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]]>The condensations of CO2 + NH4 and ATP to form carbamoyl phosphate. This is catalyzed by carbamoyl phosphate synthase-1(CPS-1). 2 ATP are required in which 1 ATP serve as the source of phosphate and second one is converted to AMP and pyrophosphate (PPi).
The carbamoyl phosphate donates its phosphate group to the ornithine to form citrulline. This reaction is catalyzed by mitochondrial ornithine trans-carbamoylase.
Transfer of second amino group from aspartate to citrulline forms arginosuccinate. This reaction requires Mg-ATP.
Breakdown of arginosuccinate by arginosuccinase forms free arginine and fumerate. The fumerate formed then enters the TCA cycle.
In the last reaction of urea cycle, the liver hydrolytic enzyme breaks the arginine to form urea and ornithine.
Energy cost of urea cycle Four ATP are consumed per molecule of urea formation
1. Urea is formed from which toxic material?
A. CO2
B. Ammonia
C. Uric acid
D. All of the above
2. Urea cycle provides intermediate for which pathway?
A. Glycolysis
B. HMP-shunt
C. TCA cycle
D. Gluconeogenesis
3. Where do urea cycle occur?
A. Muscle cell
B. Liver cell
C. Kidney
D. Heart
4. From where are the two amino groups of urea derived?
A. both derived from ammonia
B. Both derived from aspartate
C. One from ammonia and 1 from aspartate
D. None of the above
5. Which of the following is the rate limiting step of urea cycle?
A. Synthesis of citrulline
B. Synthesis of carbamoyl phosphate
C. synthesis of arginine
D. Synthesis of ornithine
6. Which of the following is the first reaction of urea cycle?
A. Formation of ornithine
B. Formation of urea
C. Formation of arginosuccinate
D. None of the above
7. How many ATP are required for the formation of carbamoyl phosphate?
A. 2
B. 3
C. 4
D. 1
8. Match the following enzyme and its product-
a. CPS-1 1. fumerate
b. Ornithine trans-carbamoylase 2. urea
c. Arginase 3. citrulline
d. Argininosuccinase 4. Carbamoyl phosphate
9. Carbamoyl phosphate donates its phosphate group to which compound to form catrulline?
A. Arginosuccinate
B. Ornithine
C. Fumerate
D. Urea
10. Which of the following compounds are formed in urea cycle?
A. Arginosuccinate
B. Ornithine
C. Fumerate
D. All of the above
11. How many ATP is/are converted to AMP and PPi to form arginosuccinate?
A. 1
B. 2
C. 4
D. 6
12. Which of the following statement is NOT true?
A. Four ATP molecules arereqired for synthesis of urea
B. Urea dispose CO2, ammonia and biotin
C. CPS-1 is the principal enzyme of urea cycle
D. High concentration of ammonia, stimulates urea cycle.
13. Why ornithine and citrulline not found in the protein structure?
A. They have excess codons
B. They lack codons
C. The secondary structure lacks hydrogen bonding
D. Both A and C
14. Which of the following is the significance of urea cycle?
A. Regulates BP
B. Regulates blood volume
C. Regulate blood flow
D. Regulate blood Ph
15. What enhances urea synthesis?
A. increased level of N-acetyl glutamate (NAG)
B. Decreased level of NAG
C. Protein rich diet
D. Both A and C
ANSWERS:
1. Ammonia
2. TCA cycle
3. Liver
4. One from ammonia and 1 from aspartate
5. Synthesis of carbamoyl phosphate
6. None of the above
7. 2
8. a – 4 b – 3 c – 2 d – 1
9. Ornithine
10. All of the above
11.1
12. Urea dispose CO2, ammonia and biotin
13. They lack codons
14. Regulate blood Ph
15. Both A and C
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REFERENCE:–
1. Pankaja Naik- Biochemistry; 4th edition; page no:- 271-273
2. U Satyanarayana, U Chakrapani-Biochemistry; 4th edition; Page no:- 337-340
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