ammonia assimilation

Question 1

normoammonemia

Answer 1

10-30 uM

Question 2

hyperammonemia

Answer 2

> 65 uM

Question 3

ammonia concentration is what throughout the human body?

Answer 3

not uniform

Question 4

central nervous system must be kept scrupulously free of _____ to prevent depletion of __________

Answer 4

NH3; a-ketoglutarate

Question 5

cystolic ammonia must be maintained at or below

Answer 5

0.05 mM

Question 6

high concentrations of ammonia are toxic to humans…

Answer 6

:NH3 crosses membrane bilayer, thereby dissipating H+ gradients; depletes a-KG, thereby greatly inhibiting TCA activity

Question 7

NH3 assimilation is catalyzed by 4 human enzymes:

Answer 7

glutamate dehydrogenase, glutamine synthetase, carbamoyl-phosphate synthetase-1, (glycine synthase)

Question 8

glutamate dehydrogenase

Answer 8

NADPH+NH3+a-KG -> glutamate+H2O+NADP+

Question 9

glutamine synthetase

Answer 9

ATP+NH3+Glutamate-> glutamine+Pi+ADP+H+

Question 10

carbamoyl-phosphate synthetase-1

Answer 10

2ATP+NH3+CO2->2HN(C=O)-O-P+Pi+2ADP+H+

Question 11

enzymes use ___ to install a good leaving group

Answer 11

ATP

Question 12

what is a poor leaving group?

Answer 12

hydroxide ion

Question 13

what is a very good leaving group?

Answer 13

phosphate

Question 14

what drives deprotonation of ammonium ion?

Answer 14

deltaGatp-hydrolysis

Question 15

NH4+ deprotonation is drive by ____________________ that re-positions a guanidinium group of a nearby arginine residue closer to the enzyme-bound NH4+

Answer 15

ATP-dependent conformational change

Question 16

____________ expels a proton from NH4+ to generate enzyme bound :NH3

Answer 16

electrostatic repulsion

Question 17

what is the main way to trap NH3 within the cytosol?

Answer 17

glutamine sythetase

Question 18

__________ is a major inter-organ nitrogen shuttle, avoiding the entry of toxic NH3 into the circulatory system

Answer 18

glutamine

Question 19

glutamine is the _________________________

Answer 19

nitrogen source in nearly all biosynthetic reactions

Question 20

cells show _____________

Answer 20

division of labor-> they carry out different metabolic tasks in different organelles or subcellular locations

Question 21

metabolite pool

Answer 21

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

Question 22

carbomoyl-P has two destinies in the liver:

Answer 22

urea and pyrimidine nucleotides

Question 23

so liver cells have two carbomoyl-P pools

Answer 23

mitochondrial CP pool->UREA
cytosolic CP pool-> pyrimidine nucleotides

Question 24

carbamoyl-phosphate synthetase 1

Answer 24

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

Question 25

what does step 1 of the carbamoyl-phosphate synthetase-1 do?

Answer 25

resembles glutamine synthetase mechanism by forming acyl-phosphate phosphoryl intermediate prior to :NH3 attack

Question 26

what does step 2 of the carbamoyl-phosphate synthetase-1 do?

Answer 26

kinase-type reaction that directly transfers the phosphoryl group to carbamate

Question 27

CPS-II: A glutamine dependent enzyme

Answer 27

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

Question 28

CPS-II forms…

Answer 28

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

Question 29

cps-II has three active sites

Answer 29

sites 1 &2 connected by long ammonia tunnel; sites 2 & 3 are connected by long carbamate tunnel