Metabolism Of Purine

METABOLISM OF PURINE

Q. 1 The purines salvage pathway is for:
 A Hypoxanthine and Xanthine
 B Hypoxanthine andAdenine
 C Adenine and Guanine
 D Xanthine and Guanine
Q. 1 The purines salvage pathway is for:
 A Hypoxanthine and Xanthine
 B Hypoxanthine andAdenine
 C Adenine and Guanine
 D Xanthine and Guanine
Ans. B

Explanation:

Hypoxanthine andAdenine


Q. 2

In humans, the end product of purine metabolism is uric acid. End product of purine metabolism in non-primate mammals is:

 A

Uric acid

 B

Ammonia

 C

Urea

 D

Allantoin

Q. 2

In humans, the end product of purine metabolism is uric acid. End product of purine metabolism in non-primate mammals is:

 A

Uric acid

 B

Ammonia

 C

Urea

 D

Allantoin

Ans. D

Explanation:

Humans convert adenosine and guanosine to uric acid. Adenosine is first converted to inosine by adenosine deaminase. In mammals other than higher primates, uricase converts uric acid to the water-soluble product allantoin. However, since humans lack uricase, the end product of purine catabolism in humans is uric acid.  

Ref: Rodwell V.W. (2011). Chapter 33. Metabolism of Purine & Pyrimidine Nucleotides. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.  

Q. 3

Nucleoside is made up of:

1. Pyrimidine
2. Histone
3. Sugar
4. Purine
5. Phosphate

 A

1,2 & 3

 B

1,3 & 4

 C

1,3 & 5

 D

2,3 & 4

Q. 3

Nucleoside is made up of:

1. Pyrimidine
2. Histone
3. Sugar
4. Purine
5. Phosphate

 A

1,2 & 3

 B

1,3 & 4

 C

1,3 & 5

 D

2,3 & 4

Ans. B

Explanation:

“The nucleoside is composed of purine purine or pyrimidine base linked to either D-ribose (in RNA) or D-2- deoxyribose (in DNA)”

“The nuclear DNA is found bounded to basic proteins called histones”.
Nucleotides
  • Nucleotides are nucleoside +P
  • The Phosphodiester bond between the nucleotides is formed mainly between 3’OH group of sugar of one nucleotide to 3’OH group of sugar of another nucleotide.

 

Ref: Harper 27/e, Page 297; Chatterjee & shinde 7/e, Page 206-07.

Quiz In Between


Q. 4

Which among the following are the substrates needed for purine salvage pathway?

 A

Hypoxanthine and Xanthine

 B

Hypoxanthine and Adenine

 C

Adenine and Guanine

 D

Xanthine and Guanine

Q. 4

Which among the following are the substrates needed for purine salvage pathway?

 A

Hypoxanthine and Xanthine

 B

Hypoxanthine and Adenine

 C

Adenine and Guanine

 D

Xanthine and Guanine

Ans. B

Explanation:

Conversion of purines, their ribonucleosides, and their deoxyribonucleosides to mononucleotides involves “salvage reactions”.
The more important mechanism involves phosphoribosylation by PRPP of a free purine (Pu) to form a purine 5′-mononucleotide (Pu-RP). Phosphoryl transfer from ATP, catalyzed by adenosine- and hypoxanthine-phosphoribosyl transferases, converts adenine, hypoxanthine, and guanine to their mononucleotides. 
 
A second salvage mechanism involves phosphoryl transfer from ATP to a purine ribonucleoside. Phosphorylation of the purine nucleotides, catalyzed by adenosine kinase, converts adenosine and deoxyadenosine to AMP and dAMP. Similarly, deoxycytidine kinase phosphorylates deoxycytidine and 2′-deoxyguanosine, forming dCMP and dGMP.
 
Ref: Rodwell V.W. (2011). Chapter 33. Metabolism of Purine & Pyrimidine Nucleotides. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.

 


Q. 5

End product of purine metabolism in non-primate mammals is:

 A

Uric acid

 B

Ammonia

 C

Urea

 D

Allantoin

Q. 5

End product of purine metabolism in non-primate mammals is:

 A

Uric acid

 B

Ammonia

 C

Urea

 D

Allantoin

Ans. D

Explanation:

D i.e. Allantoin


Q. 6

Which of the following RNA has abnormal purine bases:

 A

t-RNA

 B

m-RNA

 C

r-RNA

 D

16S RNA

Q. 6

Which of the following RNA has abnormal purine bases:

 A

t-RNA

 B

m-RNA

 C

r-RNA

 D

16S RNA

Ans. A

Explanation:

A i.e. tRNA

Quiz In Between


Q. 7

Amino acid which contributes to biosynthesis of purine ribonucleotide are all except:    

 A

Aspartate

 B

Histidine

 C

Glutamine

 D

Glycine

Q. 7

Amino acid which contributes to biosynthesis of purine ribonucleotide are all except:    

