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-ME) 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-ME has a strong affinity for lipid membranes with molar fraction partition coefficients Kxapo-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-ME. However, upon the formation of the iron(III) complex Fe-ME, the membrane affinity of the siderophore decreased substantially (KxFe-ME = 1.3 × 104 for SUVs and 9.6 × 103 for LUVs). The kinetics of membrane binding and dissociation by Fe-ME were also determined (konFe-ME = 1.01 M-1 s-1; koffFe-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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry