Simmon's Purine and Pyrimidine Nucleotide Metabolism

Question 1

What are the two purines?

Answer 1

adenine-nitrogen and guanine-oxygen

Question 2

What are the three pyrimidines?

Answer 2

thymine-mthyl
uracil-oxygen
cytosine-nitrogen

Question 3

Bases vs. nucleosides vs.nucleotides

Answer 3

base-base
nucleoside-base, sugar
nucleotide-base, sugar, phosphate

Question 4

deoxyribose vs ribose

Answer 4

deoxyribose missing oxygen on carbon 2

Question 5

Purines are nutritionally nonessential. What are the sources of each atom of the purine ring?

Answer 5

AA (aspartate, glutamine, glycine), tetrahydrofolate derivative, CO2

Question 6

Is the purine ring built up atom by atom on top of the ribose?

Answer 6

Yes

Question 7

What is the first step to making a purine?

Answer 7

activation of ribose-5-phosphate–>PRPP (5 phospho-ribose-1-pyrophosphoric acid)

Question 8

What is the source of ribose-5-phosphate

Answer 8

pentose pathway

Question 9

What is the commitment step and major regulated step to making a purine?

Answer 9

PRPP–>5-phospho-ribosylamine

Enzyme: glutamine PRPP amidotransferase

Question 10

Which nucleotide stands at the branch point leading to synthesis of AMP(adenylic acid) and GMP(guanylic acid)

Answer 10

IMP (inosinic acid)

Question 11

What are 4 ways purine synthesis is allosterically regulated?

Answer 11

1. PRPP- activation of glutamine PRPP amidotransferase
2. AMP and GMP-inhibition of glutamine PRPP amdiotransferase
3. IMP-inhibition of glutamine PRPP amidotransferase
4. AMP and GMP- inhibition from: IMP-XMP-GMP
   IMP-Adenylosuccinate-AMP
   -negative feedback

Question 12

Why would an inhibitor of purine biosynthesis be useful in slowing tumor growth?

Answer 12

Tumors need purine biosynthesis for purine nucleotides for DNA and RNA synthesis (normal cells recycle)

Question 13

What pathway does the anit-tumor agent 6-mercaptopurine inhibit?

Answer 13

1. 6-mercaptopurine is converted to nucleotide (by salvage pathway)
2. inhibits the enzymes in the purine biosynthetic pathway that catalyze step 2(main regulatory and commitment step), 12a, 12b, 13a (from IMP to AMP and GMP)

Question 14

What provides the atoms for the pyrimidine rings?

Answer 14

Glutamine, Aspartate, CO2

Question 15

What is the regulated step in pyrimidine synthesis?

Answer 15

Carbamoyl phosphate synthetase 2-pyrimidine ring structure is formed

Question 16

Is the pyrimidine ring structure formed first then attached to a ribose-5-phosphate?

Answer 16

yes

Question 17

What is the precursor for synthesis of other pyrimidine nucleotides?

Answer 17

UMP

Question 18

Which metabolic pathway is blocked by rheumatoid arthritis drug leflunomide?

Answer 18

inhibits dihyroorotate dehyrogenase

Question 19

How is dTMP formed?
Enzyme
Cosubstrate

Answer 19

dUMP–>dTMP

enzyme: Thymidylate synthase
cosubstrate: N5, N10-methylene-tetrahydrofolate (goes to dihyrofolate)

Question 20

How does 5-fluorouracil act as an antitumor agent? 2 steps

Answer 20

1. 5-fluorouracil–>f-dUMP (suicide inhibitor)

2. f-dUMP acts as a pseudosubstrate and covalently binds to and inhibits thymidylate synthase

Question 21

What happens to the dihydrofolate formed from tetrahydrofolate during the dUMP–>dTMP reaction

Answer 21

-the dihydrofolate is toxic and must be converted back to tetrahydrofolate by dihyrofolate reductase to be used in purine nucleotide synthesis again

Question 22

What does methotrexate do to dihydrofolate reductase?

Answer 22

it is a competitive inhibitor of dihydrofolate reductase and increases dihydrofolate and cuases intracellular tetrahydrofolate deficiency

Question 23

What is the enzyme responsible for the synthesis of deoxyribonucleotides and the substrates for this reaction?
What does this reaction do?
What is the cofactor?

Answer 23

Substrates: ADP, GDP, UDP, CDP

Enzyme: Ribonucleotide reductase

Cofactor: thioredoxin

Action: 2’-hydroxl group of ribose is replaced wiht a hydrogen atom

Question 24

How is deoxyribonucleotide biosynthesis regulated during the cell cycle?

Answer 24

1. Deoxyribonucleotides required for DNA synthesis–>increases greatly during S phase
2. ribonucleotide reductase and thymidylate synthase increase during late G1/early S phase (these two enzymes also correlate with tumor growth rate)

Question 25

What classes of enzymes are involved in degradation of DNA and RNA?

Answer 25

nucleases (DNA-->nucleoside) nucleotidases (nucleoside-->nucleotide--losing P) nucleoside phosphorylases (nucleoside-->bases--losing deoxyribose 1-phosphate)

Question 26

What bases are formed by degradation of DNA and RNA?

Answer 26

uracil thymine guanine hypoxanthine (from adenine)

Question 27

90% of free purine bases are reutilized in a salvage pathway, what enzymatic reaction is involved in salvaging hypoxanthine and guanine?

Answer 27

Hypoxanthine-guanine phosphoribosyltransferase (HGPRTase) Hypoxanthine+PRPP-->IMP +Pi Guanine+PRPP-->GMP+ Pi

Question 28

Lesch-Nyhan syndrome

Answer 28

genetic deficiency in HGPRTase - neurologic problems: spasticity, metal retardation, compulsive self-mutilation - affects only males - also have gout

Question 29

10% of purine bases are not salvaged and are degraded to what?

Answer 29

uric acid

Question 30

What is the enzyme that catalyzes the converstion of hypoxanthine to xanthin and xanthine to uric acid?

Answer 30

xanthine oxidase

Question 31

What is the form of uric acid in the blood?

Answer 31

urate (higher than 5.8 ph)

Question 32

What is the form of uric acid in the urine?

Answer 32

uric acid (lower than 5.8pH) Uric acid exists in both enol form and keto form (enol form can be deprotonated to form urate)

Question 33

Which form of uric acid is more soluble?

Answer 33

urate

Question 34

What is the consequence of overproduction or underexcretion of uric acid?

Answer 34

gout Hyperuricemia-(elevated urate in the blood)-deposition of sodium urate crystals in joints-->resulting tophi can elicit painful acute inflammatory responses Hyperuricosuria(elevated uric acid in urine) can lead to acid stones in the kidney)

Question 35

How does allopurinol relieve the symptoms of gout?

Answer 35

allopurinol is an inhibitor of xanthine oxidase - blocks formation of xanthine from hypoxanthine and uric acid from xanthine - reduces urate and uric acid in blood and urine-->shrinks tophi and renal uric acid stones