Bacitracins mechanisms

Godin, B., and E. Touitou. 2004. Mechanism of bacitracin permeation enhancement through the skin and cellular membranes from an ethosomal carrier. J Control Release 94 365. 

When the last condensation reaction has occurred, the linear precursor needs to be released from the enzyme. For this important last step, several mechanisms are known simple hydrolysis of the thioester (balhimycin, vancomycin), intramolecular cyclization leading to a lactam (tyrocidine, bacitracin) or a lactone (surfactin), or even reductive thioester cleavage (linear gramicidin). In some cases, the linear precursor is dimerized (gramicidin S) or even trimerized (bacillibactin, enterobactin) before cyclization (Fig. 2). Even though these reactions are critical for the compound s bioactivity, the catalytic domains responsible for the release are not found in all NRPS systems and will therefore be called modifying domains. 

Classical enzyme inhibitors such as bacitracin, bes-tatin, and amastatin have been found to be effective for improving the nasal absorption of various peptide drugs such as LHRH and calcitonin. These inhibitors having peptide like structures appear to exert their inhibitory effects by a competitive mechanism. In addition, camostat mesilate and nafamostat mesilate, which are clinically used as primary ingredients for pancreatic diseases, have been found to improve the nasal absorption of vasopressin, desmopressin, and calcitonin by inhibiting aminopeptidase and trypsin activity. 

Bacitracin contains a cyclic peptide as part of a complex structure of 12 amino acid residues (if one considers the thiazole ring as cysteine derived). Bacitracin-treated bacterial cultures accumulate precursor peptidoglycan chains, as do the penicillins. However, the mechanism is very different. Its prevention of lysine inclusion into the murein structure is probably of lesser significance. Later demonstration show that drug binding to the membrane-bound bactoprene phosphate and the metal ions (Zn2+ are particularly involved) is more likely to be important to the antibacterial mechanism, namely, inhibition of dephosphorylation of the phospholipid carrier. 

Resistance to bacitracin does not emerge rapidly in originally susceptible strains. The thermal and storage stability of bacitracin is enhanced by the presence of an equimolar concentration of Zn " and the antibacterial action is potentiated by an excess of Zn " ". It has been suggested that Zn + is also associated with the mechanism of antibacterial action. No chemical assay methods are yet available for bacitracin. For the determination of potency the cylinder plate method with Micrococcus flavus (ATCC 10 240) as test organism is used. ... 

Bernlohr and Novelli reported that, although the mechanism of antibiotic synthesis differs from protein synthesis, there appears to be a competition between the two processes for the amino acids available in the cell. During active cellular growth with high protein synthesis, practically no bacitracin was produced. In contrast, bacitracin synthesis was high, when the requirements for protein synthesis were low, as at the end of the log phase or when protein synthesis was inhibited. The production of bacitracin, and some other antibiotics, seems to be related in some way to the sporeforming metabolism . In this phase a great part of the cell wall is dissolved, while protein production remains very low. 

The peptide antibiotic bacitracin is synthesized by the protein thiotemplate mechanism present in Bacillus species... 

The underlying mechanism of this inhibition appears to be the sequestration of undecaprenyl pyrophosphate by bacitracin in the presence of a divalent cation and not the binding of the antibiotic to an enzyme (Stone and Strominger, 1971). Drug resistance in this case is related to elevated levels of undecaprenyl derivatives, rather than to a mutant enzyme in the undecaprenyl phosphate cycle. 

Stone, KJ. Strominger, J.L. (1971) Mechanism of Action of Bacitracin. Complexation with Metal Ion and C55-isoprenyl Pyrophsphate , Proceedings of the National Academy of Sciences, Washington, 68, 3223-7... 

As a model system we would like to describe the biosynthesis of bacitracin, a commercially important peptide antibiotic. Its mechanism of formation has been studied in our laboratory as well as by others. A cell free system for bacitracin production by B. Hcheniformis ATCC 10716 was reported by Ishihara et al.(29). The methods used were modified (Ref. 31) and a partially purified enzyme complex which performed de novo synthesis of bacitracin from the L-isomers of the constituent amino acids, ATP and Mg2 was isolated (Ref. 31 - 34). D-glutamic acid and D-phenylala-nine which occur in bacitracin support its synthesis (Ref. 31). This was also the case for D-aspartic acid (unpublished results). 

Thethiotemplate meahanism of formation is well established for gramicidin S, tyrocidine, bacitracin, linear gramicidin and edeine (Ref. 38). Other mechanisms for peptide synthesis have also been described (Ref. 38). 

Figure I Primary structure of bacterial (I) and fungal (II) peptide antibiotics that are synthesized by the noniibosomal thiotemplatc mechanism. (I) (a) gramicidin S, (b) tyrocidine, (c) surfactin. (d) bacitracin (II) (a) HC-toxin, (b) enniatin A. (c) cyclosporin A. The amino acid sequence of the peptides is shown, as are the names of the enzymes that catalyze synthesis. The otder of incorporation is indicated by the anows, beginning with the amino acid shown below the name of the enzyme subunit.

Bacitracin, a polypeptide antibiotic, interferes like vancomycin and ristocetin with the lipid cycle (Fig. S), preventing the nucleotide precursors from being used to form linear uncross-linked peptidoglycan strands. Badtradn s mechanism of action, however, is entirely different It spedfically inhibits the enzymatic dephosphorylation of the pyrophosphate Css polyisoprenoid alcohol which appears as the end product of the cycle reaction. As a result, the P-Css lipid carrier fails to be regenerated and is thus prevented from re-entering the cycle. The molecular basis of this inhibition has not been eluddated. 

Penicillin has no effect on protein synthesis by washed Staph, aureus under the author s experimental conditions until the concentration is raised to a value several orders above the hmiting bactericidal level. It does, however, prevent the increase in rate of protein synthesis that normally follows the addition of purines and pyrimidines to the external medium. Its action is therefore somewhat similar to that of bacitracin except that it does not inactivate the protein-synthesizing mechanism of the cell which is not synthesizing nucleic acid. For the present its action is indicated in Fig. 33 as a blocking of the pre-protein template concerned with nucleic acid synthesis but the high concentrations and partial effects obtained would suggest that, if it does act in this way, it affects a specific part of the template system rather than the whole mechanism. This will be elaborated below. 

Relationship between bacitracin synthesis and basic cellular mechanisms... 

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