Ferric parabactin

Since the spectra of ferric agrobactin and ferric parabactin do show minor differences, a number of experiments were tried in which an attempt was made to mimic these differences via examination of the 1 3 ferric complexes of model salicyl and 2,3-dihy-droxyphenyloxazolines and the 1 1 1 ferric complexes of the model oxazolines with Tait s "Compound II", -bis-(2,3-dihyd oxyben-... 

Figure 5. Circular dichroism spectra of approximately 0.2mM solutions of A, ferric enterobactin B, ferric agrobactin C, blank D, ferric agrobactin A and E, ferric parabactin in O.IM phosphate pH 7.4...

Upon adding either agrobactin or parabactin to neutral solutions of ferric enterobactin there was little change in pH. However, mixing of enterobactin with either ferric agrobactin or ferric parabactin caused the pH to faiJ. to less than 6 and ca. 1 to 2 ymoles of alkali were required to neutralize the solutions. 

Parabactin A, derived from hydrolysis of parabactin (the oxazoline group) exhibits a physiological reducibility. Therefore, oxazoline ring cleavage 148) may be a part of metabolic iron removal in Paracoccus denitrificans. A similar process was initially proposed and broadly accepted for the utilization of enterobactin 144). An esterase was found in the cell extracts of E. coli which cleaved the ester backbone of ferric enterobactin 14s> and desferri-enterobactin 146). In addition, it was demonstrated that... 

Figure 4. Titration of agrobactin and parabactin in the presence of an equimolar quantity of ferric chloride. The siderophores differed significantly only at very high...

From these data we conclude that agrobactin and parabactin form similar coordination compounds with ferric ion in which one of the atoms linked to the iron in the central bidentate portion of the complex is the tertiary N of the oxazoline ring. At neutral pH agrobactin A, in contrast to the tri-catechol enterobactin (l6), appeared to be not fully coordinated to the ferric ion. 

Similar experiments were performed by ligand exchange with the 1 1 ferric complex of nitrilotriacetate. The total proton yields per iron to the neutral pH zone were U.7 and for agrobactin and parabactin, respectively. Thus introduction of the iron (ill), either directly or by ligand exchange, gave complexes which for both agrobactin and parabactin should result in divalent anions at neutral pH. 

C. Net Electrical Charge. The iron complexes of agrobactin, agrobactin A, parabactin and enterobactin were prepared by neutralization of the ligands in the presence of ferric chloride and their electrophoretic mobilities compared with that of ferrichrome... 

D. Electronic Absorption Spectra. The absorption spectra of the complexes formed by exchange from ferric nitrilotriacetate were determined in 0.1 M phosphate, pH 7- t, with a Beckman Model 25 recording spectrophotometer over the range kOO-700 nm. Agrobactin and parabactin gave wine colored iron complexes with a broad adsorption band centered at about 500 nm. At pH 7., the... 

Table II affords the evidence that parabactin and agrobactin, but not their open-form analogues, can counteract growth retardation of P. denitrificans caused by EDDA. The simplest interpretation of these data is that the synthetic chelator, like deferri-ferrichrome A, forms a non-transportable or otherwise unavailable anionic complex with ferric ion which effectively denies iron to the cell in the absence of a strongly competitive ligand which is, in fact, utilized ( ). Table II also records the activity of these compounds with Escherichia coli RW193, an organism defective in the synthesis of enterohactin. Thus it would appear that the A,els complexes are active in coli while those with an enantio-morphic configuration around the iron, A,cis, are utilized by P. denitrificans.

Althou we have characterized the siderophore of P. denitri-ficans as parabactin (Figure 2, R=H) and not parabactin A (Figure 1, R=H), there is some question as to which form was isolated from the same organism by Tait ( 5 ) The relative stability of the oxazoline to acid hydrolysis, the spectral shifts observed by Tait in acidic media and not found in our parabactin A ( 8), and the properties he ascribes to the iron complex can only be reconciled with the structure in Figure 2, R H. Although crude preparations of ferric enterobactin contain a number of colored species, the reddish form is the tris-catecholate bluish tints are associated with oxidized/polymerized or otherwise coordinated forms of iron ( 23). Parabactin A yields a relatively inferior complex with ferric ion which fails to develop a red color even at quite alkaline pH. 

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