1.12 - Urea Cycle

Question 1

amines urea contains (2)

Answer 1

1. ammonium NH4+
2. aspartate

Question 2

what is the ammonium NH4+ ion present in urea used for in the urea cycle?

Answer 2

used to produce carbamoyl phosphate

Question 3

what is glutamate an intermediate source of? (2)

Answer 3

1. ammonium (NH4+) ion
2. aspartate

Question 4

where are ammonium (NH4+) ions provided?

Answer 4

in the mitochondrial matrix

Question 5

what does glutamate undergo in the mitochondrial matrix?

Answer 5

deamination

Question 6

how do ammonium (NH4+) ions enter the urea cycle? (2)

Answer 6

1. free ions combine with bicarbonate ions to give carbamoyl phosphate
2. this enters the urea cycle via ornithine trans carbamoylase

(in mitochondrial membrane)

Question 7

which enzyme liberates NH4+ (ammonium ion) from glutamate

Answer 7

glutamate dehydrogenase (GDH)

Question 8

which amino acids can be directly deaminated? (4)

Answer 8

1. glutamate
2. histidine
3. glycine
4. serine

Question 9

how is carbamoyl phosphate made from bicarbonate ion (HCO3-) and NH4+?

Answer 9

ammonia coupled with bicarbonate in mitochondria via carbamoyl phosphate synthetase

Question 10

how can glutamate give aspartate

Answer 10

glutamate can undergo transamination and donate the amine to axaloacetate giving aspartate

Question 11

what enzyme is present at high levels in the liver to facilitate nitrogen removal?

Answer 11

aspartate aminotransferase

Question 12

how does glutamate release the amine ammonium ion (NH4+)

Answer 12

undergoes deamination

Question 13

what are bicarbonate ions (HCO3-) the result of

Answer 13

CO2 production by TCA cycle

Question 14

ATP cost of urea cycle per urea

Answer 14

3 ATP

Question 15

urea cycle equation

Answer 15

CO2 + NH4- + 3ATP + aspartate + 2H2O -> urea + 2ADP + AMP + PPi + fumarate

Question 16

how do most other tissues get carbon skeleton from amino acids to use as fuel source?

Answer 16

use amino acid deaminase to catabolise amino acids

Question 17

where can glutamine come from?

Answer 17

any tissue in the body

Question 18

role of glutamine in amino group excretion

Answer 18

transported to the kidney where it feeds into the urea cycle

Question 19

process of ammonia ions forming glutamine (2)

Answer 19

1. ammonia ion used to produce glutamate by glutamate dehydrogenase
2. glutamine synthetase uses 2nd ammonia ion to produce glutamine

Question 20

role of glutaminase (2)

Answer 20

1. deaminates glutamine -> glutamate + ammonia
2. both the ammonium ion + glutamate can then enter urea cycle through mitochondria

Question 21

functions of liver in protein metabolism (4)

Answer 21

1. key site of amino acid catabolism (carbon skeletons recycled for gluconeogenesis)
2. site of citrulline production via urea cycle
3. synthesises fatty acids from glucose
4. synthesises ketone bodies and secretes into blood stream (fuel for peripheral tissues)

Question 22

functions of kidney in protein metabolism (4)

Answer 22

1. excretes urea produced by liver
2. metabolises glutamine and ammonia to excrete nitrogen
3. absorbs major plasma protein albumin (then broken down -> amino acids)
4. major site arginine synthesis from citrulline (intermediate produced by excretion of nitrogen - urea cycle)

Question 23

organs that can use gluconeogenesis (2)

Answer 23

1. liver
2. kidney

Question 24

urea cycle

Answer 24

ammonia/nitrogen metabolism once in liver or kidney

Question 25

molecules that provide the 2 main amines for urea synthesis (3)

Answer 25

1. glutamine
2. glutamate
3. aspartate

Question 26

how can glutamate provide both amines required for urea synthesis? (2)

Answer 26

1. 1st through glutamate dehydrogenase activity producing carbamoyl phosphate
2. 2nd through transamination into aspartate