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Biochemistry PMY-3 > Purines And Pyrimidines > Flashcards

Purines And Pyrimidines Flashcards

1
Q

What are the 3 c9mponents 9f nucleotides?

A
  1. Nitrogenous base
  2. Add a cyclic pentose monosaccharide ribose(5 Carbon sugar)
  3. Add one or more phosphate groups

Nomenclature changes with the addition of components

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2
Q

What are the nitrogenous bases?

A

Purines (2 rings)

-A and G in both DNA and RNA

Pyrimidines( 1 ring)

  • T and C bases in DNA
  • C and U bases in RNA
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3
Q

How are bases modified?

A
  • Found mainly in transfer RNA (tRNA) ADH viral viral DNA
  • about 5% of tRNA
-modifications to the bases may occur:
Methylation 
Glycosylation 
Acetylation 
Reduction
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4
Q

What are the possible pathways of purines and pyrimidines?

A
  1. De novo

2. Salvage pathways

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5
Q

Describe ribose and deoxyribose

A

Ribose is an aldopentose sugar or monosaccharide containing five carbons (C)

-base is linked to sugar by B-N-glycosidic linkage

  • ribose has -OH attached to C2’
    • base + ribose = Ribonucleoside

-Deoxyribose has -H attached to C2’
Base + deoxyribose = deoxyribonucleoside

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6
Q

What happens when one or more phosphates to a nucleoside?

A

It becomes a nucleotide

-base+ribose+ phosphate =ribonucleotide
Building blocks of RNA; Adenosine triphosphate (ATP)

  • Base+ deoxyribose+ phosphate= deoxyribonucleotide
    • building blocks of DNA; deoxyadenosine triphosphate(dATP)

Ribose sugar and phosphate are donated by PRPP in purine and pyrimidine nucleotide biosynthesis

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7
Q

Give a general description of de novo-purine synthesis pathway

A
  • constructing the purine ring by a series of reactions that add the carbon and nitrogen toa performed ribose 5-phosphate
    • carbon and nitrogen of the purine ring from amino ac8ds shown, CO2 and THF
  • ribose-5-phosphate synthesis by pentose phosphate pathway
  • in humans, all of the enzymes necessary for the de novo purine synthesis ware found in the cytoplasm of the cell
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8
Q

What must occur before de novo purine synthesis?

A

Activation of sugar must first occur

-formation of 5-phosphoribosyl-1-pyrophosphate (PRPP) is the first step in purine biosynthesis catalyzed by ribose phosphate pyrophosphokinase (PRPP synthetase)

PRPP synthetase is one of the regulated enzyme of purine nucleotide synthesis

-PRPP is involved in de novo synthesis of purines and pyrimidines, salvage of purines and pyrimidines, in the synthesis of NAD+, histidine biosynthesis and conversion of guanine to GMP

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9
Q

What is the committed p/rate-limiting step of de-novo of purines?

A

PRPP —> 5-phosphoribosylamine reaction catalyzed by:

-glutamine: phosphoribosylpyrophosphate amidotransferase (GPAT).

Uses glutamine and water and releases glutamate and PPi

Rate of reaction is controlled by concentration of Substrates:
-intracellular concentrations of glutamine and PRPP(activators)

Reaction is inhibited by the purine nucleotides

-AMP, GMP & IMP

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10
Q

How is inosine monophosphate

A

Occurs in de novo purine synthesis

Series of 9 reactions in which C and N atoms are donated sequentially to form inosine monophosphate (IMP also known as Hypoxanthine-ribose-phosphate )

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11
Q

Explain : Conversion of IMP to AMP or GMP—> separate pathways

A

Occurs in purine de novo synthesis

  • the formation of AMP from IMP requires GTP (adenylosuccinate synthetase)
    • reaction is inhibited by end product AMP
  • The formation of GMP from IMP requires ATP(GMP synthetase)
    • reaction is inhibited by end product GMP
  • this ensures that the cellular [ATP]= [GTP]
  • AMP and GMP are converted to ADP and GDP and later to ATP and GTP by nucleoside monophosphate kinases (base specific) which add phosphate groups
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12
Q

Summarize regulation of purine biosynthesis

A
  • Regulatory enzymes are PRPP synthetase and Glutamine Phosphoribosyl pyrophosphate andi transferase (phosphoribosylamidotrabsferase)
  • Purine nucleotides (ATP and GTP) inhibit PRPP synthetase and the phosphoribosylamidotrabsferase (feedback inhibition/ product inhibition)
  • PRPP is a feedforward activator of phosphoamidotransferase. High levels of PRPP activate this enzyme to increase purine nucleotide synthesis
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13
Q

What are the inhibitors of bacterial metabolism?

A

PABA analogs (sulfonamides; sulfadrugs ) inhibit bacterial folate synthesis

  • trimethoprim inhibits bacterial dihydrofolate reductase
  • sulfanoamide-trimethoprim combination is used as an antibacterial agent
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14
Q

What are the inhibitors of eukaryotic folate metabolism?

A

-methotrexate (folate analog) is used as an anti cancer agent

Methotrexate inhibits human dihydrofolate reductase and reduces the amount of tetrahydrofolate available for purine synthesis and slows down DNA replicatio/cell division in mammalian cells

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15
Q

What are the folic analogues?

A

Methotrexate(MTX) is a structural analogue of folic acid and inhibitor of dihydrofolate reductase

-trimethoprim is also an analog inhibits THF synthesis in prokaryotes

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16
Q

How does methotrexate used to treat cancer?

