Membrane affinity of the amphiphilic marinobactin siderophores

Marinobactins are a class of newly discovered marine bacterial siderophores with a unique amphiphilic structure, suggesting that their functions relate to interactions with cell membranes. Here we use small and large unilamellar L-α-dimyristoylphosphatidylcholine vesicles (SUVs and LUVs) as model membranes to examine the thermodynamics and kinetics of the membrane binding of marinobactins, particularly marinobactin E (apo-M_E) and its iron(III) complex, Fe-M_E. Siderophore-membrane interactions are characterized by NMR line broadening, stopped-flow spectrophotometry, fluorescence quenching, and ultracentrifugation. It is determined that apO-M_E has a strong affinity for lipid membranes with molar fraction partition coefficients K_x^apo-ME = 6.3 × 10⁵ for SUVs and 3.6 × 10⁵ for LUVs. This membrane association is shown to cause only a 2-fold decrease in the rate of iron(III) binding by apo-M_E. However, upon the formation of the iron(III) complex Fe-M_E, the membrane affinity of the siderophore decreased substantially (K_x^Fe-ME = 1.3 × 10⁴ for SUVs and 9.6 × 10³ for LUVs). The kinetics of membrane binding and dissociation by Fe-M_E were also determined (k_on^Fe-ME = 1.01 M^-1 s^-1; k_off^Fe-ME = 4.4 × 10^-3 s^-1). The suite of marinobactins with different fatty acid chain lengths and degrees of chain unsaturation showed a range of membrane affinities (5.8 × 10³ to 36 M^-1). The affinity that marinobactins exhibit for membranes and the changes observed upon iron binding could provide unique biological advantages in a receptor-assisted iron acquisition process in which loss of the iron-free siderophore by diffusion is limited by the strong association with the lipid phase.