Exam 1- Cushman Lec 1- Bacterial Cell wall + Beta Lactams Flashcards
How does the structure of the cell wall in Gram (+) bacteria differ from Gram (-) bacteria?
Gram (+):
-Drugs penetrate the outer layers of cell wall effectively
-Bacterial membrane is the main barrier
-B-lactamases excreted through cell wall to external environment
-B lactamases produced in larger quantities
-Thick peptidoglycan layer
-Have 1 membrane
-Peptidoglycan residue is replaced by L-lysine residue (COOH is replaced with H)
-Peptidoglycan is cross-linked by a bridge between the L-Lys strand and the terminal D-Ala of a second molecule
Gram (-):
-Outer membrane excludes drugs, preventing penetration
-Porins in outer membrane allow some drugs to pass
-B-lactamases are confined to periplasmic space
-Thin peptidoglycan layer
-Have 2 membranes (inner + outer) with periplasmic space between -have a more complex wall that is lipoidal
-Peptidoglycan contains meso-diaminopimelic acid residue (DAP)
-Peptidoglycan is cross-linked by a bridge between DAP residue of one strand and the terminal D-Ala of another
What is the role of beta lactamases?
Hydrolyze beta lactam and inactivate the drugs
Which enzyme is responsible for cross-linking peptidoglycan strands?
Transpeptidase
How does transpeptidase cross-link peptidoglycan strands?
-Covalent bond
-Creates a Gly bridge between:
Gram (-): DAP residue of one strand and D-Ala of another
Gram (+): L-Lys strand of one and terminal D-Ala of another
-Transpeptidase active site has a serine residue (CH2OH)
Why is cross-linking of peptidoglycan strands important?
Confers strength to the bacterial cell wall
-cell wall will lyse and the bacteria will die without it
What well-known antibiotic is a beta-lactamase?
Penicillin
What is the mechanism of action of beta-lactam antibiotics?
Inhibit the transpeptidases that cross-link the peptidoglycan strands in the bacterial cell wall together
How do beta-lactam antibiotics inhibit peptidglycan?
They acylate the transpeptidase Ser residue in the enzyme active site to form a stable product
-this inactivates the transpeptidase and inhibits cross-linking, causing a defective bacterial cell wall
-cell wall is now subject to osmotic stress and the bacterial cell lyses
(see next notecard)
How does penicillin fool the transpeptidase into believing it is a peptidoglycan?
-Causes the transpeptidase to think it is a D-Ala-D-Ala residue at the end of a peptidoglycan
*peptidoglycan attacks the double bonded O just like it does in peptidoglycan (carbonyl)
-Forms a tetrahedral intermediate
*electrons move from H to O, forming another double bonded O (carbonyl)
**this breaks the four-membered ring
Why are penicillins/ beta-lactams reactive?
They have a four-membered ring
*this causes a lot of ring string and therefore, reactivity
What are the mechanisms of resistance that can occur against beta-lactams?
Decreased cellular uptake of the drug
Mutation of the penicillin-binding proteins that decreases their affinity for penicillins
Presence of an efflux pump that pumps antibiotics out of the cell
Induction/elaboration of bacterial B-lactamases that catalyze the hydrolysis of the B-lactam moiety
**note that this resistance mechanism is basically the same thing as the moa mechanism. Carbonyl is attacked, forms intermediate, ring structure breaks
–creates a regenerated B-lactamase and unstable hydrolyzed penicillin
What percent of the US population is allergic to B-lactams?
6-8%
What causes the allergenicity seen with B-lactams?
The drug acts as a hapten
-acylates host proteins, raising antibodies, and resulting in an allergic reaction
If a person is allergic to one penicillin can they be given another?
NO
cross-sensitivity is common, so if a person has an allergy to one B-lactamase, they are likely to have an allergy to another
What tests are available to test for B-lactam allergy?
Topical flare and wheal tests
Under which conditions do penicillins degrade?
Acidic and Basic
How does pencillin degrade under acidic conditions?
-The side chain has echimeric assistance (participates in mechanism of reaction to assist hydrolysis)
-Side chain carbonyl attacks the main carbonyl to form a five-membered ring
*Penicillin breaks down to Penicilloic acid
*Note that the nucleophilicity of the R on the side chain determines the reactivity. If electron attracting (electronegative) the O is less nucleophilic and penicillin is more stable. If electron-donating, the O is more nucleophilic and less stable (cannot be given po)
-Creates a penillic acid
-Also can create a penicillenic acid
How does penicillin degrade under basic conditions?
OH- group attacks main carbonyl and creates Penicilloic acid
True or False: Penicillin hydrolysis products have antibiotic activity
False
-they have to antibiotic activity
True or False: Hydrolysis of the B-lactam is irreversible
True
-once the 4-membered ring is opened, it cannot close again
True or False: Electronegative substituents on the side chain carbonyl reduce the nucleophilicity of the side chain and makes the B-lactam more stable
True
-the penicillin is stabilized against hydrolysis under acidic conditions
-the first step in the hydrolysis reaction is decelerated
What element present in the side chain will make it more electronegative, and therefore stabilize the penicillin?
O
-Penicillin V is more stable than Penicillin G because V has an extra oxygen in its side chain which is electronegative and decreases the nucleophilicity of the main carbonyl
What pH is best for penicillin storage?
