Nucleotide Metabolism

What type of base is this?

pyrimidine

Where does the blue nitrogen come from?

glutamine amide

Where do the red elements come from?

aspartate

Where does the black element come from?

bicarbonate

Where does the red element come from?

aspartate amine

What base is this?

purine

Where does the green element come from?

bicarbonate

Where do the black Carbons come from?

formate

Where do the purple elements come from?

glycine

Where do the blue elements come from?

glutamine amide

What are the four phases of purine synthesis?

1. activation of ribose-5-phosphate into PRPP

2. conversion of PRPP into phosphoribosylamine (PRA)

3. PRA to construction of IMP

4. Conversion of IMP into ATP and GTP

5. 
   

What happens to purine synthesis when AMP and GMP accumulate?

synthesis is inhibited

How does purine synthesis exhibit cross regulation?

AMP synthesis is stimulated by GMP, and GMP synthesis is stimulated by AMP

What are the three phases of pyrimidine synthesis?

1. fabrication of pyrimidine ring as orotate

   1. RATE LIMITING STEP

2. attachment of orotate to PRPP to generate UMP

3. conversion of UMP to CTP and dTMP

What is the enzyme for the rate limiting step of pyrimidine synthesis?

carbamoyl phosphate synthetase 2

What inhibits and activates carbamoyl phosphate synthetase 2?

Inhibits: UMP/UTP

Activates: PRPP

What does methotrexate do?

Targets dihydrofolate reductase (DHFR) to inhibit the conversion of dietary folate to THF, which clocks nucleotide synthesis, and disrupts DNA replication in rapidly dividing cancer cells

Why does methotrexate work on cancer cells specifically?

cancer cells take up methotrexate preferentially, sparing most cells

What is the clinical use of methotrexate?

antimetabolite chemotherapy, often combined with other agents

What is the systemic use for 5-fluorouracil?

colon, esophageal, breast, cervical, pancreatic cancers

What are the topical uses for 5-fluorouracil?

warts, actinic keratoses, basal cell carcinoma

How does 5-fluorouracil work?

It targets thymidylate synthase, blocking the methylation of dUMP into dTMP, which stops DNA synthesis in rapidly dividing cells resulting in cancer cell death

What is the clinical classification of 5-fluorouracil?

antimetabolite chemotherapy

How do sulfonamides/sulfa drugs work?

Act as a competitive inhibitor for dihydropterase synthetase, blocking the incorporation of p-aminobenzoic acid (PABA) into bacterial folate synthesis pathway, disrupting folate-dependent DNA replication and protein synthesis in bacteria

Why are sulfonamides selective for bacteria?

Bacteria must synthesize folate de novo from PABA, where as humans get folate as a dietary vitamin and are unaffected by PABA blocking. So only bacteria is impacted

What are the clinical uses for sulfonamides?

urinary tract infections and bladder infections

high urinary concentration of a drug provides effective local therapy

What is 6-mercaptopurine?

antineoplastic and immunosuppressive agent; purine antagonist

How does 6-mercaptopurine work?

It disrupts phase 2 of purine synthesis by inhibiting inosine monophosphate dehydrogenase (IMPDH), which stops the synthesis of guanosine nucleotides. This disrupts cell proliferation and immune response

What happens if 6-mercaptopurine is used alongside allopurinol?

toxic accumulation because 6-mercaptopurine is metabolized by xanthine oxidase and allopurinol inhibits xanthine oxidase.

How does acyclovir work?

Viral thymidine kinase has a higher affinity for acyclovir, so it will preferentially phosphorylate acyclovir into acyclo-dGTP. This is now incorporated into viral DNA but it doesn’t have the 3’ OH needed for DNA chain elongation, so the chain is terminated. This results in the inhibition of viral DNA polymerase and DNA replication stops.

What causes the viral selectivity for acyclovir?

Virus infected cells have high viral thymidine Kinase activity that prefers acyclovir. Uninfected cells have a low basal thymidine kinase activity, meaning there is minimal acyclovir activation.

What are the clinical uses for acyclovir?

herpes simplex virus (HSV), varicella-zoster (chicken pox, shingles), cytomegalovirus

What is purine catabolism?

ribose is removed from guanosine/inosine producing guanine/hypoxanthine, which combine to form xanthine that gets converted to uric acid

What are the key enzymes of purine catabolism?

adenosine deaminase (ADA) and xanthine oxidase

A deficiency in adenosine deaminase is associated with what?

SCID

If there is excess adenosine deaminase, what could be caused?

hemolytic anemia

What is the target for gout treatment?

xanthine oxidase

What is a diagnostic marker for gout?

serum uric acid levels

What does a serum urate level >9 mg/dL increase the risk of?

gout

What is gout?

hyperuricemia leading to monosodium urate crystal deposition in joints and tissues, causing painful gouty arthritis

What are the two forms of gout and what is the difference?

primary hyperuricemia: overproduction of uric acid (excess purine degradation)

secondary hyperuricemia: underexcretion of uric acid (most common)

What can worsen uricemia in gout?

acidemia, which increases URAT1-mediated reabsorption in the kidney

What can trigger gout?

high purine diet (beans, spinach, lentils, meat, seafood), and alcohol (increases purine degradation, inhibits urate excretion, promotes acidemia)

What are the treatments for gout (give a brief discription of how they work)?

Allopurinol: inhibits xanthine oxidase

anti-inflammatory: reduce granulocyte infiltration to affected joints (NSAIDs, colchicine)

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What is pyrimidine catabolism?

pyrimidines are converted to metabolized ketogenic (U and C) or glucogenic (T), water-soluble compounds

What are the potential products produced from pyrimidine catabolism

malonyl CoA (ketogenic), succinyl CoA (glucogenic), and methylmalonyl CoA (glucogenic)

What are the salvage pathways for purines?

1. adenine phosophoribosyltransferase (APRT)

   1. generates AMP from adenine and PRPP

2. hypoxanthine-guanine phosphoribosyltransferase (HGPRT)

   1. generates GMP or IMP from guanine/hypoxanthine and PRPP

   2. 
      

3. 
   

What is lesch-nyhan syndrome?

rare x-linked disorder caused by defiiciency of HGPRT in the purine salvage pathway

What re the biochemical consequences of lesch-nyhan syndrome?

1. loss of salvage pathway

   1. results in gout, urate kidney stones, renal damage

2. accumulation of PRPP

   1. increases de novo purine synthesis, and can result in severe cognitive impairment, self-mutilation, and poor muscle control

What are the clinical manifestations of Lesch Nyhan syndrome?

1. hyperuricemia with gout and kidney stones

2. neurological dysfunction (cognitive, behavioral, motor) linked to excessive de novo synthesis and neurotransmitter degradations

<1.5% normal HGPRT activity results in what?

Lesch-Nyhan syndrome with severe neurological problems (cerebral palsy, choreoathetosis, and self-destructive biting)

>8% normal HGPRT activity indicates?

Kelley Seegmiller syndrome results with gout and kidney destruction without neurologic symptoms

8-15% HGPRT activity results in what?

variant lesch-nyhan syndrome with neurological problems ranging from clumsiness to motor dysfunction.

What drug can reduce joint and kidney problems associated with HGPRT levels, but has no effect on neurological symptoms?

Allopurinol