Iodinated vancomycin and mucopeptide biosynthesis by cell-free preparations from Micrococcus lysodeikticus

Approved Glycopeptide Antibacterial Drugs: Mechanism of Action and Resistance

The glycopeptide antimicrobials are a group of natural product and semisynthetic glycosylated peptides that show antibacterial activity against Gram-positive organisms through inhibition of cell-wall synthesis. This is achieved primarily through binding to the d-alanyl-d-alanine terminus of the lipid II bacterial cell-wall precursor, preventing cross-linking of the peptidoglycan layer. Vancomycin is the foundational member of the class, showing both clinical longevity and a still preferential role in the therapy of methicillin-resistant Staphylococcus aureus and of susceptible Enterococcus spp. Newer lipoglycopeptide derivatives (telavancin, dalbavancin, and oritavancin) were designed in a targeted fashion to increase antibacterial activity, in some cases through secondary mechanisms of action. Resistance to the glycopeptides emerged in delayed fashion and occurs via a spectrum of chromosome- and plasmid-associated elements that lead to structural alteration of the bacterial cell-wall precursor substrates.

The ABC transporter Tba of Amycolatopsis balhimycina is required for efficient export of the glycopeptide antibiotic balhimycin

All known gene clusters for glycopeptide antibiotic biosynthesis contain a conserved gene supposed to encode an ABC-transporter. In the balhimycin-producer Amycolatopsis balhimycina this gene (tba) is localised between the prephenate dehydrogenase gene pdh and the peptide synthetase gene bpsA. Inactivation of tba in A. balhimycina by gene replacement did not interfere with growth and did not affect balhimycin resistance. However, in the supernatant of the tba mutant RM43 less balhimycin was accumulated compared to the wild type; and the intra-cellular balhimycin concentration was ten times higher in the tba mutant RM43 than in the wild type. These data suggest that the ABC transporter encoded in the balhimycin biosynthesis gene cluster is not involved in resistance but is required for the efficient export of the antibiotic. To elucidate the activity of Tba it was heterologously expressed in Escherichia coli with an N-terminal His-tag and purified by nickel chromatography. A photometric assay revealed that His(6)-Tba solubilised in dodecylmaltoside possesses ATPase activity, characteristic for ABC-transporters.

Vancomycin Analogues Containing Monosaccharides Exhibit Improved Antibiotic Activity: A Combined One-Pot Enzymatic Glycosylation and Chemical Diversification Strategy

Many natural products contain carbohydrate moieties that contribute to their biological activity. Manipulation of the carbohydrate domain of natural products through multiple glycosylations to identify new derivatives with novel biological activities has been a difficult and impractical process. We report a practical one-pot enzymatic approach with regeneration of cosubstrates to synthesize analogues of vancomycin that contain an N-alkyl glucosamine, which exhibited marked improvement in antibiotic activity against a vancomycin-resistant strain of Enterococcus.

Lipid II as a target for antibiotics

Lipid II is a membrane-anchored cell-wall precursor that is essential for bacterial cell-wall biosynthesis. The effectiveness of targeting Lipid II as an antibacterial strategy is highlighted by the fact that it is the target for at least four different classes of antibiotic, including the clinically important glycopeptide antibiotic vancomycin. However, the growing problem of bacterial resistance to many current drugs, including vancomycin, has led to increasing interest in the therapeutic potential of other classes of compound that target Lipid II. Here, we review progress in understanding of the antibacterial activities of these compounds, which include lantibiotics, mannopeptimycins and ramoplanin, and consider factors that will be important in exploiting their potential as new treatments for bacterial infections.

Acceptor specificity and inhibition of the bacterial cell-wall glycosyltransferase MurG

A continuous fluorescence coupled enzyme assay was developed to study the acceptor specificity of the glycosyltransferase MurG toward different lipid I analogues with various substituents replacing the undecaprenyl moiety. It was found that most lipid I analogues are accepted as substrates and, amongst these, the saturated C14 analogue exhibits the best activity. This substrate was used to evaluate the inhibition activity of such antibiotics as moenomycin, vancomycin, and two chlorobiphenyl vancomycin derivatives. A vancomycin derivative with a chlorobiphenyl moiety on the aglycon section was identified as a potent inhibitor of MurG.

