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Flashcards in 11 Vancomycin Duncan Deck (35):
1

What is some general information of Vancomycin?

A complex antibiotic isolated from Steptomyces orientalis. Formed of unusual amino acids and sugars hence the name: Glycopeptide. Primarily active against Gram (+) bacteria; most Gram (-) species and mycobacteria are resistant

2

What is the structure of Vancomycin like?

Tricyclic structure. Linear heptapeptide, fused in 3 rings. Two sugars: terminal vancosamine. Synthesized by NRPS modular genes/proteins (NRPS)

3

What are the characteristics like of the sugars that are in Vancomycin?

Deglycosylated form is active (termed "aglycone"). Hence, sugars don't contribute to activity: but the true story is much more complex. Resistance to vancomycin occurs. Modification to vancosamine can overcome it

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4

What is the general class of vancomycin antibiotics referred to as?

Glycopeptide Antibiotics

5

What does Vancomycin do?

Inhibits cell wall biosynthesis. Inhibition prevents addition of disaccharide building block to cell wall. Inhibits the transpeptidation reaction (stage III)

6

What are the metabolic processes affected?

Blocks cell wall biosynthesis. Inhibition is non-enzymatic. Occurs by steric interference mechanism

7

What is the mechanism of inhibition from vancomycin?

The peptide protion of the molecule binds to the D-ala-D-ala tail of the pentapeptide (lock-and-key)

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8

What is the MOA of Vancomycin?

Creates steric interference for enzymes that must recognize this part of the cell wall precursor ("gum on the key"): transpeptidase, transglycosylase. Primary inhibition is at the final amino acid cross-linking step (transpeptidase; like B-lactams)

9

For Vancomycin how does it affect the role of sugars?

Causes immature peptidoglycan to form, but can't be made into the mature peptidoglycan

10

What does modifying the sugar on vancomycin do?

Mod sugar R groups counteracts resistance

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11

What is the role of the sugars on Vancomycin?

The sugars alone can exhibit antibiotic activity (when modified with the membrane-targeting chlorobiphenyl). The sugars block the incorporation of lipid II into the growing peptidoglycan (lipid II accumulates). Hence, they block the activity of transglycosylase. Thus, glycopeptides can be a bipartite, bifunctional antibiotic

12

What are the two main types of highly resistant strains to vancomycin?

VanA and VanB. Both are plasmid mediated. Both contain virtually identical genes

13

What is the biosynthetic pathway of vancomycin resistant species?

Have an altered cell wall biosynthetic pathway. The final cell wall is unaltered. However, the intermediate N-acetyl-muramic acid penta "peptide" has a D-ala-D-lac tail, instead of D-ala-D-ala. This in essence alters the shape of the "lock" that vancomycin recognizes. VISA, VRE (mechanism of resistance may differ). The genetic basis for this property (for VRE): plasmid-encoded, requires two different functionalities, synthetic, and degradative

14

What are the genes in the plasmid like for the vancomycen resistant enterococci?

Has new genes: VanH and VanA that make a new tail. VanX cleaves D-ala-D-ala whenever it sees it do stop it from competing with the new tail synthesis

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15

What is VanA vs. VanB resistance?

VanA plasmid provides resistance to vancomycin AND teicoplanin. VanB plasmid provides resistance to vancomycin but NOT teicoplanin. VanB remains sensitive to teicoplanin because the Van HAX cassette is NOT activated (no cell wall precursor change occurs). Teicoplanin does NOT activate the RS sensor system in VanB-type plasmid (the S sensor is a histadine kinase, the R regulator is a transcription factor)

16

What is a good way to overcome vancomycin resistance?

Modify the sugar with a lipid moiety. Mirrors the structure of the natural product teicoplanin. Synthetic lipoglycopeptides primarily target the transglycosylase step

17

What is Telavancin?

Binds primarily to the cell membrane. Inhibits the transglycosylase and transpeptidase. Uses high affinity binding to Lipid II to recruit to cell membrane (10x potency of vancomycin, leading to improved bacteriocidal activity). Once there, lipid tail interacts wtih membrane in a lipid II-independent fashion to inhibit membrane function. Cell membrane depolarization, intracellular inhibitions, all contributing to cell death

18

What is some general information on Bacitracin?

An antibiotic. Formed of 12 amino acids, with 7 in a ring, plus a tail. Bacitracin inhibits cell wall biosynthesis. Primarily active against Gram (+) bacteria. Principally used in topical OTC antibiotic preparations.

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19

What is Bacitracin biosynthesis?

Bacitracin is synthesized by a complex molecular machine called a: Non-Ribosomal Protein Synthetase. Extremely large. 3 domains, each adding a different A.A, going down the line and slowly extending. 3 genes making 3 proteins

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20

What metabolic processes are affected by Bacitracin?

