Membrane dynamics of the amphiphilic siderophore, acinetoferrin

Acinetobacter haemolyticus is an antibiotic resistant, pathogenic bacterium responsible for an increasing number of hospital infections. Acinetoferrin (Af), the amphiphilic siderophore isolated from this organism, contains two unusual trans-2-octenoyl hydrocarbon chains reminiscent of a phospholipid structural motif. Here, we have investigated the membrane affinity of Af and its iron complex, Fe-Af, using small and large unilamellar phospholipid vesicles (SUV and LUV) as model membranes. Af shows a high membrane affinity with a partition coefficient, K_x = 6.8 × 10⁵. Membrane partitioning and trans-membrane flip-flop of Fe-Af have also been studied via fluorescence quenching of specifically labeled vesicle leaflets and ¹H NMR line-broadening techniques. Fe-Af is found to rapidly redistribute between lipid and aqueous phases with dissociation/partitioning rates of k_off = 29 s^-1 and k_on = 2.4 × 10⁴ M^-1 s^-1, respectively. Upon binding iron, the membrane affinity of Af is reduced 30-fold to K_x′ = 2.2 × 10⁴ for Fe-Af. In addition, trans-membrane flip-flop of Fe-Af occurs with a rate constant, k_p = 1.2 × 10^-3 s ^-1, with egg-PC LUV and a half-life time around 10 min with DMPC SUV. These properties are due to the phospholipid-like conformation of Af and the more extended conformation of Fe-Af that is enforced by iron binding. Remarkable similarities and differences between Af and another amphiphilic siderophore, marinobactin E, are discussed. The potential biological implications of Af and Fe-Af are also addressed. Our approaches using inner- and outer-leaflet-labeled fluorescent vesicles and ¹H NMR line-broadening techniques to discern Af-mediated membrane partitioning and trans-membrane diffusion are amenable to similar studies for other paramagnetic amphiphiles.