Peptidoglycan biosynthesis in Micrococcus luteus (sodonensis): transglycosidase and phosphodiesterase activities in membrane preparations

Two enzyme activities involved in the biosynthesis of peptidoglycan in Micrococcus luteus (sodonensis), a transglycosidase and a phosphodiesterase, have been demonstrated in isolated membrane preparations. The transglycosidase activity promotes the in vitro synthesis of an uncross-bridged peptidoglycan that is completely susceptible to lysozyme. This in vitro-synthesized peptidoglycan consists of 76% "soluble" and 24% "insoluble" material. The soluble peptidoglycan is primarily a single low-molecular-weight species of approximately 20 disaccharide peptide units. "Insoluble" peptidoglycan, which likely represents newly synthesized material incorporated into an existing cell wall, was solubilized by butanol extraction, and the two were compared. The phosphodiesterase activity demonstrated in this system cleaves uridine diphosphate-N-acetylmuramyl-L-alanyl-D-isoglutamyl-L-lysyl-D-alanyl-D-alanine to yield N-acetylmuramyl-L-alanyl-D-isoglutamyl-L-lysyl-D-alanyl-D-alanine plus uridine 5'-monophosphate plus inorganic phosphate. This phosphodiesterase activity, not detected under normal transglycosidase assay conditions, is a recycling mechanism and acts indirectly through formation and subsequent cleavage of a lipid-linked intermediate.

Amidase activity involved in peptidoglycan biosynthesis in membranes of Micrococcus luteus (sodonensis)

Membrane suspensions prepared from Micrococcus luteus (sodonensis) in both the exponential and stationary phases of growth contained a transglycosidase activity capable of synthesizing linear peptidoglycan. Exponential-phase membranes also contained an N-acetylmuramyl-L-alanine amidase activity which degraded the peptidoglycan as it was formed. The product of this amidase was purified and found to be free pentapeptide. The amidase was specific for peptidoglycan and could not attack lower-molecular-weight substrates even though the susceptible bond was present. Crude cell wall preparations isolated from exponential-phase cells also contained high levels of amidase. This cell wall-bound amidase would preferentially degrade in vitro-synthesized peptidoglycan over its own cell wall. Amidase activity could be solubilized from both cell walls and membranes by Triton X-100 treatment, butanol extraction, or LiCl extraction. Both membrane- and cell wall-derived amidases, solubilized by LiCl extraction, appeared to be of high molecular weight (greater than 150,000). Once solubilized, these wall- and membrane-derived amidases could attack the cross-bridged peptidoglycan of purified native cell walls, whereas bound amidases could not.

Peptidoglycans synthesized by a membrane preparation of Micrococcus luteus

By incubation of cell-free particulate preparations from Micrococcus luteus with nucleotidic precursors uridine 5'-diphosphate-N-acetylglucosamine and uridine 5'-diphosphate-N-acetylmuramic acid-L-Ala-D-iso-Glu-L-Lys-D-Ala-D-Ala, several types of peptidoglycans were obtained: soluble peptidoglycan, insoluble peptidoglycan bound to the membrane and solubilized by trypsin, and peptidoglycan, which remained insoluble after the action of trypsin. The structure of each type of peptidoglycan was studied by action of lytic enzymes and separation of the fragments on Sephadex. Soluble peptidoglycans consist of a mixture of un-cross-linked polymers of various molecular weights. Trypsin-solubilized peptidoglycans are also a mixture of polymers of various sizes. They contain a preponderance of un-cross-linked material and some bridges with dimer peptides. Insoluble peptidoglycans, after the action of trypsin, contain about 50% of un-cross-linked peptide residues; in the other moiety, peptide units are cross-linked by D-Ala leads to L-Lys and D-Ala leads to L-Ala bonds which characterize the natural peptidoglycan. Therefore, the cell-free particulate preparation possesses the whole enzymatic system necessary for synthesis of cross-linked peptidoglycan.

