Synthesis, Structure, and Molecular Dynamics of Gallium Complexes of Schizokinen and the Amphiphilic Siderophore Acinetoferrin

ArticleinJournal of the American Chemical Society 126(38):12065-75 · October 2004with12 Reads
DOI: 10.1021/ja048145j · Source: PubMed
A new general synthesis of the citrate-based siderophores acinetoferrin (Af) and schizokinen (Sz) and their analogues is described. The molecular structure of gallium schizokinen, GaSz, was determined by combined (1)H NMR, Hartree-Fock ab initio calculations, DFT, and empirical modeling of vicinal proton NMR spin-spin couplings. The metal-coordination geometry of GaSz was determined from NOE contacts to be cis-cis with respect to the two chelating hydroxamates. One diaminopropane adopts a single chairlike conformation while another is a mixture of two ring pucker arrangements. Both amide hydrogens are internally hydrogen bonded to metal-ligating oxygen atoms. The acyl methyl groups are directed away from each other with an average planar angle of ca. 130 degrees. The kinetics of GaSz racemization were followed by selective, double spin-echo inversion-recovery (1)H NMR spectroscopy over the temperature range of 10-45 degrees C. The racemization proceeds by a multistep mechanism that is proton independent between pD 5 and 12 (k(0) = 1.47 (0.15 s(-1))) and acid catalyzed below pD 4 (k(1) = 2.25 (0.15) x 10(4) M(-1) s(-1)). The activation parameters found for the two sequential steps of the proton independent pathway were DeltaH(++) = 25 +/- 3 kcal M(-1), DeltaS(++) = 25 +/- 7 cal M(-1) K(-1) and DeltaH(++) = 17.1 +/- 0.2 kcal M(-1), DeltaS(++) = 0.3 +/- 2.7 cal M(-1) K(-1). The first step of the proton-independent mechanism was assigned to the dissociation of the carboxyl group. The second step was assigned to complex racemization. The proton-assisted step was assigned to a complete dissociation of the alpha-hydroxy carboxyl group at pD < 4. The ab initio modeling of gallium acinetoferrin, GaAf, and analogues derived from the structure of GaSz has shown that the pendant trans-octenoyl fragments are oriented in opposite directions with the average planar angle of ca. 130 degrees. This arrangement prevents GaAf from adopting a phospholipid-like structural motif. Significantly, iron siderophore complex FeAf was found to be disruptive to phospholipid vesicles and is considerably more hydrophilic than Af, with an eight-fold smaller partition coefficient.
    • "Whereas gallium, in contrast with iron, is not a micronutrient, Ga(III) is very similar to Fe(III) in chemical properties (trivalent, and of similar ionic radius, although not biologically reducible) and has frequently been used as a Fe(III) analogue in iron uptake experiments in Gram-negative bacteria. Like Fe(III), it is complexed by synthetic chelators such as HEDTA, DTPA and HBED (Martell and Smith 2004), and by DFB (Martell and Smith 2004 ) and schizokinen (Fadeev et al. 2004). Ga(III) acts as an inhibitor of Fe(III) transport in several pathogenic Gram-negative bacteria , apparently by competing with Fe(III) for the periplasmic iron-binding protein (Weaver et al. 2008). "
    [Show abstract] [Hide abstract] ABSTRACT: Iron-limited cyanobacterial cells are generally considered to acquire extracellular iron through a siderophore-dependent system, although evidence has started to accumulate that other, as yet poorly characterized, iron acquisition systems may also play a role. Iron-limited cells of the cyanobacterium Anabaena flos-aquae (Lyng.) Brèb. are well known to produce the relatively low Fe(III) affinity dihydroxamate siderophore schizokinen. In this set of experiments we show that iron-limited A. flos-aquae cells (i) acquired iron at substantial rates in the absence of the schizokinen and (ii) acquired iron from a bacterial siderophore (the trihydroxamate molecule desferrioxamine B (DFB)), and also a synthetic chelator (N, N-bis(2-(bis(carboxymethyl)amino)ethyl)glycine (DTPA)), with substantially higher affinities for Fe(III) than schizokinen, indicating that a schizokinen-independent iron acquisition pathway was operating. We suggest that there exists a siderophore-independent iron acquisition system that is able to acquire Fe(III) from high stability Fe(III)-chelates, which are not accessible to iron-limited cells via the schizokinen-based system. As well, we present two possible models for iron acquisition by iron-limited A. flos-aquae cells. Both of these models suggest that there are two major routes for Fe(III) entry into the periplasm of iron-limited cells: (1) the well-characterized siderophore (schizokinen) dependent process and (2) a siderophore-independent process that is able to access Fe(III) sources not biologically available to the schizokinen system.
