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.

The bioinorganic chemistry of iron in oxygenases and supramolecular assemblies

“…Fig. 4 also depicts the dynamic interactions of marinobactin E and its iron complex with phospholipid vesicles that have recently been elucidated (76). Rates of iron chelation could be measured by stopped-flow spectroscopy by observing the UV absorption of the iron complex.…”

“…It is tempting to suggest a functional relationship wherein the membrane association is affected by iron binding to facilitate the receptor-mediated uptake process (77). The strong membrane affinity of marinobactin E would facilitate its retention by the bacterial cell or colony and protect against diffusive loss whereas the weaker binding of the iron complex would facilitate diffusive sharing of the iron resources among the cells in the colony (76).…”

Dipeptide metabolites from the marine derived bacterium Streptomyces acrymicini

“…Additionally, both CH 2 -5 at δ 3.53 and NH-4 proton at δ 8.10 showed long-range couplings to the carbonyl group at δ 169.9. Therefore, in order to account for the NMR data obtained in DMSO-d 6 , the structure should have a lactam group including the ornithine δ-amino group and the glycine acid group, corresponding to 8-amino- [1,4]diazonane-2,5-dione (1). We have been unable to establish the stereochemistry of the ornithine residue in 1 due to the small amount of compound isolated.…”

“…A related macrocyclic dipeptide has been recently reported as part of the macrobactins, a group of amphiphillic siderophores recently isolated from Marinobacter sp. 5,6 The second dipeptide isolated from S. acrimycini was the linear leucyl-4-hydroxyproline (3), isolated in a mixture with N-acetyltyramine (4). Due to the small quantity of the mixture (~1 mg), we have not attempted to separate both compounds in order to avoid any loss of material.…”

“…Due to the small quantity of the mixture (~1 mg), we have not attempted to separate both compounds in order to avoid any loss of material. Since we have been also able to isolate a pure sample of 4, 7 we could analyse the MS and NMR data of 3 and 4, in order to assign the H COSY spectrum, which showed sequencial couplings from H-5 (δ 4.38) to H-4a (δ 1.93) and H-4b (δ 2.02), from these two hydrogens to H-3 (δ 4.27), which was in turn coupled with both H-2a (δ 3.21, overlapped by the H 2 O signal in DMSO-d 6 ) and H-2b (δ 3.47). Since the spectra were obtained in DMSO-d 6 , we have been able to observe a vicinal coupling between H-3 and the hydroxyl proton at δ 5.08 (d, J 3 Hz).…”