Antibiotic Chemistry

Question 1

what structure does the beta lactam ring mimic

Answer 1

D-Ala-D-Ala dipeptide involved in cross linking of peptidoglycan

Question 2

how does the beta lactam ring work

Answer 2

highly strained (electrophilic) ring behaves as a substrate mimic for the penicillin binding proteins and undergoes an acyl-substitution reaction with an active site nucleophile (usually serine side chain)
stable acyl-complex is formed that blocks enzyme active site and removes nucleophile (irriversible reaction)

Question 3

why does the beta lactam antibiotic reaction work

Answer 3

enzyme containing acyl-intermediate is inactive which results in bacterial cell lysis
high levels of antibiotic reduce lysis
only occurs in living cells as they produce peptidoglycan

Question 4

what mechanisms are involved in resistance to beta lactam antibiotics

Answer 4

target modification, amidases, beta lactamases

Question 5

how does the target modification mode of resistance work

Answer 5

modifies penicillin binding proteins

important for methicillin but has been used by MRSA

Question 6

how does the amidases mode of resistance work

Answer 6

removes the penicillin side chain

Question 7

what are the features of beta lactamases

Answer 7

evolved from PBPs; use of antibiotics puts selective pressure on bacteria to produce beta lactamases
genes for beta lactamases can be encoded on bacterial chromosomes or plasmids

Question 8

what are some examples of beta lactamases (penicillinases)

Answer 8

TEM-1 and TEM-2

Question 9

what beta lactams are class A

Answer 9

penicillinases TEM1 and TEM2

Question 10

what beta lactamases are in class B

Answer 10

metallo-beta-lactamases like NDM1

Question 11

what beta lactamases are in class C

Answer 11

cephalosporinases

Question 12

what beta lactamases are in class D

Answer 12

extended spectrum beta lactamases, hydrolysing beta lactam antibiotics with more variable structures

Question 13

what happens during the development of semi-synthetic penicillins

Answer 13

the side chain is changed during biosynthesis; epimers have different antibiotic activities

Question 14

what are epimers

Answer 14

molecules with different stereochemical centres

Question 15

what structure are penicillins semi-synthetically synthesised from

Answer 15

6-aminopenicillanic acid

Question 16

what are cephalosporins and cephamycins made from

Answer 16

penicillin N (last intermediate in penicillin synthesis)
semi-synthetic ones also made using an acylase to remove side chains

Question 17

how are semi-synthetic cephem antibiotics made

Answer 17

using an acylase to remove the side-chain to give the cephem core

Question 18

what are some examples of irreversible beta lactamase inhibitors

Answer 18

sulbactam, bromopenicillin, thienamycin, clavulanic acid

Question 19

how do irreversible beta lactamase inhibitors work

Answer 19

the inhibitor behaves as a substrate, reacting with penicillinase active site Ser residue
rearrangement of acyl-enzyme intermediate results in enzyme inactivation, these inhibitors are called ‘suicide substrates’
most do not work well in vivo due to difficulties penetrating the cell wall

Question 20

what is clavulanic acid

Answer 20

irriversible inhibitor of class A beta lactamases
weak abx activity of its own but potentiates effect of penicillin by preventing degredation due to beta lactamases
no effective against class B as they do not use an active site residue behaving as a nucleophile
not effective against classes C and D as beta lactamases do not bind to the enzyme

Question 21

how are multidrug resistant plasmids transferred between bacteria

Answer 21

by conjugation

Question 22

what is the process of bacterial conjugation

Answer 22

1. bacteria with plasmid uses pilus to attach to other bacterium
2. donor and recipient merge
3. relaxosome and transferosome allow transfer of plasmid
4. both cells separate and now contain the plasmid and become new donors

Question 23

what are the mechanisms of antibiotic resistance

Answer 23

inactivate/modify drug, alter drug target site, alter drug uptake/exit

Question 24

what chromosomal bacteria do beta lactamases inactivate

Answer 24

s.aureus, p.aeruginosa

Question 25

what plasmid-mediated bacteria do penicillinases inactivate

Answer 25

e.coli

Question 26

what is the beta lactamase action in gram negative bacteria

Answer 26

found in periplasmic space (between cell wall and cell membrane)

Question 27

what is the beta lactamase action in gram positive bacteria

Answer 27

beta lactamase released into medium around cell

Question 28

how do beta lactamases work

Answer 28

they open the beta lactam ring so antibiotics do not work

Question 29

how do beta lactams alter target site

Answer 29

by altering PBPs thorugh the MecA gene

Question 30

how is altered uptake affecting beta lactamase resistance

Answer 30

reducing number or size of porins

reducing permeability and preventing uptake

Question 31

how did vancomycin resistance occur

Answer 31

altered target site
change in terminal amino acids of peptide chain D-ala-D-ala to D-ala-D-lactate
from stable 5H bond to unstable 4H bond

Question 32

what resistance occurs in aminoglycosides

Answer 32

drug inactivation by changing acetyl groups

Question 33

what resistance occurs in flouroquinolones

Answer 33

altereg drug target

chromosomal mutations in DNA gyrase or topoisomerase IV inhibit binding of abx to target site

Question 34

how does resistance occur in tetracyclines

Answer 34

efflux pumps
expressed by gram positive and negative bacteria, actively pump substrates out of bacterial cell
confer resistance to single antibiotic or multiple unrelated agents
basal expression confers intrinsic resistance
upregulated expression confers acquired resistance

Question 35

what is an inducible efflux pump

Answer 35

only ‘switched on’ in the presence of the substrate it pumps