6-6.8
What molecules can catalyze penicillin degradation reactions, and should therefore be kept away from penicillin solutions?
Heavy metal ions
How does lipophilicity of the penicillin side chain affect protein binding?
More lipophilic side chains result in more protein binding
How does protein binding affect the action of B-lactams?
Protein binding reduces bioavailability
-reduces the effective concentration of the free drug
Penicillins are excreted by which routes?
Renal
Biliary
For penicillins excreted by the kidneys, what percent of excretion is glomerular vs tubular?
Glomerular filtration: 10%
Tubular secretion: 90%
How is penicillin excretion affected by kidney disease/failure?
Penicillin half-life increases
What are the 2 mechanisms of tubular secretion?
One for anions and one for cations
*Note that penicillins are anions
How does probenecid affect penicillin excretion?
Penicillins are anionic and so is probenecid
-these two drugs compete with each other for secretion mechanisms
-therefore, when probenecid is administered with penicillin it increases the half-life of penicillin
What are the 5 nomenclatures to know for B-Lactam ring systems?
These are added to the four-membered ring of penicillins:
Penam (5-membered ring with S and no double bond)
Penem (5-membered ring with S + double bond)
Carbapenem (5-membered ring with no S but has a double bond)
Cephem (6-membered ring with S + double bond)
Monobactam (no extra ring, just NH)
What are the B-lactam sensitive penicillins?
Benzylpenicillin (Penicillin G)
Phenoxymethyl Penicillin (Penicillin V)
What points are important to know about Benzylpenicillin (Penicillin G)?
Works on: Gram + cocci
B-lactam sensitivity: Yes
Admin: Parenteral
Precautions: In individuals with history of allergies/asthma
What is the main difference between Penicillin G and V?
V is more stable in acid (can give po)
-due to extra oxygen on side chain (more electrophilic)
What are the B-lactamase-resistant parenteral penicillins?
Methicillin
Nafcillin
What causes methicillin to be resistant to B-lactamase?
Steric hinderance
-unable to have nucleophilic attack by the enzyme on the B-lactam carbonyl
What are important points to know about methicillin?
-Unstable to acid in the stomach, must be administered by injection
-Discontinued due to resistance
Why is methicillin so unstable in stomach acid?
The oxygen on the side chain, instead of reducing electronegativity of the main carbonyl, increases it
-oxygen in the side chain moves electrons to the carbonyl and makes it more electronegative
-this is due to the resonance effect
What is the most important bug to remember that is resistant to methicillin?
MRSA
(methicillin-resistant staphylococcus aureus)
-because of mutation in transpeptidase
What gene codes for the mutated penicillin-binding protein (transpeptidase) in MRSA?
mecA
(methicillin resistance gene)
What penicillin binding protein is coded for by mecA?
PBP2A
*this is the protein found in MRSA
*it can cross-link peptidoglycan but does not react with methicillin and many other B-lactams
What are the important points to remember about Nafcillin?
-Not sensitive to beta lactamase
-Slightly more stable than methicillin in acid
What are the B-lactamase-Resistant oral penicillins?
Oxacillin
Cloxacillin
Dicloxacillin
Why are oxacillin, cloxacillin, and dicloxacillin B-lactamase resistant?
They all have an isoxazole structure and large 6-membered ring that sterically hinders the carbonyl
Which B-lactamase resistant penicillin is the only one still used orally?
Dicloxacillin
What are the B-Lactamase-Sensitive, Broad-Spectrum, Oral Penicillins?
Ampicillin
Amoxicillin
What organisms are sensitive to ampicillin?
Gram-
(Salmonella, Shegella, Proteus mirabilis, Escherichia coli, Haemophilis influenzae, Neisseria gonorrhea)
How is ampicillin able to be transported into Gram - bacteria?
The porin channels are hydrophilic and transport ionic compounds
-ampicillin has a charged amino group at physiological pH, allowing it to be transported through porins
Why is ampicillin stable in acid?
The amino group is protonated in the stomach
-the charged nitrogen group becomes more electron-attracting
-this decreases nucleophilicity of the carbonyl and it does not participate in ring-opening
What points are important to remember about amoxicillin?
-Analog of ampicillin
—only difference is the addition of a hydroxyl group to the aromatic ring
**Better oral absorption than ampicillin
What are the B-lactamase inhibitors?
Clavulanic Acid
Sulbactam
Tazobactam
Avibactam
When are B-lactamase inhibitors used?
In combination with B-lactamase-sensitive penicillins
-enhance the activity of these penicillins against resistant strains
What is the MOA of the B-lactamase inhibitors?
Acylate the serine hydroxyl group in the active site of the B-lactamase
What is the brand name of Potassium Clavulanate + Amoxicillin?
Augmentin
What is the brand name of Sulbactam + Ampicillin?
Unasyn
What is the brand name of Sulbactam + Pipericillin?
Zosyn
What is the bran name of Avibactam + Ceftazidime?
Avycaz
What is the B-Lactam-Sensitive, Broad-Spectrum, Parenteral Penicillin?
Piperacillin
What bacteria is piperacillin active against?
Gram -
Gram +
Why does piperacillin have enhanced potency over the B-lactam-sensitive oral penicillins?
The added side chain fragment on piperacillin resembles a longer section of the peptidoglycan chain than ampicillin
