amino acid catabolism and the urea cycle Flashcards
glutamate dehydrogenase
abundant in liver
localizes to mitochondrial matrix
if glutamate levels high, reaction goes to oxidative deamination
allosterically regulated by energy status: ATP and GTP allosteric inhibitors, ADP and GDP activators
overall stoichiometry of urea synthesis
CO2 + NH4 + 3ATP + aspartate + H2O –> urea + 2 ADP + 2 Pi + AMP + PPi + fumarate
net production of PPi makes reverse reaction unfavorable
although some energy is required, when AA are used as fuel for the TCA cycle more ATP is generated than is consumed (eg alanine –> pyruvate –> acetyl CoA –> CO2 => 12 ATP; need 3 ATP for urea synthesis)
urea cycle steps
1: carbamoyl phosphate synthetase I uses CO2, NH3 and 2 ATP to make carbamoyl phosphate - releases 3H+, 2ADP and Pi
2: ornithine trascarbamoylase combines L-Ornithine with carbamoyl phosphate to make L-citrulline - releases Pi
(note : these two steps occur in the mitochondrial matrix)
3: L-Citrulline is transported out of the mitochondrion by transporter
4: argininosuccinate synthase uses the L-citrulline and an L-asparate (made from oxaloacetate) to make argininosuccinate - uses ATP and releases AMP + PPi
5: arginosuccinate lyase converts argininosuccinate to fumarate and L-arginine (note: fumarate is used to make the asparatate used in step 4)
6: arginase converts L-arginine to urea and L-ornithine
7: L-ornithine is transported into the mitochondiral matrix to be used in step 2
generation of L-aspartate in urea cycle
argininosuccinate lyase makes fumarate and L-arginine from argninosuccinate
the fumarate is hydrated to malate
the malate is oxidized to oxaloacetate
oxaloacetate is transaminated to aspartate (converting glutamate to aKG in the process)
carbamoyl phosphate synthetase I (CPS1)
step 1 in urea cycle
irreversible
rate limiting step
allosterically activated by N-acetyl glutamate
in mitochondrial matrix
uses CO2, NH3, and 2 ATP to make carbamoyl phosphate
release 3H+, 2 ADP, and Pi in the process
ornithine transcarbamoylase
step 2 in urea cycle
in mitochondrial matrix
combines carbamoyl phosphate and L-ornithine to make L-citrulline
argininosuccinate synthase
step 4 in urea cycle
in cytoplasm
combines L-citrulline with L-aspartate to make argininosuccinate
uses ATP - releases AMP and PPi
argininosuccinate lyase
step 5 of urea cycle
in cytoplasm
reversible
converts argininosuccinate to fumarate and l-arginine
arginase
step 6 in urea cycle
in cytoplasm
irreversible
converts l-arginine to urea and l-ornithine
n-acetyl glutamate
allosterically activates carbamoyl phosphate synthase I and therefore helps regulate rate of urea cycle
controlled by AA levels, specifically arginine
when AA levels increased (such as after a meal) activity of N-acetylglutamate synthetase increases => increase in activity of carbamoyl phosphate synthetase I and increased production of urea
glutamine synthetase
enzyme in most tissues
uses ammonia and glutamate to make glutamine:
glutamate + NH4 + ATP –> glutamine + ADP + Pi
glutamine then transferred to liver where hepatic glutaminase can make ammonia and glutamine
method for transporting extrahepatic amino groups to the liver
hepatic glutaminase
in liver
glutamine + H20 –> glutamate + NH4
glucose-alanine cycle
method for transporting extra-hepatic amino groups to the liver
muscle amino groups are used to transaminate pyruvate => alanine
alanine transported to liver via bloodstream
amino groups from hepatic alanine used to transaminate aKG to make glutamate
glutamate is an excellent nitrogen source for urea synthesis
cycle very active under starvation conditions
inborn errors in urea cycle enzymes
can be mutations in all five of the urea cycle genes
disorders characterized by hyperammonemia, encephalopathy, respiratory alkalosis
disposal of excess AA
after protein rich meal or when low ATP/GTP levels, AA catabolized to CO2 and NH3 by AA transferases, TCA cycle, and oxidative deamination of glutamate
