium ion deprotonation?
Delta G atp-hydrolosis; deprotonation driven by enzyme conformation change
what is the main ammonia-assimilating rxn in mitochondria?
Carbamoyl-phosphate synthetase 1; it is a highly energy-dependent rxn; it is a multi-step rxn with enzyme-bound intermediates
Why is glutamine vital for the efficient biosynthesis of nitrogen-containing metabolites?
it allows glutamine-dependent biosynthetic enzymes to generate ammonia in situ. The ammonia is then used as a nucleophile without ever touching water.
What is CPS-2?
it is a glutamine- dependent enzyme that uses a transfer tunnel to move UNPRROTONATED NH3 from glutamine-hydrolysis site to the biosynthetic subunit. This avoids toxicity of ammonia in our cells.
What is the 1st subunit of CPS-2 and what does it do?
Glutamine-hydrolyzing subunit; it hydrolyzes glutamine to glutamate and NH3
What is the 2nd subunit of CPS-2?
Biosynthetic subunit;
