Main barrier keeping drugs out of cell in gram positive
Gram positives excrete...
B lactamase through the cell wall into the external environment
Outer membrane barrier property in gram negative?
excludes drugs, but some may stil get in through porins in the OUTER cell membrane
In gram negatives, where is the beta lactamase located?
in the periplasmic space
type with thicker PG
Gram negatives have an ____ that contains ____ which may facilitate entry
G- Peptidoglycan contains _______
meso-diaminopimelic acid residue
G+ peptidoglycan contains ____ instead of the ____ that gram negative contains
Crosslinking occurs in G- between the ___ and ____
DAP and D-ala
G+ crosslinking occurs between...
L-Lys and terminal D-ala
S. aureus crosslink composition
5 glycine residues
The OH from ___ attacks the ___ carbon of peptide-d-ala-d-ala
OH from Transpeptidase attacks the amide carbon
Transpeptidase mechanism: ____ is formed as an intermediate which is more reactive
After the ester linkage is broken, a ____ intermediate is formed.
Final products of transpeptidase mechanism
Peptide- D-Ala-Gly-Gly -Peptide
acylates the transpeptidase's serine residue in the enzyme active site.
--> inactivates the enzymes and inhibits cross linking.
reactivity of B lactam system is due to...
highly strained 4-membered ring
NCC bind of the B lactam is compressed to ___ from _____
compressed to 90 from normal 120 angle
Bond angle of a tetrahedral carbon
Stress can be releived in beta lactam strain by conversion of...
carbonyl to the tetrahydral intermediate
The beta lactal carbonyl is more reactive because...
its more like a ketone carbonyl than an amide carbonyl, because of the steric inhibition of overlap of the non-bonded electrons of the nitrogen with the adjacent carbonyl.
Penicillins assume a _____ structure with _____
folded ring structure with an sp3 hybridized nitrogen
There are many ____ which explains the heterogenecity of bacterial responses
penicillin binding proteins
The bacterial transpeptidases do not catalyze rxns with host cell proteins because...
the bacterial substrate contains unnatural D-ala residues that arent found in host cell proteins
Four mechanisms of resistance to beta-lactam
decreased cellular uptake
mutation of the PBPs
Presence of efflux pump
**Induction/elaboration of B-lactamases**
Four steps in B lactamase mechanism
[B lactamase + Penicillin] -->
Tetrahedral intermediate -->
Acylated B lactamase -->
[Regenerated enzyme + hydrolyzed penicillin]
Fast step in beta lactamase mechanism =
Hydrolysis of the acylated B-lactamase intermediate
__% of population has beta-lactam AB's
How are beta lactams allergenic?
Drug acts as hapten - acylates host cell proteins
(generates covalent products that are "foreign")
Why can't the allergic reaction of beta lactam be overcome via structural manipulation of the compound?
the reaction originates from the pharmacore of the drug
Can you use other penicillin-type drug on a penicillin-allergic person?
____ tests for beta lactam allergy are recommended if there is doubt about allergy
Topical flare and wheal test
Main degradation products of Pen G
- Benzylpenicillenic acid
- Benzylpenicillic acid
- Benzylpenicilloic acid
Penicillin hydrolysis products have no _______
Hydrolysis of B lactam is ___
(once the ring is opened, no closure)
Electronegative substituents have what effect when they're on the side chain carbonyl?
They reduce the nucleophilicity of the side chain amide carbonyl oxygen atom.
- Stabilizes the penicillin against hydrolysis under acidic conditions
- Increases oral bioavailability
Which is more stable, Pen G or Pen V?
Pen V is more stable to acid hydrolysis
*because the electronegativity of the ether oxygen decreases the nucleophilicity of the amide carbonyl
Solutions of penicillins should be used promptly and stored how?
In refrigerator at pH 6.0-6.8
_____ catalyze penicillin degradation
heavy metal ions
Penicillins with ______ are more highly protein bound
lipophilic side chains
Protein binding effects (2)
- Reduces BA
- Protects from degradation
Why isn't half life affected by protein binding?
dissociation rates from protein are fast,
and the renal excretion rates are the rate-limiting factor
Highest and lowest penicillins protein bound
cloxacillin = >90% protein bound
Ampicillin = 25-30% protein bound
Penicillin excretion routes
Glomerular vs tubular filtration of penicillins
Effect of kidney disease on half life of penicillin
prolonged half life
Two mechanisms for tubular excretion
Penicillins are anionic or cationic?
Competition with probenecid (anion) for secretion when coadministered with penicillin
- Beta-lactamase sensitivity?
- Gram+ and N.gonorrhoeae and H. influenzae
- Yes, sensitive
- Acute allergy
Methicillin has no beta lactamase sensitivity because
steric hindrance of nucleophilic attack by the enzyme on the beta lactam carbonyl.
methoxy groups are attached ortho to the amide--hinder nucleophilic attack on the lactam.
Why must methicillin be injected?
Electron donation to the amide carbonyl oxygen by the methoxy groups = amide carbonyl oxygen more nucleophilic
= unstable in acidic conditions
Methicillin is mainly used for
B-lactamase producing S. Aureus
Methicillin is an effective...
inducer of beta lactamase
What causes MRSA to be methicillin resistant
mutation in mecA in the penicillin binding protein gene (the protein is PBP2')
Cephapirin is a ___ AB
has a 6-membered sulfur containing ring instead of a 5 membered ring.
Some have an acetate in the 3 position = inactivated by hydrolysis by esterases (produces hydroxymethyl acid)
Ceph's MOA vs Pen MOA
Carbapenems are ______
carbon analogs of penicillins
Carbapenems have a ____ group instead of ____ that is present in penicillin's thiazolidine ring
What does ring strain have to do with sulfur and carbapenems?
The methylene in Carbapenems instead of the sulfur causes increased ring strain because the methylene is smaller
2 things imipenem reacts with
beta lactamases (inhibits them)
imipenem is hydrolyzed by _____
But this can be overcome by ___________
coadministration of cilastatin (dehydropeptidase-1 inhibitor)
Imipenem and cilastatin is special because... (2)
it has broader spec than any other AB available
active against both gram + and -
imipenem and cilastatin are used to treat
GI, GU, Bone, Skin, and Endocardium
ROA for imipenem-cilastatin
Aztreonam disodium is a ____ drug that is inspired by _____
In Aztreonam, a _______ group takes the place of the c2 carboxyl group (as in pen/ceph)
Spectrum of Aztreonam
Almost all gram-
What (Aztreonam) property activates the beta lactam ring to hydrolysis and to reaction with PBP's?
The electronegativity of the sulfamic acid
Aztreonam is used to treat...
severe Gram- infections, especially those by penicillin-resistant organisms acquired in hospitals
Example of cross reactivity between pen and cephs
(has identical sidechain)