Week 4 - Bacteria and Bacterial targets /drugs Flashcards
Identify biochemical targets for antimicrobial activity
- Bacterial DNA (plasmids)
- Protein synthesis (ribosome)
- Metabolism
- Bacterial cell wall
- Bacterial cell membrane
How do antimicrobials i.e. antibiotics work?
- Target specific features only found in bacteria cells = destroy bacteria selectively (selective inhibition)
Explain how bacterial DNA is targeted
- Bacterial DNA is supercoiled by DNA gyrase (enzyme)
(left handed super helix = -ive supercoiling, in hot environments DNA is right handed = +ive supercoiling = tighter coil = prevent denature) - DNA gyrase: breaks DNA strand, loops 2nd DNA through break, reseals break
- Supercoiling makes DNA compact, easier for proteins to travel, DNA replication etc.
- DNA gyrase inhibitors
- Stops gyrase process after DNA strand has been broken
- Broken DNA strands are detected = apoptosis triggered - DNA strand breaking drugs
- Require anaerobic environment
- Nitro group is reduce forming a imidazole radical which extracts H from DNA
-Leads to DNA strand breaking + cell death
What drugs are used to target bacterial DNA?
- DNA gyrase inhibitors ~ Quinlones
- Nalidixic acid: mainly active against gram -ive bacteria
- Fluroquinlones: increased activity against gram -ive and +ive and were more potent
- Levofloxacin: has broad spectrum, very potent + can target pneumonia, UTI - DNA strand breaking drugs
- Nitroimidazele: treats aerobic bacteria + protozoal infections e.g. BV, acute oral infections
Explain how proteins synthesis is targeted (in bacteria)
- Bacteria ribosome is different shape + size (70S) compare to human ribosome (80S)
- Bacteria = 50S and 30S and Human = 60S and 40S
- Inhibiting synthesis = cell become bacteriostatic (growth slows down)
Transcription is targeted
What drugs are used to target protein synthesis?
- Oxyazoldinones
- Binds to 50S subunit = stops it from binding to 30S subunit
- Has chiral centre (important for anti-bacteria activity)
- Has -e donating N = improves safety profile
- Used against MRSA - Tetracylines
- Interfere with tRNA binding
- bind to 30S + stop aminoacyl tRNA from binding to ribosome
- Has broad spectrum BUT widespread resistance - Macrolides
- Binds to 50S subunit = stops growth of peptide chain
- Active against gram +ive bacteria
- e.g. erythromycin - binds + blocks exit tunnel for peptide chain = chain is pushed back in wrong direction = bond formation is ruined
- e.g. chloramphenicol - binds to same region as eryth. + has broad spectrum, toxic effects on bone marrow (don’t us in infants)
Explain how metabolism is targeted (in bacteria)
- Folate synthesises DNA bases
- Dihydrofolate reductase (DHFR enzyme) = target
- folate is in human + bacteria cells BUT have diff. structure + a.a sequence - Dihydropteroate synthase (DHPS)
- humans don’t have DHPS
- synthesises dihydrofolate by adding PABA to pteridine diphosphate
Targeting DNA Synthesis:
- Slows growth of bacteria
- Reduces folate
What drugs are used to target metabolism?
- Dihydrofolate Reductase (DHFR) Inhibitors
- Mimic pteridine of folate
- Potent against bacteria DHFR
- Used for UTIs, cancer arthritis - Sulfa Drugs
- Mimic PABA = DHPS becomes full with drug
- Sulfanilamide (not very acidic), sulfapyridine (more acidic, + aromatic ring), Sulfamethoxazole (treats UTI, have lower + safer doses due to sequential blocking)
Explain how synthesis of bacterial cell wall is targeted
- Human cells don’t have cell wall = TARGET
- Cell wall protects membrane from rupture (resitshigh osmotic pressure in cell)
- Cell wall provides strength to cell (membrane is fragile)
- Cell wall retains shape + volume of cell
Cell wall has cross linking between peptide chains (transpeptidase (enzyme) forms these links)o form networks
What are the differences in cell wall structure between Gram-positve and Gram-negative
Gram +ive:
- Have thick cell wall - thick peptidoglycan layer
- Cell wall is strong
- Porous cell wall = drugs can penetrate to membrane
- Stains purple
Gram -ive:
- Have 2 lipid cell membranes with peptidoglycan inbetween (in periplasm)
- Outer membrane (OM) = barrier to cell wall + inner membrane (IM)
- Porin protein channels (in periplasm) = drugs can penetrate across OM to IM
What drugs are used to target synthesis of bacterial cell wall?
- Penicillin G
- Deactivates transpeptidase (enzyme)
- Has a B-lactam ring (square ring)
- transpeptidase attacks ring + adds acyl group to drug
- transpeptidase becomes inactive = cross linking in cell wall can’t occur = cell weakens + dies
- Allergy to drug is common
- Active against gram +ive and -ive (limited)
- NOT orally active - Ampicillin (semi-synthetic penicillin)
- Has an e- withdrawing side chain (= acid resistance in stomach = orally active)
- deactivates carbonyl (decrease its nucleophilicity)
- Made from 3 a.acids - Cephalosporins
- Have B-lactam ring = targets transpeptidase
- Less likely to cause allergic reactions
- Good activity against gram -ive - Vancomycins
- Is a cyclic glycopeptide
- Is very rigid
- Separate peptidoglycan (dimer inserts between peptidoglycan)
= peptides unable to crosslink = cell burst + dies
- Used in gram +ive infections
B-lactamases:
- Inactivate drugs with B-lactam ring (i.e. penicillin, ampicillin)
- they open B-lactam ring + inactivate drug
- larger side chain groups (i.e. amoxycillin) prevent B-lactamase binding + inactivating drug
Side Chain Groups:
- Hydrophobic groups favour gram +ive bacteria
- Hydrophilic groups favour gram -ive
Explain how cell membrane is targeted
- Controls flow of ions, drugs and substance into / out of cell
What drugs are used to target cell membrane?
- Gramicidin A
Is a peptide
- Forms helix (pore) in membrane
- Allows uncontrolled flow of ions = cell dies (bursts)
- Also targets HUMAN cells (= non-selective)
- Only used topically
- Active against gram +ive bacteria
List extraction methods of medicinal products
Removing Solvent:
1. Rotary Evaporation
- Use vacuum + heat to evaporate solvent
- Solvent condenses + collected in diff. flask
= Left with dry, fully isolated drug
- Lyophilisation
- Freeze mixture till its solid
- Apply vacuum to sublimate solid to gas
- Keep dreaming water vapour away
= Solid product let in flask- Used for vaccines, viruses, proteins etc.
- Fractional Distillation
- Separates compound from mixture
- Diff. fractions have diff. BP - Crystallisation
- Dissolve compound in solvent
- Non-soluble = undissolved solid
- Soluble = dissolved + can be crystallised using vacuum
List techniques for analysis and purification of drugs
Chromatography
1. Reverse phase silica gel
- non-polar interactions of analyte + stationary phase
- gel contain porous beads which capture small molecules only i.e. impurities
2. Normal phase silica
3. Ion exchange
- electrostatic interaction of analyte + stationary phase
