NUCLEOTIDES LECTURE 4

Question 1

why is uracyl in DNA bad and how can you control it?

Answer 1

mimics deamination of cytosine and leads to mutations following repair/synthesis
a specific UTPase hydrolyzes dUTP to decrease its concentration and prevent its incorporation
dUTP can also come from the deamination of dCTP
uridylate kinase is not specific for ribose vs deoxyribose which can lead to production of dUDP and then dUTP

Question 2

how is dUTP controlled by dUTPase?

Answer 2

dUTPase degrades dUTP to dUMP
dUMP is then converted to dTMP by thymidylate synthase

Question 3

characteristic of thymidylate synthase

Answer 3

last step of synthesis of dTMP from dUMP
important target for drugs against cancer and bacteria because it is require for DNA synthesis but not for RNA synthesis
synthesis at the level of monophosphate

Question 4

how does thymidylate synthase work

Answer 4

THF goes to DHF (dihydrofolate), oxidation reaction
methylene is reduced to methyl

Question 5

which enzyme reduces DHF back to THF?

Answer 5

dihydrofolate reductase

Question 6

what is the thymidylate synthase cycle

Answer 6

Question 7

what does FdUMP do?

Answer 7

5-fluorodeoxyuridylate
suicide inhibitor
messes up thymidylate synthase

Question 8

how do methotrexate, aminopterin and trimethoprim work?

Answer 8

inhibitors of DHF reductase
analogs of THF, mimic the substrate and bind very tightly to the enzyme
competitive inhibitor
aminopterin and methotrexate are anticancer
trimethoprim is antibacterial

Question 9

how do sulfonamides (sulfa drugs) work?

Answer 9

Question 10

where is THF used

Answer 10

methionine cycle and purine de novo pathway

Question 11

what is the salvage pathway?

Answer 11

recovery of bases for making new nucleotides
more divergent and varied

Question 12

what are some salvage pathway enzymes?

Answer 12

Question 13

what does hypoxanthine/guanine phosphoribosyl transferase do and what does the loss of it lead to?

Answer 13

hypoxanthine+PRPP-> inosinate + PPi
guanine+PRPP-> guanylate + PPi
goes up two levels in the cube
genetic loss of HGPRT activity leads to Lesch Nyhan disease which leads to gout and mental retardation
there are other enzymes for adenine+uracil

Question 14

how are nucleosides formed in the salvage pathway?

Answer 14

Question 15

overview of the de novo pathway

Answer 15

Question 16

purine degradation pathway

Answer 16

Question 17

what does loss of adenosine deaminase lead to?

Answer 17

leads to severe combined immunodeficiency

Question 18

what is gout and how is it treated?

Answer 18

deposition of uric acid in joints
painful, prevalent in middle aged males
thought to be due to excess meat, but more likely due to kidney disfunction
metabolic imbalance: too much purine synthesis, high PRPP levels
treated by inhibiting xanthine oxidase (the step before making uric acid)

Question 19

uric acid characteristics

Answer 19

final degradation product of purines
the way that birds use to eliminate nitrogen
more conservation of water
purine de novo synthesis discovered by studying pigeons
uric acid is less soluble at low pH, birds lower it to precipitate the uric acid and get rid of it
naturally obtained from guano
used as fertilizer and to make gunpowder
used to make facial cream
ammonia is the cheapest energetically but it is toxic and requires water
uric acid takes lots of energy but isnt toxic and no need of water