    Full-text · Article · Mar 2012
    • "We suggest that superoxide, however , is likely unimportant for iron uptake by ironlimited Anabaena cells. Cell-associated, amphiphilic siderophores have been isolated from gram-negative marine bacteria (Martinez et al. 2000, Martin et al. 2006) and bacterial pathogens (Fadeev et al. 2004, Ratledge 2004) and have also been postulated to occur in cyanobacterial outer membranes (Wilhelm and Trick 1994, Wilhelm et al. 1998). Martinez and Butler (2007) demonstrated that some of the siderophoremediated iron uptake by the marine bacterium Marinobacter was mediated by iron binding to siderophores (marinobactins) that partitioned into the bacterial outer membrane. "
    [Show abstract] [Hide abstract] ABSTRACT: Iron acquisition by iron-limited cyanobacteria is typically considered to be mediated mainly by siderophores, iron-chelating molecules released by iron-limited cyanobacteria into the environment. In this set of experiments, iron uptake by iron-limited cells of the cyanobacterium Anabaena flos-aquae (L.) Bory was investigated in cells resuspended in siderophore-free medium. Removal of siderophores decreased iron-uptake rates by ∼60% compared to siderophore-replete conditions; however, substantial rates of iron uptake remained. In the absence of siderophores, Fe(III) uptake was much more rapid from a weaker synthetic chelator [N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA); log Kcond = 28.64 for Fe(III)HEDTA(OH)−] than from a very strong chelator [N,N′-bis(2-hydroxybenzyl)-ethylenediamine-N,N′-diacetic acid (HBED); log Kcond = 31.40 for Fe(III)HBED−], and increasing chelator:Fe(III) ratios decreased the Fe(III)-uptake rate; these results were evident in both short-term (4 h; absence of siderophores) and long-term (116 h; presence of siderophores) experiments. However, free (nonchelated) Fe(III) provided the most rapid iron uptake in siderophore-free conditions. The results of the short-term experiments are consistent with an Fe(III)-binding/uptake mechanism associated with the cyanobacterial outer membrane that operates independently of extracellular siderophores. Iron uptake was inhibited by temperature-shock treatments of the cells and by metabolically compromising the cells with diphenyleneiodonium; this finding indicates that the process is dependent on active metabolism to operate and is not simply a passive Fe(III)-binding mechanism. Overall, these results point to an important, siderophore-independent iron-acquisition mechanism by iron-limited cyanobacterial cells.
    Full-text · Article · Sep 2010
    • "(Persmark et al. 1993). Finally, corrugatin produced by Pseudomonas corrugata (Risse et al. 1998) is one other amphiphilic peptide siderophore that is reported as a single siderophore, as opposed to a suite like the marinobactins, aquachelins, amphibactins and ornibactins: Investigations of the amphiphilicity of the marinobactins , the amphibactins and acinetoferrin to date have centered on the self-assembling characteristics (e.g. the marinobactins (Martinez et al. 2000) and the partitioning of the marinobactins, amphibactins and acinetoferrin into 1,2-dimyri- stoyl-sn-3-glycero-phosphocholine (DMPC) vesicles (Xu et al. 2002; Fadeev et al. 2004; Luo et al. 2005). As expected, the siderophores with longer fatty acids partition more than those with shorter fatty acids and siderophores with saturated fatty acids partition more than those with the cis double bonds, but otherwise the same chain length. "
    [Show abstract] [Hide abstract] ABSTRACT: Iron is essential for the growth of nearly all microorganisms yet iron is only sparingly soluble near the neutral pH, aerobic conditions in which many microorganisms grow. The pH of ocean water is even higher, thereby further lowering the concentration of dissolved ferric ion. To compound the problem of availability, the total iron concentration is surprisingly low in surface ocean water, yet nevertheless, marine microorganisms still require iron for growth. Like terrestrial bacterial, bacteria isolated from open ocean water often produce siderophores, which are low molecular weight chelating ligands that facilitate the microbial acquisition of iron. The present review summarizes the structures of siderophores produced by marine bacteria and the emerging characteristics that distinguish marine siderophores.
    Article · Sep 2005
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