INHIBITORS OF FOLATE METABOLISM

Question 1

What do bacteria use folate for?

Answer 1

DNA synthesis

Question 2

How does folate metabolism differ in humans vs bacteria?

Answer 2

• Humans - consume folate and it is converted to tetrahydrofolate
• Bacteria - synthesise the precursor (dihydropteroare diphosphate + p-aminobenzoic acid (PABA)) and using 2 enzymes, this is converted to tetrahydrofolate
  
  • Bacteria uses dihydropteroate synthase - humans don’t have this enzyme

Question 3

How is folate metabolism in humans vs bacteria similar?

Answer 3

Humans and bacteria both utilis dihydrofolate reductase

Question 4

What is dihydrofolate reductase?

Answer 4

An enzyme used in the final stage to make tetrahydrofolate

Question 5

What is tetrahydrofolate for?

Answer 5

• Tetrahydrofolate is essential for C1 metabolism.
• It is converted to 5,10-methylenetetra-hydrofolate, which is involved in DNA synthesis

Question 6

Why is the enzyme dihydropteroate synthetase a good antibacterial drug target?

Answer 6

Because humans don’t have this enzyme

Question 7

What does dihydropteroate synthetase do in (and only in) bacteria?

Answer 7

Catalyzes the combination of dihydropteroare diphosphate + p-aminobenzoic acid (PABA)

Question 8

What is the pka of para-aminobenzoic acid?

Answer 8

pKa 5

pKa is the pH at which 50% is ionised.

Question 9

Prontosil was shown to be a prodrug for which drug?

Answer 9

Sulfanilamide

Question 10

How was sulphanilamide made less foul tasting?

Answer 10

An elixir was produced, using diethylene glycol as a solvent (but it was this solvent that killed 107 people)

• Frances Kelsey roved the deaths were due to diethylene glycol

Question 11

Why is diethylene glycol toxic?

Answer 11

Because the liver oxidises the hydroxyl groups to toxic aldehydes

Question 12

How does sulphanilamide work and what are its limitations?

Answer 12

Mimics para-aminobenzoic acid

• PABA is converted by dihydropteroate synthetase = no tetrahydrofolate made

Question 13

What are the limitations of sulfanilamides?

Answer 13

• PABA: 5
• sulphanilamide: 10
• Difference in pKa means ionisation isn’t ideal and reduces its potential as a drug (sulphanilamide needs to be more acidic)

Question 14

What is the optimum pKa (acidic) of the sulfonamide group for bacterial activity?

Answer 14

pKa = 6-7

Question 15

How can sulfanilamides be made more acidic?

Answer 15

The R-group is the only molecule that can be changed

• N must be secondary
• Aromatic ring must be para-substituted with a para-amino group (NH2)

Question 16

What are substituted-sulphanilamides used for?

Answer 16

Treating gram-positive bacterial infections e.g. pneumococci and meningococci

Question 17

What are the 4 main substituted sulphanilamides?

Answer 17

• sulfapyridine
• sulfadimidine
• sulfadiazine
• sulfamethoxazole

Question 18

What is sulfadiazine used to treat?

Answer 18

Toxoplasmosis

Question 19

Describe the pathway of tetrahydrofolate biosynthesis in bacterial cells, including the enzymes in evolved in each step

Answer 19

Question 20

What does tetrahydrofolate break down into (that bacteria use)? (3)

Answer 20

• thymidines
• purines
• methionine

Question 21

What does trimethoprim inhibit?

Answer 21

Dihydrofolate reductase

Question 22

Why is trimethoprim selectively toxic?

Answer 22

Because its affinity for dihydrofolate reductase in bacteria is 50,000x stronger than for human dihydrofolate reductase

Question 23

Co-trimoxazole is a combination of which two medicines?

Answer 23

Trimethoprim
Sulphamethoxazole

Question 24

What is pyrimethamine?

Answer 24

Pyrimethamine is another dihydrofolate reducatse inhibitor.

Question 25

How does trimethoprim differ from pyrimethamine?

Answer 25

Pyrimethamine is more hydrophobic than trimethoprim. This is because of the Chlorophenyl ring

Question 26

The pyrimidine ring in pyrimethamine has DHFR activity. True or false?

Answer 26

True

Question 27

Does pyrimethamine have a greater effect on bacteria or protozoa?

Answer 27

Greater effect on protozoa

• pyrimethamine is too hydrophobic and gets stick in the bacterial cell wall = not an antibacteria, but protozoa doesn’t have a cell wall = good antimalaria drug

Question 28

What is methotrexate used (include at different doses) for and how does it work?

Answer 28

• Very low doses - treat rheumatoid arthritis and IBD (sometimes for the termination of pregnancy)
• Normal doses - anticancer agent
• Targets dihydrofolate reductase preventing folate metabolism and therefore DNA synthesis (IN HUMANS)

Question 29

Why doesn’t methotrexate work in bacterial cells?

Answer 29

Too polar to act on bacterial cells (too polar for passive diffusion)

Question 30

What are the downsides of methotrexate use?

Answer 30

• Moderately toxic
• If used concomitantly with penicillin, elimination is inhibited (overdose)

Question 31

What needs to be taken if a patient is on methotrexate?

Answer 31

Folic acid supplements (as the body will stop producing folate)

Question 32

Name the structure

Answer 32

Sulphanilamide

Question 33

Name the structure

Answer 33

Trimethoprim

Question 34

Name the structure

Answer 34

Pyrimethamine

Question 35

Name the structure

Answer 35

Methotrexate