Amino Acids (Lecture 26)

Question 1

where are the reactions in there cycle compartmentalized?

Answer 1

mitochondrial matrix

cytosol

Question 2

what is the source of NH4 in the urea cycle?

Answer 2

glutamine that enters the liver and is transformed into glutamate via glutaminase

glytaminate will undergo a demaniation via glutamte dehydrogenase to release NH4

Question 3

once NH4 is found in the mitochondrial matrix, which to elements are combined to make carbonyl phosphate?

Answer 3

NH4 + 2ATP + CO2 (HCO3-)

Question 4

Which enzyme is responsible for carbonyl phosphate, the first substrate of the urea cycle?

Answer 4

carbamoyl phosphate synthetase (CPS1)

Question 5

under what conditions will CPS1 be activated?

Answer 5

high [ATP], since it needs 2 ATP to synthesize carbonyl phosphate

Question 6

explain the mechanism of CPS1

Answer 6

bicarbonate (source of CO20- will prime its carbon with an input of ATP

this will form carboxyphsophate (primed intermediate)

an NH3 will replace the phosphate group to form carbamate

carbamate will be primed again via ATP to form the high energy intermediate, carbomoyl phosphate

Question 7

why is CPS1 irreversible?

Answer 7

the rate limiting step

Question 8

how is CPS1 regulated?

Answer 8

allosteric activated by NAG (N-acetlyglutamate)

NAG is produced by NAGS (NAG synthase), which is stimulated by argenine

NAG proportional to the amount of aa (arginine, glutamine)

abundance of aa activates CPS1

Question 9

CPS1 vs CPS2

Answer 9

CPS1:

• urea synthesis
• needs NAG activation
• uses NH4+
• mitochondrial

CPS2:

• pyrimidine synthesis
• cytoplasmic
• no need for NAG
• uses glutamine as an indirect donor of NH3

Question 10

why are CPS1 and CPS 2 compartmentalized?

Answer 10

ensures that both products are made (both pathways work at the same time)

Question 11

what are some hyperammonenia causes?

Answer 11

deficiency in CPS-1 (missense mutation) causes death.

if heterozygous for deficiency in CPS1, could survive but an over intake of aa could be toxic

NAGS deficiency (nonsense mutation) causes a delay or poor activation of CPS1

GDH gain of function mutation (in GTP allosteric binding site), more NH4 will be produced, thus clogging the CPS1

Question 12

step 1 of the urea cycle

Answer 12

ornithine is imported int the mitochondrial matrix via a membrane transporter

orthinine combines with carbonyl phosphate by transpiring the carbonyl group onto ornithine via orthinine transcarbomoylase . this synthesizes citruline

citruline is then exported into the cytoplasm

Question 13

mutations in mitochondrial transporters and OTC can cause:

Answer 13

hyperammonemia

Question 14

what it the membrane transporter?

Answer 14

ORC1 , which transports citruline and ornithine via an antiport mechanism

Question 15

step 2 of the urea cycle:

Answer 15

arginosuccinate synthase (ASS) incorporates the second amine group in urea

ASS primes the citruline’s carbons with ATP to form citrullyl-AMP. PPi is also released in this reaction and as we know, this may serve as a driving point because PPi is hydrolyzed into 2Pi

a nucleophilic attack by an alpha amine group of aspartate will release AMPO to form arginosuccinate

Question 16

mutations in ASS can cause:

Answer 16

hypperammnemia

Question 17

step 3 or the urea cycle:

Answer 17

arginosuccinate synthesizes arginine and releases fumarate (CAC intermediate) via arginosuccinase

Question 18

step 4 of the urea cycle:

Answer 18

arginine is hydrolyzed into ornithine via arginase

this step will release urea, which will be transported tot he kidneys via the blood and eventually excreted via the urine

Question 19

how is aspartate incorporated into the urea cycle?

Answer 19

in the mitochondria, glutamate and oxaloactate will undergo a transamination via aspartate aminotransferase in order to synthesize alpha KG and aspartate.

the aspartate is carrying the NH3

aspartate will then serve as a substrate (step 2)

Question 20

where are the NH3 coming from?

Answer 20

both from glutamate

• oxidative deamination of glutamate via GDH
• aspartate, which is the transamination of glutamate

Question 21

what is the source of oxaloacetate?

Answer 21

CAC

Question 22

what’s the fate of fumarate?

Answer 22

replenish the CAC to synthesize more oxaloacetate

Question 23

what is the energy cost of the urea cycle?

Answer 23

3ATPs used
GDH produces 2.5 ATP
and the fumarate generated can replenish the CAC to produce 2.5 ATP