Probing a homoleptic PbS3 coordination environment in a designed peptide using 207Pb NMR spectroscopy: implications for understanding the molecular basis of lead toxicity.

Lead is a ubiquitous environmental contaminant; nearly 5% of American children are affected by lead poisoning (a blood lead level (BLL) of 10 mgdL 1 or higher). Even lower BLLs have been shown to cause many subtle health effects in children. Lead, which is found in paint and soil, causes toxicity by several possible mechanisms. Pb interacts with several zinc enzymes or proteins (such as carbonic anhydrase, acetylcholine esterase, Cys2His2 “zinc-finger” proteins, and acid phophatases) [2,3] and calcium ion binding proteins (calmodulin, calbindin, and troponin C). Inhibition of protein function is induced by alternative coordination number and structural preferences. Pb is a chemically interesting toxin in that it can replace calcium and sometimes zinc in “hard” active sites that are oxygen/nitrogen rich; it can also attack softer ligands, such as all-sulfur-containing zinc ion coordination sites. Among the sulfur-rich targets for Pb are glutathione and metallothioneines, which cause perturbations of essential metal ion homeostasis. Aminolevulinic acid dehydratase (ALAD), a zinc-dependent enzyme, is inhibited by a femtomolar concentrations of Pb. ALAD is found in yeast and mammals and is involved in the second step of heme biosynthesis. Pb-poisoned ALAD blocks the synthesis of hemoglobin, causing anemia in mammals. Furthermore, toxic levels of aminolevulinic acid can result. The crystal structure of ALAD contains an unusual Zn(Cys)3H2O site, where Zn 2+ is substituted by Pb in a trigonal pyramidal geometry. The high affinity of Pb to cysteine thiolates is presumably due to the high enthalpy of Pb S bond formation and the preferred PbS3 coordination geometry in thiolate-rich sites of proteins. A number of peptides and small-molecule synthetic models have been used to understand the chemistry of the Pb-poisoned ALAD. UV/Vis and EXAFS studies on the metalloregulatory protein Pb-PbrR691 and Pb model compounds reveal that Pb binds in a PbS3 environment. [13]