Hydroxylated Phytosiderophore Species Possess an Enhanced Chelate Stability and Affinity for Iron(III)

Graminaceous plant species acquire soil iron by the release of phytosiderophores and subsequent uptake of iron(III)-phytosiderophore complexes. As plant species differ in their ability for phytosiderophore hydroxylation prior to release, an electrophoretic method was set up to determine whether hydroxylation affects the net charge of iron(III)-phytosiderophore complexes, and thus chelate stability. At pH 7.0, non-hydroxylated (deoxymugineic acid) and hydroxylated (mugineic acid; epi-hydroxymugineic acid) phytosiderophores form single negatively charged iron(III) complexes, in contrast to iron(III)-nicotianamine. As the degree of phytosiderophore hydroxylation increases, the corresponding iron(III) complex was found to be less readily protonated. Measured pKa values of the amino groups and calculated free iron(III) concentrations in presence of a 10-fold chelator excess were also found to decrease with increasing degree of hydroxylation, confirming that phytosiderophore hydroxylation protects against acid-induced protonation of the iron(III)-phytosiderophore complex. These effects are almost certainly associated with intramolecular hydrogen bonding between the hydroxyl and amino functions. We conclude that introduction of hydroxyl groups into the phytosiderophore skeleton increases iron(III)-chelate stability in acid environments such as those found in the rhizosphere or the root apoplasm and may contribute to an enhanced iron acquisition

Nicotianamine Chelates Both Fe(III) and Fe(II). Implications for Metal Transport in Plants

Nicotianamine (NA) occurs in all plants and chelates metal cations, including Fe(II), but reportedly not Fe(III). However, a comparison of the Fe(II) and Zn(II) affinity constants of NA and various Fe(III)-chelating aminocarboxylates suggested that NA should chelate Fe(III). High-voltage electrophoresis of the FeNA complex formed in the presence of Fe(III) showed that the complex had a net charge of 0, consistent with the hexadentate chelation of Fe(III). Measurement of the affinity constant for Fe(III) yielded a value of 10(20.6), which is greater than that for the association of NA with Fe(II) (10(12.8)). However, capillary electrophoresis showed that in the presence of Fe(II) and Fe(III), NA preferentially chelates Fe(II), indicating that the Fe(II)NA complex is kinetically stable under aerobic conditions. Furthermore, Fe complexes of NA are relatively poor Fenton reagents, as measured by their ability to mediate H(2)O(2)-dependent oxidation of deoxyribose. This suggests that NA will have an important role in scavenging Fe and protecting the cell from oxidative damage. The pH dependence of metal ion chelation by NA and a typical phytosiderophore, 2′-deoxymugineic acid, indicated that although both have the ability to chelate Fe, when both are present, 2′-deoxymugineic acid dominates the chelation process at acidic pH values, whereas NA dominates at alkaline pH values. The consequences for the role of NA in the long-distance transport of metals in the xylem and phloem are discussed