Surface seawater iron levels of 0.1-1 nM are one to four orders of magnitude lower than the estimated bacterial growth requirement of 0.05-1 µM Fe3+. Therefore, marine bacteria use specialized iron(Ill) transport mechanisms in order to obtain the iron(lll) they require. One iron(lll) uptake mechanism is the production of siderophores: low-molecular weight, strongly iron(lll) chelating organic molecules secreted by many microorganisms to solubilize and transport iron. To date, only a few marine siderophore structures have been determined. The focus of this project is the determination of the structures and metal-binding properties of marine bacterial siderophores.
The structures of four closely related tri-hydroxamate siderophores, the aquachelins, from the marine bacterium Deleya aquamarina have been determined using amino acid analysis and peptide sequencing by tandem mass spectrometry. NMR spectroscopy and chiral GC analysis of partially hydrolyzed peptide fragments completed the structure characterization. Aquachelins A, B, C and D consist of a heptapeptide L-threo-B-hydroxy aspartate, D-serine, L-serine, D-glutamine, D-N5-acetyl, N6-hydroxy-ornithine, D-serine, L-N5-acetyl, N6-hydroxy-ornithine amide-bonded through the N-terminus to one of four fatty acids: (Z)-5-dodecenoic acid, dodecanoic acid, (Z)-7-tetradecenoic acid or tetradecanoic acid.
The siderophore from a marine Vibrio species was found to be aerobactin, a citrate-dihydroxamate siderophore that is also produced by several terrestrial species. This is the first instance of the production of the same siderophore by organisms from widely disparate environments and genetic classifications.
The coordination chemistry of two previously discovered marine siderophores, alterobactins A and B, from Alteromonas luteoviolacea was investigated. The stability constant for iron(Ill) coordination to alterobactin A was estimated using a square wave voltammetric method to be 1049-53. This independent electrochemical technique confirms the stability constant previously estimated by spectrophotometric competition with EDTA. Additionally, the stability constant for the complex of Fe(Ill) and alterobactin B was determined at pH 6 and 8.2, in the two pH-dependent coordination modes of the iron(Ill) complex.