1.12 - Urea Cycle

Question 1

amines urea contains (2)

Answer 1

1. ammonium NH4+
2. aspartate

Question 2

what is glutamate an intermediate source of? (2)

Answer 2

1. ammonium (NH4+) ion
2. aspartate

Question 3

where are ammonium (NH4+) ions provided?

Answer 3

in the mitochondrial matrix

Question 4

what does glutamate undergo in the mitochondrial matrix?

Answer 4

deamination

Question 5

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

Answer 5

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

(in mitochondrial membrane)

Question 6

which enzyme liberates NH4+ (ammonium ion) from glutamate

Answer 6

glutamate dehydrogenase (GDH)

Question 7

which amino acids can be directly deaminated? (4)

Answer 7

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

Question 8

how can glutamate give aspartate

Answer 8

glutamate can undergo transamination and donate the amine to oxaloacetate giving aspartate (via aspartate aminotransferase)

Question 9

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

Answer 9

aspartate aminotransferase

Question 10

what are bicarbonate ions (HCO3-) the result of

Answer 10

CO2 production by TCA cycle

Question 11

ATP cost of urea cycle per urea

Answer 11

3 ATP

Question 12

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

Answer 12

use amino acid deaminase to catabolise amino acids

Question 13

where can glutamine come from?

Answer 13

any tissue in the body

Question 14

role of glutamine in amino group excretion

Answer 14

transported to the kidney where it feeds into the urea cycle

Question 15

process of ammonia ions forming glutamine (2)

Answer 15

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

Question 16

role of glutaminase in preparing glutamine for urea cycle (2)

Answer 16

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

Question 17

functions of liver in protein metabolism (4)

Answer 17

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 18

functions of kidney in protein metabolism (4)

Answer 18

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 19

organs that can use gluconeogenesis (2)

Answer 19

1. liver
2. kidney

Question 20

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

Answer 20

1. glutamine
2. glutamate
3. aspartate

Question 21

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

Answer 21

1. 1st through glutamate dehydrogenase activity (NH4+ + bicarbonate ions => carbamoyl phosphate
2. 2nd through transamination into aspartate