09 Beta-Lactams Duncan Flashcards
How do host cells destroy bacteria on their own?
Host cell brings in O2 – iNOS –> NO, which destroys the bacteria
What is the common pathway shared by all bactericidal antibiotics?
Hydroxyl radical. Destroys membranes, proteins, causes cell death
What do bactericidal antibiotics require to work?
An intact TCA and electron transport chain in order to make superoxide, which either leads to iron release or H2O2, ultimately leading to the hydroxyl radical
What are some general characteristics of B-Lactams?
Disrupt the cell wall of many different types of bacteria. Potent, wide spectrum of activity, low toxicity to humans
What is the general Penicillin Structure?
B-Lactam ring. Thiazolidine ring. R group to determine specificity, range
What is required for penicillin biosynthesis?
Cysteine and Valine. Penicillin biosynthesis occurs via the action of a Non-Ribosomal Protein Synthetase (NRPS). NRPS produce many of the commonly (therapeutically) used antibiotics
What are the characteristics of the Penicillin R group?
R group varies with the producer strain, the growth emdium, or at the hands of the synthetic chemist. In the naturally occuring penicillin, penicillin G, the R group is benzene
What are some different ways to modify the R group?
Precursing (putting side chain that you want into a medium and hoping it incorporates). Growth medium (hope to see something you want grows). Synthetic or semi-synthetic. Prototype semi-synthetic penicillins
What are some reasons for changing the R group?
Transport properties: interactions with human and bacterial physiology (PepT1 transporter). Interaction with bacterial proteins. Range of species affected. To counteract development of resistance
What are the principals and mechanisms of Drug-Target interactions?
Lock (the drug) fits into key (target’s bidning site). Binding affinity can be increased. Binding range can be increased. Binding inhibition can be overcome. ADRs can be minimized
What are the “targeted” R groups in Aminopenecillins to modify binding?
Amino side chain preceding benzene; increased G- activity; B-lactamase sensitive; acid stable
What are the “targeted” R groups in Methylpenicillins to modify binding?
Methyl group on benzene; resistance to S. aureus B-lactamase; acid labile; 95-98% protein bound
What are the “targeted” R groups in Isoxazolylpenicillins to modify binding?
Novel side chain; resistance to lactamase; acid stable; orally active; 95-98% protein bound
What are the “targeted” R groups in Ureidopenicillins to modify binding?
Urea group; strain specificity (e.g. Klebsiella, Pseudomonas; B-lactamase sensitive)
What are the “targeted” R groups in Carboxypenicillins to modify binding?
Carboxy side chain preceding benzene; strain specificity (e.g. Klebsiella, Pseudomonas; B-lactamase sensitive; ticarcillin gives higher blood levels)
What the the biochemical action of penicillins?
Penicillins (and other B-lactams) inhibit the cross-linking of bacterial cell walls
In the peptidoglycan wall, what are the virtical columns made of?
Cross bridges of glycine and side chain of L-alanine-D-Isoglutamine-L-Lysine-D-Alanine
In the peptidoglycan wall, what are the horizontal rows made of?
N-acetyl-glucosamine and N-acetyl-muramic acid
What are the three stages that cell wall biosynthesis occurs in?
1) Sugar precursors made inside the cell. 2) Disaccharide building block made. 3) Disaccharide linked and cross-linked to growing peptidoglycan (where B-lactams act)
How do antibiotic inhibitors of stage 1 (i.e. Fosfomycin) work?
Fosfomycin acts as a mimic of PEP (phosphoenolpyruvate (transfers lactic acid to UDP-GluNac)). Recognized by, and inhibits, MurA. Epoxide “O” forms permanent covalent bond with enzyme cysteine
What does Transglycosylase do?
Moves the immature peptidoglycan from Bactoprenol phosphate and joins it to the other peptidoglycans
What does Transpeptidase do?
Transpeptidase catalysis of glycine-alanine bond. Creates the mature peptidoglycan
What is Penicillins Activity?
Penicillins bind to transpeptidase and form a permanent covalent bond, thus inactivating transpeptidase (penicillin is a suicide substrate for transpeptidase)
What is Penicillins basis for activity?
Penicillins shape “looks like” D-ala-D-ala
What are the enzymes inolved in cell wall biosynthesis?
Transpeptidase carboxypeptidases. Penicillin-binding proteins (PBPs). Murein hydrolysis. B-lactamases (amidohydrolyases)
What activity do PBPs have?
Several PBPs, for example PBP1a and PBP2, possess both transglycosylase and transpeptidase activity. Numerous PBPs have been identified; one bacterium will have several types
What is the gene structure of PBP1a, 1B, 1C-like genes like?
