Antibiotic Chemistry Flashcards
what structure does the beta lactam ring mimic
D-Ala-D-Ala dipeptide involved in cross linking of peptidoglycan
how does the beta lactam ring work
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)
why does the beta lactam antibiotic reaction work
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
what mechanisms are involved in resistance to beta lactam antibiotics
target modification, amidases, beta lactamases
how does the target modification mode of resistance work
modifies penicillin binding proteins
important for methicillin but has been used by MRSA
how does the amidases mode of resistance work
removes the penicillin side chain
what are the features of beta lactamases
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
what are some examples of beta lactamases (penicillinases)
TEM-1 and TEM-2
what beta lactams are class A
penicillinases TEM1 and TEM2
what beta lactamases are in class B
metallo-beta-lactamases like NDM1
what beta lactamases are in class C
cephalosporinases
what beta lactamases are in class D
extended spectrum beta lactamases, hydrolysing beta lactam antibiotics with more variable structures
what happens during the development of semi-synthetic penicillins
the side chain is changed during biosynthesis; epimers have different antibiotic activities
what are epimers
molecules with different stereochemical centres
what structure are penicillins semi-synthetically synthesised from
6-aminopenicillanic acid
what are cephalosporins and cephamycins made from
penicillin N (last intermediate in penicillin synthesis) semi-synthetic ones also made using an acylase to remove side chains
how are semi-synthetic cephem antibiotics made
using an acylase to remove the side-chain to give the cephem core
what are some examples of irreversible beta lactamase inhibitors
sulbactam, bromopenicillin, thienamycin, clavulanic acid
how do irreversible beta lactamase inhibitors work
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
what is clavulanic acid
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
how are multidrug resistant plasmids transferred between bacteria
by conjugation
what is the process of bacterial conjugation
- bacteria with plasmid uses pilus to attach to other bacterium
- donor and recipient merge
- relaxosome and transferosome allow transfer of plasmid
- both cells separate and now contain the plasmid and become new donors
what are the mechanisms of antibiotic resistance
inactivate/modify drug, alter drug target site, alter drug uptake/exit
what chromosomal bacteria do beta lactamases inactivate
s.aureus, p.aeruginosa
what plasmid-mediated bacteria do penicillinases inactivate
e.coli
what is the beta lactamase action in gram negative bacteria
found in periplasmic space (between cell wall and cell membrane)
what is the beta lactamase action in gram positive bacteria
beta lactamase released into medium around cell
how do beta lactamases work
they open the beta lactam ring so antibiotics do not work
how do beta lactams alter target site
by altering PBPs thorugh the MecA gene
how is altered uptake affecting beta lactamase resistance
reducing number or size of porins
reducing permeability and preventing uptake
how did vancomycin resistance occur
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
what resistance occurs in aminoglycosides
drug inactivation by changing acetyl groups
what resistance occurs in flouroquinolones
altereg drug target
chromosomal mutations in DNA gyrase or topoisomerase IV inhibit binding of abx to target site
how does resistance occur in tetracyclines
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
what is an inducible efflux pump
only ‘switched on’ in the presence of the substrate it pumps
