The elements of plant micronutrients.

Amino acid R groups in proteins provide a limited repertoire of functional groups for catalyzing biochemical transformations. The use of inorganic elements, particularly the first row transition metals, expands greatly the range of chemistry that can be catalyzed in a cell. Zinc ions are key for enzymatic catalysis of reactions that require an electrophile, while iron, manganese, copper, nickel, and molybdenum are brokers of redox transformations. These elements are therefore essential nutrients for plants. They are referred to as micronutrients because they are less abundant (by 1–4 orders of magnitude) compared to the macronutrients like sulfur and phosphorus. Even among the micronutrients, the amount of individual transition metals in plant tissues varies over several orders of magnitude, with iron being the most abundant (approximately 100 mg/g) and molybdenum the least (Fig. 1). Each metal has unique chemical properties including ligand preferences, coordination geometries, and redox potentials, which are exploited for diverse, yet highly specific, chemistry. In the photosynthetic electron transfer chain, the midpoint potentials of the metal centers span nearly 1.5 V. The import of metals in biology is evident from the association of approximately 30% to 40% of proteins with a metal (Waldron et al., 2009). Transition metal-protein associations are highly specific, at least in vivo, because mismetallation can block activity or yield undesirable chemistry. In vitro, the associations occur according to thermodynamic preferences described by the Irving-Williams series: For divalent ions, copper and zinc ions bind most tightly relative to manganese and iron ions (Waldron and Robinson, 2009). The in vivo specificity is achieved by kinetic control of metal ion assimilation, distribution, and storage and of metalloprotein assembly, or in other words, metal metabolism. For nickel and copper proteins in bacteria, specificity can be achieved by direct protein to protein transfer via metallochaperones coupled with structural reorganization and stabilization of the resulting holoprotein so that the metal is kinetically trapped. For a periplasmic manganese protein in bacteria, one study showed that correct metallation is achieved by restricting holoprotein formation to the cytoplasm where the concentration of another competing metal ion is reduced by sequestration in binding protein (Tottey et al., 2008). Compartmentation of metal ions is, therefore, a key consideration in metalloprotein biogenesis pathways. Nonessential metals like cadmium, mercury, and silver can compete with the essential transition metals for uptake and metalloprotein assembly pathways because protein flexibility can reduce the selectivity of metal-binding sites. A consequence of the redox reactivity and the promiscuous binding of transition metals to thiol, thioether, imidazole, and carboxylate ligands is that inappropriate accumulation of metals is harmful in biology. Therefore, metal metabolism is under homeostatic regulation (plant homeostasis pathways reviewed in Palmer and Guerinot, 2009). Studies of metalloregulation in bacteria, involving nutrient acquisition pathways as well as metal resistance pathways, indicate that regulators for each metal have coevolved in a single organism as a set of regulators with ranked relative affinities for various metals (Waldron and Robinson, 2009). These regulators determine the ranges (between deficiency and excess) of the number of atoms of metal available in a bacterial cell to match the number of metal-binding sites. The problem is more complex in a eukaryotic cell where subcellular compartmentation of metals and metalloproteins is another consideration. In multicellular organisms, there is the question of how regulation at the cellular level is integrated into the context of the whole organism. The subject of metal-protein interactions in plants needs more attention at all levels of study, from molecular to cellular to whole plant. From a practical perspective, this is important for effective cross-species transfer of metalloenzymes and metal-binding domains to ensure that they are populated with the correct metals. Plants are a dietary source of minerals for a large fraction of the human population and zinc and iron deficiency are pressing problems in human nutrition. The understanding of metalloprotein biochemistry and plant mineral nutrition, therefore, has global impact. In the coming years, a greater understanding of human nutrient metabolism will have an impact on the design of micronutrient stores in crop plants. 1 This work was supported by the Department of Energy (grant no. DE–FG02–04ER15529) and the National Institutes of Health (grant no. GM42143). * E-mail [email protected]. www.plantphysiol.org/cgi/doi/10.1104/pp.110.161810