What are the various shapes of cell wall?
Different extents and types of cross linking in different cell surface regions. Growth and division mean changes in shape occur over time
What are the physiological effects of B-lactams?
Inhibit cell division. Cause cell lysis. Bacteriocidal
What are the mechanisms of sensitivity and resistance?
Recognition of PBPs. Accessibility of PBPs (G+ vs. G-). B-lactamses
How does the mechanism of “recognition of PBPs” work?
Species-specific. Different affinities for different penicillin varieties. ID of good targets: low vs. high molecular weight. Low MW often have only carboxypeptidase activity, and little consequence for pen killing. High MW are multifunctional or multi-protein complexes, often with both TG and TP activity, and are prime pen targets
How are the characterizations of PBPs made?
In vitro binding, followed by SDS-PAGE to identify which varieties of B-lactam work (bind) best. Identifying useful, and useless, forms of B-lactams to treat an infection
How does Methicillin resistance in Staphylococcus occur?
Occurs when mutation in MecA gene occurs. MecA encodes a major PBP, PBP2A; mutated version sometimes referred to as PBP2’. Significantly reduces binding affinity for B-lactams (need to develop new varieties of B-lactams that bind to PBP2’)
What are the 4 main ways of antibiotic resistance?
1) Antibiotic efflux pump. 2) Antibiotic degrading enzyme. 3) Target altering enzyme. 4) Antibiotic altering enzyme
What are some general thoughts about resistance?
Resistance is bad. Resistance occurs following use of virtually any drug, over time (“Natural” selection for survivors). Two broad categories of resistance (pre-treatment resistance, post-treatment (induced) resistance). Post-treatment “requires” SOS response
What can be done in the resistance and the SOS response in order to stop the development of resistance?
Can block RecA, causing a cleavage of LexA (which would be used to transcribe error-prone DNA cauesd by DNA damage)
What are the characteristics of the SOS response?
Initiated by DNA damage: a repair system. Usually entails new DNA synthesis. Induces novel, error-prone DNA polymerases. Error-prone DNA synthesis (more proteins with mutations –> some selectivity advantageous –> resistance). Also reduces growth rate (to allow time for repair). Antibiotic effectiveness is reduced proportional to growth rate (most obvious for B-lactams, where changing cell wall is required for growth)
What are Amidohydrolase and penicillinase?
B-lactamses in bacteria. Penicillinase acts very similarly to transpeptidase (binds to B-lactam, cleaves B-lactam ring). The difference is, instead of forming a permanent covalent bond, the enzyme dissociates and then can do it all over again and again. Amidohydrolase cleaves the R group
What are the different classes of B-lactamases?
TEM and SHV. Extended Spectrum B-lactamases (ESBLs). AmpC or C class
What are TEM and SHV?
“Earliest variety; lowest range of activity. Multiple forms of each type exist; different genes
What are Extended Spectrum B-lactamases (ESBLs)?
Active against a wider range of B-lactams: can degrade 3rd generation cephs and monobactams
What are AmpC or C Class?
Initially chromosomal, inducible. Now plasmid encoded; rapid & easy transmission. Not inhibited by clavulanate, sulbactam, tazobactam
How do humans counterattack B-lactamases?
Clavam (oxygen in the 5 atom ring). Binds to B-lactamase, and blocks its activity. Humans got this with a little help from our “friends”, in this case a type of fungi
What are Tazobactam and Sulbactam?
B-lactamase inhibitors
What is the basic structure of Cephalosporins?
What are the characteristics of the different generation CEPHs?
1st: the original isolate, and forms with equivalent activity. 2nd: increased activity against certain kinds of microorganisms. 3rd: activity against resistant microorganisms. 4th: newer. 5th: newest
What is sensitivity and resistance to CEPHs like?
Basically, same considerations as described for the penicillins apply. B-lactamses cleave CEPHs; sometimes termed cephalosporinases. Resistance can arise through evolutoin or obtaining these B-lactamases; sensitivity can be re-established through modifications in the side chains that prevent B-lactamase activity
What are carbapenems?
B-lactam class. Most active of the anti-microbial B-lactams. Most resistant to degradation by “-ases”. Avoids resistance. Combined with cilastatin to reduce renal metabolism. Double bond in 5 membered ring makes these penems rather than penams
What is Aztreonam (Azactam)?
B-lactam. A semi-synthetic monobactam with high activity against gram negative bacteria, and high resistance to common B-lactamases; not recognized as “penicillin” by allergic system
