Binding parameters and microbiological activity of macrolides, lincosamides and streptogramins against Staphylococcus aureus

Lincomycin and clindamycin conformations. A fragment shared by macrolides, ketolides and lincosamides determined from TRNOE ribosome-bound conformations

Two important lincosamide antibiotics, lincomycin and clindamycin were studied in the complex state with the bacterial ribosome after a conformational analysis by 1H and 13C NMR spectroscopy and molecular modelling of the unbound molecules. Lincosamide-ribosome interactions were investigated using two-dimensional transferred nuclear Overhauser effect spectroscopy (TRNOESY), resulting in a bound structure compatible with the experimental NMR data. The results compared with the conformational analysis of the substrates in solution indicate that specific conformations are preferred in the bound state. Clindamycin, the more bioactive antibiotic studied, displayed a stronger NMR response than lincomycin showing that in lincosamide-ribosome interactions, a low affinity binding level is associated to the tight binding one and is related to biological activity. This study shows that conformation plays an essential role for the low affinity binding site. Superimposition of lincosamide, macrolide and ketolide bound structures exhibited conformational similarities in a particular fragment which is in agreement with a hypothesis of partial overlapping lincosamide and macrolide binding sites.

Transferred nuclear Overhauser effect study of macrolide-ribosome interactions: correlation between antibiotic activities and bound conformations

The study of macrolide-ribosome interactions has been investigated using two-dimensional transferred nuclear Overhauser effect spectroscopy (TRNOESY). A new medically important macrolide antibiotic, roxithromycin, with the replacement of the 9-keto group in erythromycin by a 9-oxime chain, was studied in the complex state with the bacterial ribosome. Analysis of transferred nuclear Overhauser effect (TRNOE) experiment resulted in a set of constraints for all protons pairs. These constraints were used in structure determination procedures based on molecular modelling to obtain a bound structure compatible with the experimental NMR data. The results compared with the conformational analysis of the substrate in solution indicate that only one specific conformation is preferred in the bound state while in the free state the sugar ring moities were relatively disordered. The bioactive macrolide antibiotics studied roxithromycin and erythromycin which displayed a strong NMR response, are metabolized in RU39001 and erythralosamine respectively which do not retain antimicrobial activity. The inactive major metabolites were used to define if TRNOEs observation may be characteristic of a biological activity. These control experiments gave essentially blank TRNOESY spectra. This study shows that Mg2+ does not play a direct role for the low affinity binding site studied by TRNOE what is in agreement with an hypothesis of two distinct binding levels, with a low affinity binding level necessary for the tight binding one.

Binding studies of macrolides, lincosamides and streptogramins to Streptococcus G group using [3H]-erythromycin

Parameters of [3H]-erythromycin binding to Streptococcus are determined in vivo using both equilibrium and kinetic methods. This binding is saturable, reversible and independent of energetic systems. Whatever the methods used, the binding parameters are identical as 14 nM for the dissociation constant of the complex erythromycin-Streptococcus and a density of binding sites of 11,865 molecules/cell. Other macrolides, streptogramins and lincosamides competitively displaced bound [3H]-erythromycin suggesting that these compounds share common binding sites on the bacteria. In parallel, the MIC values of these antibiotics against Streptococcus are determined by agar dilution method in Mueller-Hinton medium with 5% of horse blood in order to compare the binding and microbiological parameters. A strong correlation (n = 0.863) has been found between the corresponding inhibition constants and MIC values. Such binding studies could be used in conjunction with microbiological assays for primary screening of active analogous or other compounds with interfere with [3H]-erythromycin binding to the bacteria.