ammonia assimilation Flashcards
normoammonemia
10-30 uM
hyperammonemia
> 65 uM
ammonia concentration is what throughout the human body?
not uniform
central nervous system must be kept scrupulously free of _____ to prevent depletion of __________
NH3; a-ketoglutarate
cystolic ammonia must be maintained at or below
0.05 mM
high concentrations of ammonia are toxic to humans…
:NH3 crosses membrane bilayer, thereby dissipating H+ gradients; depletes a-KG, thereby greatly inhibiting TCA activity
NH3 assimilation is catalyzed by 4 human enzymes:
glutamate dehydrogenase, glutamine synthetase, carbamoyl-phosphate synthetase-1, (glycine synthase)
glutamate dehydrogenase
NADPH+NH3+a-KG -> glutamate+H2O+NADP+
glutamine synthetase
ATP+NH3+Glutamate-> glutamine+Pi+ADP+H+
carbamoyl-phosphate synthetase-1
2ATP+NH3+CO2->2HN(C=O)-O-P+Pi+2ADP+H+
enzymes use ___ to install a good leaving group
ATP
what is a poor leaving group?
hydroxide ion
what is a very good leaving group?
phosphate
what drives deprotonation of ammonium ion?
deltaGatp-hydrolysis
NH4+ deprotonation is drive by ____________________ that re-positions a guanidinium group of a nearby arginine residue closer to the enzyme-bound NH4+
ATP-dependent conformational change
____________ expels a proton from NH4+ to generate enzyme bound :NH3
electrostatic repulsion
what is the main way to trap NH3 within the cytosol?
glutamine sythetase
__________ is a major inter-organ nitrogen shuttle, avoiding the entry of toxic NH3 into the circulatory system
glutamine
glutamine is the _________________________
nitrogen source in nearly all biosynthetic reactions
cells show _____________
division of labor-> they carry out different metabolic tasks in different organelles or subcellular locations
metabolite pool
cells make and store metabolites where they’ll be needed; pool is defined as reservoir of a metabolite, as located in a specific sub-cellular compartment and kept from another pool of the same metabolite, often by a membrane barrier
carbomoyl-P has two destinies in the liver:
urea and pyrimidine nucleotides
so liver cells have two carbomoyl-P pools
mitochondrial CP pool->UREA
cytosolic CP pool-> pyrimidine nucleotides
carbamoyl-phosphate synthetase 1
main ammonia-assimilating step in mitochondria; first committed step in urea cycle; highly energy-dependent reaction uses 2 ATP molecules; multi-step mechanism with enzyme-bound intermediates
what does step 1 of the carbamoyl-phosphate synthetase-1 do?
resembles glutamine synthetase mechanism by forming acyl-phosphate phosphoryl intermediate prior to :NH3 attack
what does step 2 of the carbamoyl-phosphate synthetase-1 do?
kinase-type reaction that directly transfers the phosphoryl group to carbamate
CPS-II: A glutamine dependent enzyme
uses a transfer tunnel to move unprotonated :NH3 from glutamine-hydrolysis site to ‘biosynthetic’ site; tunnel prevents any protonation of ammonia; covalently bound glutamate acts as a lid covering the glutamine site, preventing entry of h2o or h3o+; used throughout metabolism
CPS-II forms…
y-glutamine thiolester, releasing deprotonated :NH3, which traverses an aprotic ammonia tunnel to the site of carbamate synthesis. the latter then travels thru the carbamate tunnel to the site of carbamoyl-P synthesis & release
cps-II has three active sites
sites 1 &2 connected by long ammonia tunnel; sites 2 & 3 are connected by long carbamate tunnel
