DNA replication/telomere

Question 1

fluoroquinolones

Answer 1

Bacterial gyrase (Topoisomerase type II) is the target enzyme for fluoroquinolones
Gyrase is heterotetramer (GyrA2GyrB2)
To make the cut Tyr122 on GyrA forms covalent linkage with the 5’P of DNA. The DNA is subsequently religated.
GyrB carries out ATP hydrolysis.
Fluoroquinolones prevent the reversible ligation step carried out by GyrA.
Novobiocin competes with ATP for binding to GyrB.

Question 2

anticancer drugs

and their targets

Answer 2

Mammalian Topo I is the target enzyme for anticancer drugs
Topoisomerase I makes a cut on one strand of the DNA and binds to 3’ end of the phosphodiester back bone
After it relieves the supercoiling, DNA is religated
Topotecan (Hycamtin) is FDA approved for ovarian cancer and small cell lung cancer
Topotecan stabilizes Topo I-DNA complex and prevents the religation step (inhibition of replication)

Question 3

antibiotic resistance and resistance to quinolone

Answer 3

Bacterial genomes are small
Replication and doubling times are shorter
Errors in replication are likely to occur
These errors lead to mutations and produce mutant bacteria
If mutant bacteria are able to survive in the presence of antibiotic (selective advantage):
Mutant bacterium will be selected and the susceptible bacteria will perish
Resistance may develop in a matter of days or weeks if only one gene is involved in resistance
If multiple genes are involved then likelihood of all mutational hits in the same bacterium will be remote.

Point mutations:
N-terminal end of GyrA (most mutations).
C-terminal end of GyrB (few mutations).

Question 4

Telomerase and Human Cancer

Answer 4

Normal tissue- No telomerase. controlled length of life of cell
Tumor- Telomerase activity–> extended life/growth more than normal
==> ex. kidney, skin, colon cancer
Germ cells- Telomerase activity

trials of telomerase inhibitors as therapeutic agents for cancer (GRN163L and RHPS4) are thought to block the replicative potential of cancer cells through inhibiting telomerase by binding to the active site or preventing attachment of telomerase

Question 5

HIV and targets

Answer 5

HIV: Genome size:
Two identical copies of 9749 nucleotide RNA
Replication requires that the viral RNA be converted into DNA:
–>Priming by a tRNA (tRNA-lys) and cDNA formation by reverse transcriptase (RNA dependent DNA polymerase)
Integration of cDNA copy of viral RNA into host genome by integrase enzyme.
An important enzyme for DNA synthesis:
Reverse transcriptase
Reverse transcriptase is more error prone than DNA polymerase.

targets of medication to prevent HIV infection: fusion, reverse transcriptase, protease, integrate
–> without these essential enzymes, virus cannot replicate genetic info and become active

Zidovudine (ZDV)=is an alalogue of deoxythymidine–> prevents DNA chain elongation by reverse transcriptase (absence of 3’OH)

Question 6

5-Fluorouracil

Answer 6

is an analogue of uracil or thymine (competitive inhibitor of thymidylate synthase)
used for cancer chemotherapy. Inhibits DNA synthesis by inhibiting thymidylate synthase: an enzyme for TMP synthesis.

Capecitabine (Xeloda) is an orally-administered chemotherapeutic agent . It is enzymatically converted to fluorouracil in the body.