the urea cycle

Question 1

true/false: all pathways for amino acid degradation include the separation of the alpha-amino group

Answer 1

true

Question 2

what are the two themes of amino acid catabolism

Answer 2

N theme - catabolism of amino groups
C theme - catabolism of carbon skeletons

Question 3

how are amino acids broken down in order to enter the citric acid cycle?

Answer 3

amino acids are broken down to acetyl-coA which then gets into the citric acid by getting converted to citrate, or doesn’t get to the citric acid cycle and becomes ketone bodies

Question 4

when amino acids are used to replenish intermediates in the citric acid cycle, this is referred to as:
a) cataplerosis
b) anaplerosis
c) aminoplerosis
d) ketosis

Answer 4

b

Question 5

this enzyme catalyzes the combination of free ammonia with glutamate to yield glutamine; requires ATP and it is critical in order to transport toxic ammonia to the liver

Answer 5

glutamine synthetase

Question 6

this enzyme catalyzes the conversion of glutamine to glutamate and NH4+

Answer 6

glutaminase

Question 7

_____ muscles produce pyruvate, lactate and ammonia

Answer 7

skeletal

Question 8

this enzyme interconverts pyruvate and alanine via transamination with glutamate

Answer 8

alanine aminitransferase

Question 9

in the _________ cycle, alanine carries ammonia and the carbon skeleton from pyruvate to the liver

Answer 9

glucose-alanine

Question 10

in the glucose-alanine cycle, _______ is excreted

Answer 10

ammonia

Question 11

in the glucose-alanine cycle, _______ is used to produce glucose which is returned to the muscle

Answer 11

pyruvate

Question 12

involves muscle protein being degraded to provide more glucose to generate additional ATP for muscle contraction.

Answer 12

the glucose alanine cycle

Question 13

amino acids are not stored in appreciable concentrations. however, which amino acid would be found in significant concentration in the blood of basketball players after a long and intense workout in a gym?
a) histidine
b) alanine
c) glutamine
d) cysteine

Answer 13

b

Question 14

what is the primary role of the glucose-alanine cycle?
a) converting glucose to alanine
b) delivering amino groups from the muscle to the liver
c) delivering de novo synthesized glucose from the liver to the muslce
d) enhancing protein degradtion
e) preventing glucose accumulation in the muscle

Answer 14

b

Question 15

urea is produced from ammonia in 5 enzymatic steps: this enzyme brings in ammonia from deamination reactions

Answer 15

step 1: carbamoyl phosphate synthetase I

Question 16

urea is produced from ammonia in 5 enzymatic steps: this enzyme brings carbamoyl phosphate into the urea cycle. resulting citrulline can enter cytosol

Answer 16

step 2: ornithine transcarbamoylase

Question 17

urea is produced from ammonia in 5 enzymatic steps: this enzyme brings in second amino group from aspartate

Answer 17

step 3: argininosuccinate synthetase

Question 18

urea is produced from ammonia in 5 enzymatic steps: this enzyme removes length of carbon skeleton as fumerate

Answer 18

step 4: argininosuccinase

Question 19

urea is produced from ammonia in 5 enzymatic steps: this enzyme cleaves arginine to form urea and ornithine (back at the beginning of the cycle)

Answer 19

step 5: arginase

Question 20

urea is produced from ammonia in 5 enzymatic steps: this is the rate limiting step. the reaction has two activation steps that require ATP. the second step of the reaction uses ammonia from deamination reaction

Answer 20

the carbamoyl phosphate synthetase I reaction

Question 21

what is the first step of the carbamoyl phosphate swynthetase I reaction?

Answer 21

bicarbonate is phosphorylated by ATP (activation step)

Question 22

what is the second step of the carbamoyl phosphate synthetase I reaction

Answer 22

ammonia from deamination reactions displaces it phosphoryl group from bicarbonate to generate carbamate

Question 23

what is the third step of the carbamoyl phosphate synthestase I reaction

Answer 23

`carbamate is phosphorylated from ATP to yield carbamoyl phosphate

Question 24

______ catalyzes the formation of citrulline and Pi from ornithine and carbamoyl phosphate

Answer 24

ornithine transcarbamoylase

Question 25

the urea cycle:
a) can be used to make uric acid by oxidation of urea in the liver
b) occurs primarily in the kidneys
c) can be used ti generate NH4+ from urea
d) uses two unusual alpha-amino acids as intermediates

Answer 25

d

Question 26

where does the urea cycle occur?

Answer 26

in the mitochondria of the liver cells

Question 27

what is the main function of the urea cycle

Answer 27

to remove toxic ammonia from the body

Question 28

true/fasle: the formation of argininosuccinate requires atp

Answer 28

true

Question 29

______ catalyzes the condensation of the amino group from aspartate and the ureido group from citrulline to form argininosuccinate

Answer 29

argininosuccinate synthetase

Question 30

what is the intermediate of the formation of argininosuccinate

Answer 30

citrullyl-AMP intermediate

Question 31

true/false: in order to form argininosuccinate, citrulline needs to pass from the mitochondria and go into the cytosol

Answer 31

true

Question 32

________ catalyzes the reversible cleavage of argininosuccinate to form arginine and fumerate

Answer 32

argininosuccinase

Question 33

what happens to fumerate once it formed from argininosuccinate

Answer 33

fumerate gets converted to malate and joins the pools of other citric acid cycle intermediates

Question 34

is the formation of arginine reversible or irreversible

Answer 34

reversible

Question 35

______ catalyzes the cleavage of arginine to form urea and ornithine.

Answer 35

arginase

Question 36

where does the formation of urea from arginine occur

Answer 36

cytosol

Question 37

where does the formation of arginine from argininsuccinate occur

Answer 37

cytosol

Question 38

what happens to ornithine once it is formed from arginine

Answer 38

ornithine gets transported into the mitochondria to initiate another round

Question 39

two nitrogen containing groups are used to make urea in liver cells. which two molecules of the urea cycle contribute to these two groups?
a) carbamoyl phosphate and aspartate
b) alanine and glutamine
c) glutamate and glutamine
d) carbamoyl phosphate and glutamate
e) arginine and aspartate

Answer 39

a

Question 40

what links the citric acid cycle and the urea cycles together

Answer 40

aspartate-argininosuccinate shunt - links the amino groups and the carbon skeletons together

Question 41

which molecule that is produced in the cytosol by the urea cycle can be used by the citric acid cycle but cannot pass the inner mitochondrial membrane?
a) malate
b) argininosuccinate
c) ornithine
d) NAD+
e) fumerate

Answer 41

e

Question 42

does N-acetylglutamate inhibit or stimulate carbamoyl phosphate synthetase I

Answer 42

stimulates

Question 43

N-acetylglutamate synthase catalyzes the formation of N-acetylglutamate from ______ and _____

Answer 43

acetyl-coa and glutamate

Question 44

how is the urea cycle regulated
a) a diet rich in carbohydrates and lipoids results in downregualtion of urea cycle enzymes
b) arginine stimulate the urea cycle by activated N-acetylglutamate synthase
c) starvation or ingestion of protein-rich diets up regulates urea cycle enzymes
d) carbamoyl phosphate synthetase I is allosterically regulated by N-acetylglutamate
e) all of the above

Answer 44

e

Question 45

what are the two main forms of regulation of the urea cycle

Answer 45

1. at the level of enzyme synthesis for the urea cycle enzymes and carbamoyl phosphate synthetase I
2. by allosteric regulation of carbamoyl phosphate synthetase I