 A

Aspartate

 B

Histidine

 C

Glutamine

 D

Glycine

Ans. B

Explanation:

 

Biosynthesis of purine

  • N1 of purine is derived from amino group of aspartate
  • N3 and N9 are obtained from amide group of glutamine
  • C4, C5 and N7 of the purine ring of nucleotides are contributed by glycine



Q. 8

Final product of purine metabolism is:

 A

Uric acid

 B

Creatinine

 C

Xanthine

 D

Phoshphates

Q. 8

Final product of purine metabolism is:

 A

Uric acid

 B

Creatinine

 C

Xanthine

 D

Phoshphates

Ans. A

Explanation:

 

Nucleotides are derived from biosynthetic precursors of carbohydrate and amino acid metabolism, and from ammonia and carbon dioxide.

The liver is the major organ of de novo synthesis of all four nucleotides.

Degradation in humans, however, is only complete for pyrimidines (C, T, U), but not purines (G, A), which are excreted from the body in form of uric acid



Q. 9

Nitrogen-9 of purine ring is provided by ‑

 A

Glycine

 B

Aspartate

 C

Glutamine

 D

CO2

Q. 9

Nitrogen-9 of purine ring is provided by ‑

 A

Glycine

 B

Aspartate

 C

Glutamine

 D

CO2

Ans. C

Explanation:

 

In de novo synthesis, purine ring is formed from variety of precursors is assembled on ribose-5-phosphate. Precursors for de novo synthesis are –

i)      Glycine provides C4, C5 and N7

ii)     Aspartate provides N1

iii)    Glutamine provides N3 and N9

iv)   Tetrahydrofolate derivatives furnish C2 and C8

v)    Carbon dioxide provides C6

Quiz In Between


Q. 10

First product of purine metabolism

 A

Uric acid

 B

Xanthine

 C

P-alanine

 D

CO2

Q. 10

First product of purine metabolism

 A

Uric acid

 B

Xanthine

 C

P-alanine

 D

CO2

Ans. B

Explanation:

 

  • Humans catabolize purines to uric acid.
  • But, first purines are catabolized to xanthine, which is further catabolized to purine.

Q. 11

First purine nucleotide, which is synthesized in purine biosynthesis ‑

 A

AMP

 B

GMP

 C

IMP

 D

UMP

Q. 11

First purine nucleotide, which is synthesized in purine biosynthesis ‑

 A

AMP

 B

GMP

 C

IMP

 D

UMP

Ans. C

Explanation:

 

The biosynthesis of purine begins with ribose-5-phosphate, derived from pentose phosphate pathway (PPP).

First intermediate formed in this pathway, 5-phosphoribosyl-pyrophosphate (PRPP), is also an intermediate in purine salvage pathway.


Q. 12

Salvage pathway of purine biosynthesis is important for ‑

 A

Liver

 B

RBCs

 C

Kidney

 D

Lung

Q. 12

Salvage pathway of purine biosynthesis is important for ‑

 A

Liver

 B

RBCs

 C

Kidney

 D

Lung

Ans. B

Explanation:

 

Purine nucleotide synthesis occurs by two pathways :

1.De novo synthesis

2.Salvage pathway

Liver is the major site of purine nucleotide biosynthesis (de novo).

Certain tissues cannot synthesize purine nucleotides by de novo patyway, e g. brain, erythrocytes and polymor­phonuclear leukocytes.

These are dependent on salvage pathway for synthesis of purine nucleotides by using exogenous purines, which are formed by degradation of purine nucleotides synthesized in liver.

Quiz In Between


Q. 13

Purine metabolism produces –

 A

β – alanine

 B

NH3

 C

CO2

 D

Uric acid

Q. 13

Purine metabolism produces –

 A

β – alanine

 B

NH3

 C

CO2

 D

Uric acid

Ans. D

Explanation:

Ans. is ‘d’ i.e., Uric acid


Q. 14

PRPP glutamyl amidotransferase is increased to increase purine synthesis in ‑

 A

Liver

 B

RBC

 C

Brain

 D

Polymorphs

Q. 14

PRPP glutamyl amidotransferase is increased to increase purine synthesis in ‑

 A

Liver

 B

RBC

 C

Brain

 D

Polymorphs

Ans. A

Explanation:

 

PRPP glutamyl amidotransferase is one of the rate limiting enzyme in de novo synthesis of purines. o The de novo synthesis of purines is most active in liver.

Brain, RBCs (erythrocytes) and polymorphonuclear leokocytes (polymorphs) cannot synthesize purine nucleotides by de novo pathway.

Biosynthesis of purine neucleotides

Two important purine nucleotides are synthesized : (i) adenosine monophosphate (AMP) and (ii) guanosine monophosphate (GMP). Then AMP and GMP are converted to other purine nucleotides like ADP, ATP, GDP, GTP etc. Purine nucleotides can be synthesized by two pathways – (I) De novo synthesis and (2) Salvage pathway.

