Antibiotic and antifolate targeting

Question 1

Give an example of an antibiotic that inhibits bacterial cell wall synthesis

Answer 1

Penicillin
Inhibits peptidoglycan cross-links forming during cell wall synthesis - cell walls being constantly remodelled. Therefore imbalance between cell wall degradation and synthesis develops and cells rapidly die

Question 2

Give an example of an antibiotic that inhibits bacterial transcription

Answer 2

Rifampin

Allosterically inhibits RNApol, so no mRNA

Question 3

Give an example of an antibiotic that inhibits bacterial protein synthesis

Answer 3

Tetracyclines

Question 4

Give an example of an antifolate

Answer 4

Methotrexate
Is structurally similar to folate so acts as a competitive inhibitor to the enzyme dihydrofolate reductase (DFR), which activates folate into its active form tetrahydrofolate

Question 5

How does an antibiotic that inhibits bacterial cell wall synthesis work?

Answer 5

Disrupt cell wall synthesis

Blocks peptidoglycan cross-linking via competitive inhibition

Question 6

How does an antibiotic that inhibits bacterial transcription work?

Answer 6

Inhibit DNA gyrase

Inhibits RNA polymerase

Question 7

How does an antibiotic that inhibits bacterial protein synthesis work?

Answer 7

Inhibits 50S or 30S ribosomal subunit e.g. By blocking the a site of the ribosome so that tRNAs cannot fit and be added to the growing peptide chain

Question 8

How does an antifolate work?

Answer 8

Block the formation of the active form of folate (Vitamin B9), tetrohydrofolate, by inhibiting the enzyme dihydrofolate reductase (DFR). Tetrahydrofolate is a cofactor of methyltransferases involved in serine, methionine, thymidine and purine biosynthesis.
Inhibition therefore inhibits cell division, DNA/RNA synthesis/repair and protein synthesis

Question 9

Describe the mechanism that allows cells to become resistant to these drugs

Answer 9

1. Can stop the antibiotic form getting into the cell : transport channels, biofilm barrier
2. Once inside the cell can:
   (i) Destroy the antibiotic
   (ii) Modify the drug: methylation
   (iii) Physically stop it binding to the target protein: mutate the target
   (iv) Confuse the drug: make more target (e.g. DHFR (a methyltransferase)
   (v) Kick the drug out before it works: insertion of efflux pumps into the cell membrane

Question 10

Describe how cells acquire resistance to antibiotics?

Answer 10

Intrinsic resistance e.g. no cell wall (spread vertically down generations)
Acquired resistance (horizontal transfer):
(i) genetic mutation
(ii) DNA transfer:
- conjugation: transfer of transposons or plasmids through pili
- Transduction: bacteriophage viruses
- Transformtion: acquiring “free” DNA from their environment

Question 11

Are antifolates specific for mammalian cells?

Answer 11

You can use them on bacteria too, but they are not as effective (tend to use a different drug). This is because bacteria can synthesise folate (so don’t NEED the transporters etc to uptake folate into the cell), whereas mammals can’t synthesise folate.
Works on the prinicple that cancer cells divide more than normal cells BUT it will effect all dividing cells, therefore it has quite nasty side effects - especially on rapidly dividing cells (gut epithelia, hair folicles and white blood cells)

Question 12

What is the clinical use of antifolates?

Answer 12

Cancers
Some autoimmune diseases like psoriasis and rheumatoid arthritis
Clinical abortions

Question 13

Why is antibiotic resistance increasing?

Answer 13

Over-use of antibiotics means more SELECTIVE PRESSURE for the survival or resistant strains of bacteria.

Question 14

How do mammalian cells acquire resistance to antifolates?

Answer 14

Natural selection - cancer cells divide more quickly than normal mammalian cells as they tend to skip the cell checkpoints G1 and G2