Biosynthesis of Nucleotides: Salvage Pathway and MCQs for GPAT, NEET UPSC, CSIR NET Exam

Purine and pyrimidine are dietary non-essential components. Dietary nucleic acid and nucleotides do not provide essential components for the synthesis of endogenous nucleic acids.

In humans, the synthesis of nucleic acid is done by 2 methods: de-novo synthesis method and salvage method

Biosynthesis of purine nucleotides means synthesis of AMP and GMP.

Salvage pathway for the synthesis of nucleic acids:

Conversion of free purine to nucleotide is called salvage pathway; where salvage means property saved from losses.

Metabolic degradation of nucleotide and nucleic acids produces purine bases like adenine, guanine etc.

This pathway is simpler and consumes very less energy.

This pathway is found in such tissues which are incapable of synthesis of nucleotide by De-novo method like in brain and RBCs

Reactions of salvage pathway:

There are two mechanisms for salvage pathway

• Salvage reaction by phosphoribosylation of purine bases:- in this type of reaction, the ribose phosphate group of PRPP is transferred to the purine from the corresponding nucleotide. This type of reaction is catalyzed by two enzyme
  1. Adenine phosphoribosyl transferase- it forms AMP
  2. Hypoxanthine guanine phosphoribosyl-transferase- it forms IMP and GMP
• Salvage reaction by phosphorylation of purine bases:- this type of reaction involves direct phosphorylation of a purine nucleoside to purine nucleotide through ATP. This reaction is catalyzed by kinase enzyme

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Significance of salvage pathway

Salvage pathway provides urine nucleotides to the tissue which are not capable of performing the De-novo synthesis of purine nucleotide. For eg:

• Brain has low level of PRPP amidotransferase enzyme hence depends on salvage pathway
• RBCs and polymorphonuclear leukocytes cannot synthesize 5-phosphoribosylamine; and therefore utilize exogenous purine to form nucleotides.

Multiple Choice Questions (MCQs)

1. Salvage pathway is used for the synthesis of which type of nucleotides?
A. Purine

B. Pyrimidine

C. Both

D. None

2. Purine and pyrimidine are nutritionally classified as which type of components?
A. Essential

B. Non-essential

C. Semi-essential

D. None of the above

3. Synthesis of purine nucleotide can be done through how many process?
A. 2

B. 3

C. 4

D. 1

4. Which out of de-novo and salvage method consumes less energy?
A. Salvage pathway

B. De-novo synthesis method

C. Both

D. None

5. What is the meaning of the word salvage?
A. Reuse of property

B. Use of intermediates

C. Both A and B

D. None of the above

6. In which organ, salvage pathway takes place?
A. Heart

B. Brain

C. Eye

D. All of the above

7. Which of the following is formed by salvage pathway?
A. CMP

B. UMP

C. TMP

D. None of the above

8. Adenine phosphoribosyl transferase enzyme catalyzes the formation of which nucleotide?
A. ATP

B AMP

C. ADP

D. All of the above

9. Match the following enzyme and the product it forms-

A. Adenine phosphoribosyl transferase                                                                               1. GMP

B. Hypoxanthine-guanine phosphoribosyl transferase on hypoxanthine                    2.  AMP

C. Hypoxanthine-guanine phosphoribosyl transferase on guanine                               3. IMP

D. Kinase                                                                                                                                     4. Purine nucleotide

10. Which type of enzyme performs the second type of reaction of salvage pathway?
A. Oxidases

B. Kinases

C. Reductase

D. Lipases

11. Which compound forms IMP through Hypoxanthine guanine phosphoribosyltransferase enzyme?
A. Hypoxanthine

B. Guanine

C. Adenine

D. Uracil

12. Which of the following is the advantage of salvage pathway over de-novo synthesis method?
A. Consume less energy

B. Simpler

C. One step reaction

D. All of the above

13. Which of the following statement is NOT true?

A. Kinases are present in all cells

B. IMP is also known as inosinate

C. Salvage pathway contains two type of reactions

D. Brain forms purine nucleotide with the help of salvage pathway

14. Why purine nucleotide is not synthesized in the RBCs through de-novo method?
A. Low level of PRPP

B. 5-phosphoribosylamine is not synthesized

C. GAR is not synthesized

D. All of the above

15. The ribose phosphate group of purine nucleotide is taken from which compound?
A. PRPP

B. GAR

C. AIR

D. FGAM

ANSWERS:-

1. Purine nucleotide
2. Non-essential
3. 2
4. Salvage pathway
5. Reuse of property
6. Brain
7. None of the above
8. AMP
9. A – 2 B – 3 C – 1 D – 4
10. Kinases
11. Hypoxanthine
12. All of the above
13. Kinases are present in all cells
14. 5-phosphoribosylamine
15. PRPP

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REFERENCE:- 1. Pankaja Naik- Biochemistry; 4^th edition; page no:- 327-329.