De novo pathway (De novo synthesis)

In de novo pathway, the purine nucleotides are synthesized from amphibolic intermediates. Amphibolic intermediates are the intermediary metabolites of amphibolic pathways (eg. citric acid cycle) which have dual purposes, i e. they serve in catabolism as well as in anabolism.

In de novo synthesis, purine ring is formed from variety of precursors is assembled on ribose-5-phosphate. Precursors for de novo synthesis are

i)  Glycine provides C4 C5 and N7

ii)  Aspartate provides NI

iii)  Glutamine provides N3 and N9

iv)  Tetrahydrofolate derivatives furnish C2 and Cs

v)  Carbon dioxide provides C6


Q. 15

Salvage pathway of purine nucleotide synthesis are used by all except ‑

 A

Brain

 B

Liver

 C

RBC

 D

Leukocytes

Q. 15

Salvage pathway of purine nucleotide synthesis are used by all except ‑

 A

Brain

 B

Liver

 C

RBC

 D

Leukocytes

Ans. B

Explanation:

 

Purine nucleotide synthesis occurs by two pathways :-

1.De novo synthesis

2.Salvage pathway

Liver is the major site of purine nucleotide biosynthesis (de novo).

Certain tissues cannot synthesize purine nucleotides by de novo patyway, e g. brain, erythrocytes and polymorphonuclear leukocytes.

These are dependent on salvage pathway for synthesis of purine nucleotides by using exogenous purines, which are formed by degradation of purine nucleotides synthesized in liver.

Quiz In Between


Q. 16

Salvage pathway of purine nucleotide synthesis are used by all except ‑

 A

Brain

 B

Liver

 C

RBC

 D

Leukocytes

Q. 16

Salvage pathway of purine nucleotide synthesis are used by all except ‑

 A

Brain

 B

Liver

 C

RBC

 D

Leukocytes

Ans. B

Explanation:

Ans. is ‘b’ i.e., Liver

  • Purine nucleotide synthesis occurs by two pathways :
  • De novo synthesis
  • Salvage pathway
  • Liver is the major site of purine nucleotide biosynthesis (de novo).
  • Certain tissues cannot synthesize purine nucleotides by de novo patyway, e g. brain, erythrocytes and polymorphonuclear leukocytes.
  • These are dependent on salvage pathway for synthesis of purine nucleotides by using exogenous purines, which are formed by degradation of purine nucleotides synthesized in liver.

Q. 17

In humans, end product of purine metabolism 

 A

Allantoin

 B

Uric acid

 C

CO2

 D

None

Q. 17

In humans, end product of purine metabolism 

 A

Allantoin

 B

Uric acid

 C

CO2

 D

None

Ans. B

Explanation:

Ans. is ‘b’ i.e., Uric acid


Q. 18

Salvage purine synthesis refers to ‑

 A

Synthesis of purine from ribose-5-phosphate

 B

Synthesis of purine from pyrimidine

 C

Synthesis of purine nucleotides from purine bases

 D

None of the above

Q. 18

Salvage purine synthesis refers to ‑

 A

Synthesis of purine from ribose-5-phosphate

 B

Synthesis of purine from pyrimidine

 C

Synthesis of purine nucleotides from purine bases

 D

None of the above

Ans. C

Explanation:

Ans. is ‘c’ i.e., Synthesis of purine nucleotides from purine bases

  • Two important purine nucleotides are synthesized : (i) adenosine monophosphate (AMP) and (ii) guanosine monophosphate (GMP). Then AMP and GMP are converted to other purine nucleotides like ADP, ATP, GDP, GTP etc. Purine nucleotides can be synthesized by two pathways – (1) De novo synthesis and (2) Salvage pathway. De novo pathway (De novo synthesis)
  • In de novo pathway, the purine nucleotides are synthesized from amphibolic intermediates. Amphibolic intermediates are the intermediary metabolites of amphibolic pathways (eg. citric acid cycle) which have dual purposes, i e. they serve in catabolism as well as in anabolism.
  • In de novo synthesis, purine ring is formed from variety of precursors is assembled on ribose-5-phosphate. Precursors for de novo synthesis are ‑
  1. Glycine provides Ca, C5 and N7
  2. Aspartate provides N,
  3. Glutamine provides N3 and N9
  4. Tetrahydrofolate derivatives furnish C2 and C8
  5. Carbon dioxide provides Co

Salvage pathway of purine nucleotide synthesis

  • Free purine bases (adenine, guanine and hypoxanthine) and purine nucleosides are formed in cells during the metabolic degradation of nucleic acids and nucleotides.
  • These free purine bases and purine nucleosides are reused in the formation of purine nucleotides. 
  • This is called salvage pathway (salvage means property saved from loss).
  • Salvage synthesis requires far less energy than de novo synthesis.

Quiz In Between



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