Siderophores metal binding

FIGURE 7 Examples of Fe ligands. (A) The bacterial hexadentate siderophore enterobactin. (B) The planar porphyrin phytochelatin. M denotes the metal binding site. Reprinted with permission from Stumm W., and J. J. Morgan. 1996. Aquatic Chemistry Chemical Equilibria and Rates in Natural Waters. Copyright 1996 Wiley, New York. 

As mentioned previously, siderophores must selectively bind iron tightly in order to solubilize the metal ion and prevent hydrolysis, as well as effectively compete with other chelators in the system. The following discussion will address in more detail the effect of siderophore structure on the thermodynamics of iron binding, as well as different methods for measuring and comparing iron-siderophore complex stability. The redox potentials of the ferri-siderophore complexes will also be addressed, as ferri-siderophore reduction may be important in the iron uptake process in biological systems. 

In some cases, exudates enable the plankton to control their environment. For example, diatoms release compounds, called oxyUpins, that induce natural abortions and growth reduction in the zooplankton that are their primary predators. Other exudates complex with trace metals, serving to reduce the bioavailability of toxics, such as copper, and enhance the bioavailability of micronutrients, such as iron. Examples of iron-binding extracellular DOM are the siderophores (Figure 5.11). 

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