Effect of divalent cations on the structure of the antibiotic daptomycin

Daptomycin, a cyclic anionic lipopeptide antibiotic, whose three-dimensional structure was recently solved using solution state NMR ( Ball et al. 2004; Jung et al. 2004; Rotondi and Gierasch 2005), requires calcium for function. To date, the exact nature of the interaction between divalent cations, such as Ca2+ or Mg2+, has not been fully characterized. It has, however, been suggested that addition of Ca2+ to daptomycin in a 1: 1 molar ratio induces aggregation. Moreover, it has been suggested that certain residues, e. g. Asp3 and Asp7, which are essential for activity (Grunewald et al. 2004; Kopp et al. 2006), may also be important for Ca2+ binding ( Jung et al. 2004). In this work, we have tried: ( 1) to further pinpoint how Ca2+ affects daptomycin structure/ oligomerization using analytical ultracentrifugation; and ( 2) to determine whether a specific calcium binding site exists, based on one-dimensional C-13 NMR spectra and molecular dynamics ( MD) simulations. The centrifugation results indicated that daptomycin formed micelles of between 14 and 16 monomers in the presence of a 1: 1 molar ratio of Ca2+ and daptomycin. The 13 C NMR data indicated that addition of calcium had a significant effect on the Trp1 and Kyn13 residues, indicating that either calcium binds in this region or that these residues may be important for oligomerization. Finally, the molecular dynamics simulation results indicated that the conformational change of daptomycin upon calcium binding might not be as significant as originally proposed. Similar studies on the divalent cation Mg2+ are also presented. The implication of these results for the biological function of daptomycin is discussed.