Membrane-bound DD-carboxypeptidases from Bacillus megaterium KM general properties, substrate specificity and sensitivity to penicillins, cephalosporins and peptide inhibitors of the activity at pH5

1. The membrane from Bacillus megaterium KM contained a dd-carboxypeptidase with optimum activity under the following conditions: pH5.2, bivalent cation, 3mm; ionic strength, 40mm; temperature, 35 degrees C. It was inactivated by treatment with p-chloromercuribenzoate but was fairly insensitive to 2-mercaptoethanol. 2. The enzyme was inhibited by penicillins and cephalosporins. The inhibition of this enzyme was partially reversed on dialysis but 0.2m-2-mercaptoethanol could neither prevent nor reverse the inhibition. 3. The enzyme was extremely sensitive to changes in the configuration and size of the side chain of the C-terminal dipeptide of the substrate. An aliphatic side chain of a well-defined length and polarity was required in the residue that precedes the C-terminal dipeptide. 4. The enzyme was inhibited by a wide range of analogues of the peptidic portion of the natural substrate.

Reversal by a specific peptide (diacetyl-alpha gamma-L-diaminobutyryl-D-alanyl-D-alanine) of vancomycin inhibition in intact bacteria and cell-free preparations

Vancomycin inhibited the growth of Bacillus megaterium, Staphylococcus aureus and Micrococcus lysodeikticus, and in cell-free preparations from B. megaterium it inhibited the formation of mucopeptide and enhanced the accumulation of the lipid intermediate in the biosynthetic pathway. All these inhibitory processes were reversed by the presence of a synthetic peptide analogous to un-cross-linked mucopeptide side chains, namely diacetyl-l-diaminobutyryl-d-alanyl-d-alanine. A considerable amount of vancomycin was found in recovering cells, whether recovery was caused by peptide or took place naturally because a low initial concentration of antibiotic was used. In cell-free preparations pretreated with vancomycin, continued inhibition of mucopeptide synthesis depended on the presence of cell-wall material. This inhibition was also reversible by added peptide.

In vivo and in vitro action of new antibiotics interfering with the utilization of N-acetyl-glucosamine-N-acetyl-muramyl-pentapeptide

Recent literature on the antibiotics enduracidin, moenomycin, prasinomycin, and 11.837 RP suggested an interaction with murein synthesis. Incubation of sensitive strains from Bacillus cereus and Staphylococcus aureus in a "wall medium" containing labeled l-alanine showed that all four antibiotics inhibited the incorporation of alanine into murein and gave rise to accumulation of radioactive uridine diphosphate-N-acetyl-muramyl (UDP-MurNAc)-pentapeptide. Peptidoglycan was synthesized when the particulate enzyme of B. stearothermophilus was incubated with the murein precursors UDP-N-acetyl-glucosamine (UDP-GlcNAc) and UDP-MurNAc-pentapeptide. The newly formed polymer was less accessible for lysozyme and more strongly bound to the acceptor than the same product from the Escherichia coli particulate enzyme. After incubation in the presence of penicillin, a greater part of the peptidoglycan was lysozyme sensitive and more loosely bound to the acceptor. The antibiotics enduracidin, moenomycin, prasinomycin, and 11.837 RP inhibited peptidoglycan synthesis by the B. stearothermophilus particulate enzyme. The rate of synthesis of GlcNAc-MurNAc(-pentapeptide)-P-P-phospholipid was independent from the addition of these antibiotics, but its utilization was strongly inhibited. With the present results, it is not possible to distinguish the mechanisms of action of enduracidin, moenomycin, prasinomycin, and 11.837 RP from the mechanisms of action of vancomycin and ristocetin.