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-alpha-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-ME. 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 x 10(5) for SUVs and 3.6 x 10(5) for LUVs. This membrane association is shown to cause only a 2-fold decrease in the rate of iron(Ill) binding by apo-ME. However, upon the formation of the iron(III) complex Fe-ME, the membrane affinity of the siderophore decreased substantially (K-x(Fe-ME) = 1.3 x 10(4) for SUVs and 9.6 x 10(3) for LUVs). The kinetics of membrane binding and dissociation by Fe-ME were also determined (kon(Fe-ME) = 1.01 M-1 s(-1); k(off)(Fe-ME) = 4.4 x 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 x 10(3) 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.