A

Folate analogs are used to treat leukemia’s and other cancers

Methotrexate is the most commonly used analog

Inhibit the reduction of dihydrofoloate to tetrahydrofolate by dihydrofoloate reductase thus limits purine synthesis and DNA replication

Cancer cells can become resistant to. MTX by upregulating the expression of dihydrofolate

17
Q

What is the most common vitamin deficiency?

A

Folic acid

Bacteria synthesis their own folate: required vitamin in mammalian diet

18
Q

Folate deficiency in humans:

A

Cause birth defects (spina bifida and anencephaly )

Megaloblastic anemia (hemoglobin levels are low and bone marrow shows many large, abnormal, immature erythrocytes)

19
Q

Sulfonamides are…

A

PABA (p-aminobenzoate) analogues that competitively inhibit the synthesis of folate in bacteria.

Purine synthesis requires tetrahydrofolate as coenzyme thus slow this pathway in bacteria

20
Q

How much purine bases are recycled?

A

80%

21
Q

What 2 enzymes are involved in salvage pathway?

A
  • hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

- adenine phosphoribosyltransferase (APRT)

22
Q

How are free bases derived for salvage?

A

Hydrolysis of cellular dna and rna

23
Q

What does lack of HGPRT lead to? Give details

A

Lesch Nyhansyndrome (x-linked)

Blood Uris acid levels are elevated

Purine bases are not reused, causing increased degradation of the purines to form Uris acid

Characterized by:

  • orange crystals in baby’s diapers first sign
  • development problems
  • behavioral disturbances(self mutilation, biting of lips and fingers)
  • Gout(hyperurecemia, unrated crystals in the joints)
  • urticaria acid stones in kidney
24
Q

Contrast CPS 1 and CPS 2

A

CPS 1-mitochondria
Cps2 - cytosol

CPS 1- urea cycle
CPS 2- pyrimidine synthesis

CPS 1- source of nitrogen is ammonia
CPS 2- source of nitrogen is y-amide group of glutamine

CPS 1- REGulated by: N-acetylglutamate
CPS 2- Activater: PRPP
Inhibitir- UTP

25
Q

What is the first reaction in the degradation of purine nucleotides to Uric acid and disease associated

A

Adenosine converted to inosine
Adenosine deamination deficiency (ADA): autosomal recessive cause of severe combined immunodeficiency (SCID) untreated children usually dies before 2 years from overwhelming infection

26
Q

What is the second reaction in the degradation of purine nucleotides to Uric acid and disease associated

A

Inosine becomes hypoxanthine

Purine nucleoside phosphorylase deficiency (PNP)

Autosomal recessive disorder that is less severe than ADA, more rare

Causes recurrent infections and neurodevelopmental delay

27
Q

What is the final step of purine degradation?

A

Guanosine to guanine via purine Nucleotide phosphorylase

Xanthine oxidase converts hypocpxathine (and also guanine) to xanthine then uric acid

28
Q

Explain gout

A

Hyperuricemia: underexcretion of Uric acid —> deposition of mono sodium urate crystals leading to recurrent attacks of acute arthritic joint inflammation, swelling and redness

Crystal deposition seen in soft tissues and kidney.

Treatment with allopurinol inhibits xanthine oxidase = increased hypoxanthine and xanthine, more soluble than Uric acid

29
Q

What is the long term treatment of gout?

A
  • allopurinol: inhibits xanthine oxidase and decreases the formation of Uric acid
  • hypoxanthine and xanthine are more soluble than urate
30
Q

What is the only purine to be salvaged?

A

Adenosine

31
Q

What is the rate limiting enzyme of adenosine extracellular concentrations?

A

Adenosine kinases

Selective inhibition of this improves management of inflammation, epilepsy and chronic pain

32
Q

What is the rate limiting step of pyrimidine synthesis?

A

Formation of carbamoyl phosphate from CPS II using glutamate and CO2

No folic acid needed

Remember: thymidine requires one-carbon group donation from methylene tetrahydrofolate (THF)

33
Q

How is pyrimidine synthesis regulated?

A

CPS II activated by ATP (ensures equal concentration of purine and pyrimidine nucleotide) and PRPP

CPS 1- inhibited by UTP(endproduct) via feedback inhibition

Aspartate transcarbamoylase is the regulated domain of bacterial(prokaryotic) pyrimidine nucleotide synthesis

34
Q

What causes orot8c aciduria?

A
  • OPRT and OMP decarboxylase are different activities present on the same polypeptide chain. This enzyme is also known as UMP synthase
  • deficiency of OPRT and/or OMP decarboxylase(UMP synthase)
  • these children have elevated orotic acid levels in blood and urine
  • Deficiency of pyrimidines results in decreased RBC formation; megaloblastic anemia, reduced rates of DNA synthesis and cell division
35
Q

How is CTP formed?

A
  • UMP is converted to UDP and then UTP by nucleoside monophosphate kinase and nucleoside diphosphate kinase
  • UTP is converted to CTP by CTP synthetase
  • glutamine is the nitrogen donor
  • uracil and cytosine (UDP and CDP) are synthesized with ribose sugar
36
Q

How is dTMP synthesized from dUMP?

A

Thymidine synthase uses THF to produce dTMP from dUMP

Fluorouracil is a substrate analog that will irreversibly to the enzyme and inhibit thymidylate synthase activity

Biochemistry PMY-3 (37 decks)

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