Inhibition occurs "during" stage III reactions. Stage III involves membrane-associated Lipid Intermediates. The lipid used in biosynthesis is bactoprenolphosphate (aka Undecaprenylphosphate). Following its use, it is released as bactoprenolPYROphosphate. Before it can act again, it must be converted back into bactoprenolphosphate (this process is called "recycling"). Bacitracin blocks bactoprenolpyrophosphate recycling

21

What is the mechanism of inhibition of Bacitracin?

Phosphate cleavage (recycling) requires an enzyme, phosphomonoesterase. However, bacitracin does not affect phosphomonoesterase activity. Rather, bacitracin binds directly to the pyrophosphate end of undecaprenyl-pyrophosphate (it binds to the substrate, not the enzyme. the mechanism is steric interference). Binding is with a 1:1 stoichiometry, and requires both a pyrophosphate and a lipid tail

22

What does Bacitracin require for inhibition?

Requries both the lipid tail AND pyrophosphate

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23

What are some other determinants of Bacitracin activity?

The histidine appears to be a very critical component in binding. Binding requires the participation of a metal ion to stabilize it (metal chelators diminish activity). The histidine complexes with both the metal and UDPP by ionic bonds. The amino group on isoleucine 1 participates --> binding to UDPP involves non-covalent, ionic bonds, and hence is pH sensitive (binding is maximal at pH 7.0-7.5)

24

What is the general introduction of Polymixins?

Formed of amino acids, coupled to lipid tail. Polymixins inhibits cell membrane function. Primarily active against Gram (-) bacteria. Toxic to kidney function, hence used principally in topical (OTC) antibiotic preparations

25

What is the general structure of Polymixins?

A positive ring structure of 7 amino acids. A fatty acid tail between 7-14 carbon atoms long

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26

What are the metabolic processes affected by Polymixin?

Polymixins primarily affect the integrity of cell membranes (secondary effects). Membranes equal "skin" (cell wall would equal "clothes"). Membrane structure: phospholipid bilary, proteins

27

What is The Rule of Similars?

Simialr molecular domains like to assocaited with each other. Conversely, opposite types of domains tend to repel each other

28

What are the specific aspects of membranes relevant to polymixins?

For considering polymixins' actions, the phospholipids are important; i.e. a primary target. Phospholipids possess two distinct domains (polar heads (phosphate groups), hydrophobic carbon chain tails). Phospholipid bilayer perturbations affect membrane (1) barrier and (2) protein functions. Gram negative microorganisms have "exposed" outer membrane, including exposed LPS

29

What is the MOA of Polymixins?

Disrupt membrane by mimicking the molecular structure of a phospholipid (i.e. they are "like"). Initially there is outer membrane disruption leading to penetration to the inner cell membrane, its disruption, and cell death. Polymixins initially bind to the lipopolysaccharide (LPS) component of the Gram (-) outer membrane. Polymixins then insert into the cell membrane, disrupting its orderly structure leading to altered permeability, and disrupting essential protein functions

30

What is Sensitivity and Resistance to Polymixin like?

Proportional to the total amount of phospholipids in the cell membrane (bacteria with high total amounts are the most sensitive). Gram (-) are most sensitive because their cell membrane/LPS is most accessible. Gram (+) less sensitive because the cell wall provides a physical barrier. Resistance also occurs with LPS changes

31

What is the general introduction to antibiotics that poke holes in microbial membranes?

These very effectily kill bacteria (or fungi). Most are not clinically useful because they have numerous deleterious side effects, though they could be modified to reduce toxicity. Gramicidin, Magainins, Defensins, Lantibiotics

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32

What is Plectasin?

A new antibiotic. 40 amino acid peptide stabilized by 3 S-S bridges. Cell killing kinetics resemble vanco, B-lactams, bacitracin; not like polymixin, novispirin, gramicidin. No evidence for membrane pores. Forms 1:1 complex with Lipid II; steric inhibition

33

What is Daptomycin?

Natural product. Structure: a cyclic lipopeptide. Recommended use: active against most resistant Gram (+) pathogenic bacteria (non-resistant too)

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34

What is Daptomycin's MOA?

Lipid portion inserts into membranes. Like-associates-with-like rule. Cationic amino side chains can interact with phosphate head groups of phospholipids. CALCIUM dependent binding process. Destroys membrane integrity. Depolarizes membrane via K leakage (forms pores). Intracellular processes, such as protein synthesis, become inhibited, as secondary effects

35

What is sensitivity and resistance like for Daptomycin?

Virtually all Gram (+) are sensitive. Very little inherent resistance. Gram (-) are not susceptible, because of the outer cell membrane and penetration issues