Siderophores secondary

Another factor that can possibly affect the redox potential in biological systems is the presence of secondary chelating agents that can participate in coupled equilibria (3). When other chelators are present, coupled equilibria involving iron-siderophore redox occur and a secondary ligand will cause the siderophore complex effective redox potential to shift. The decrease in stability of the iron-siderophore complex upon reduction results in a more facile release of the iron. Upon release, the iron(II) is available for complexation by the secondary ligand, which results in a corresponding shift in the redox equilibrium toward production of iron(II). In cases where iron(II) is stabilized by the secondary chelators, there is a shift in the redox potential to more positive values, as shown in Eqs. (42)—(45). 

The nonprotein amino acid /3-A -methylamino-L-alanine (BMAA) is a neurotoxin found in various species of marine cyanobacteria. This nonprotein amino acid occurs both as a free amino acid and bound to proteins.Siderophores are secondary metabolites generally produced by bacteria under iron-deficient conditions. These molecules sequester and transport ferric ion via active transport in bacteria. Some known siderophores are nonribosomal peptides that contain nonprotein amino acids with terminal amines or hydroxylamine side chains, such as exochelin Many of the known nonprotein amino... 

A microbial strain may produce more than one siderophore. There are variations in fatty acid chains of a lipophilic part or in the amino acids making up the backbone, as well as released intermediates of the biosynthetic chain. These variations belong all to the same structural pattern. However, there is also the possibility that so-called secondary siderophores are encountered. They constitute a different structural type, usually less complex in their constitution but also less efficient in binding Fe " than the primary ones. Secondary siderophores will be produced when the demand for iron is not so severe or in case there is a genetic defect impeding the production of the primary ones. Examples will be found throughout the review. 

Corrugatin (34) (Fig. 9) is the siderophore of Pseudomonas corrugata (302). It was also found as secondary siderophore of several pyoverdin producing Pseudomonas strains as P. fluorescens, occasionally in slightly modified forms such as... 

The stmcture of pyochelin (for a detailed bibliography, see (57)), a secondary siderophore of Pseudomonas aeruginosa and of Burkholderia cepacia was established (75) as 2-(2-o-hydroxyphenyl-2-thiazolin -yl)-3-methylthiazolidine-4-carboxylic acid. It consists of a mixture of two easily interconvertible stereoisomers (pyochelin I and II) differing in the configuration of C-2". They can be separated by chromatography, but in methanolic solution (not in DMSO) the equilibrium (ca. 3 1) is restored quickly. For a discussion of the mechanism of isomerization, see (57, 577). 

Pseudomonine (Fig. 27, 76) is produced by Pseudomonas fluorescens strains (7, 228) and by P. entomophila, where it can act as a secondary siderophore (209). The substituents on C-4 and C-5 of the isoxazolinone ring are in trans positions (333). The complex formation has not been studied. In vitro enzyme-catalyzed synthesis studies (333,388) showed that initially the intermediate pre-pseudomonine (Fig. 24, 79) is formed, which non-enzymatically rearranges to pseudomonine. 

The NRPS-based biosynthesis of the myxochelins - catecholate-type siderophores produced by a number of myxobacterial strains - has also been investigated. The biosynthetic pathway, the only known myxobac-terial secondary metabolite gene cluster involved in iron acquisition, has been identified in S. aurantiaca Sg al5, S. cellulosum So ce56, and M. xanthus DK1622 (unpublished data), and six genes are found to be involved... 

Certain secondary metabolites act as metal transport agents. One group is composed of the siderophores (also known as sideramines) which function in uptake, transport, and solubilization of iron. Siderophores are complex molecules which solubilize ferric ion which has a solubility of only 10 18 mo 1/1 at pH 7.4... 

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