NMR structural studies of the antibiotic lipopeptide daptomycin in DHPC micelles

Daptomycin is a cyclic anionic lipopeptide that exerts its rapid bactericidal effect by perturbing the bacterial cell membrane, a mode of action different from most other currently commercially available antibiotics (except e.g. polymyxin and gramicidin). Recent work has shown that daptomycin requires calcium in the form of Ca2+ to form a micellar structure in solution and to bind to bacterial model membranes. This evidence sheds light on the initial steps in the mechanism of action of this novel antibiotic. To understand how daptomycin goes on to perturb bacterial membranes, its three-dimensional structure has been determined in the presence of 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) micelles. NMR spectra of daptomycin in DHPC were obtained under two conditions, namely in the presence of Ca2+ as used by Jung et al. [D. Jung, A. Rozek, M. Okon, R.E.W. Hancock, Structural transitions as determinants of the action of the calcium-dependent antibiotic daptomycin, Chem. Biol. 11 (2004) 949–57] to solve the calcium-conjugated structure of daptomycin in solution and in a phosphate buffer as used by Rotondi and Gierasch [K.S. Rotondi, L.M. Gierasch, A well-defined amphipathic conformation for the calcium-free cyclic lipopeptide antibiotic, daptomycin, in aqueous solution, Biopolymers 80 (2005) 374–85] to solve the structure of apo-daptomycin. The structures were calculated using molecular dynamics time-averaged refinement. The different sample conditions used to obtain the NMR spectra are discussed in light of fluorescence data, lipid flip-flop and calcein release assays in PC liposomes, in the presence and absence of Ca2+ [D. Jung, A. Rozek, M. Okon, R.E.W. Hancock, Structural transitions as determinants of the action of the calcium–dependent antibiotic daptomycin, Chem. Biol. 11 (2004) 949–57]. The implications of these results for the membrane perturbation mechanism of daptomycin are discussed.