title: Membrane affinity of the amphiphilic marinobactin siderophores.
abstract: 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-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)(-)(M)E = 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(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)(-)(M)E = 1.3 x 10(4) for SUVs and 9.6 x 10(3) for LUVs). The kinetics of membrane binding and dissociation by Fe-M(E) were also determined (k(on)(Fe)(-)(M)E = 1.01 M(-)(1) s(-)(1); k(off)(Fe)(-)(M)E = 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.
authors.affiliation: Department of Chemistry, Princeton University, New Jersey 08544, USA.
authors.forename: Guofeng
authors.initials: G
authors.lastname: Xu
authors.forename: Jennifer S
authors.initials: JS
authors.lastname: Martinez
authors.forename: John T
authors.initials: JT
authors.lastname: Groves
authors.forename: Alison
authors.initials: A
authors.lastname: Butler
mesh_terms: D002462:Cell Membrane / Q000737:chemistry / Q000378:metabolism*; D002851:Chromatography, High Pressure Liquid; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry / Q000378:metabolism; D005290:Ferric Compounds / Q000737:chemistry / Q000378:metabolism; D020632:Halomonas / Q000737:chemistry; D007700:Kinetics; D008567:Membranes, Artificial; D019906:Nuclear Magnetic Resonance, Biomolecular; D017262:Siderophores / Q000737:chemistry / Q000378:metabolism*; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D014461:Ultracentrifugation
chemical_list: D005290:Ferric Compounds; D008567:Membranes, Artificial; D017262:Siderophores; D004134:Dimyristoylphosphatidylcholine