The 1H NMR structure of bovine Pb2+-osteocalcin and implications for lead toxicity

Structural information on the effect of Pb2+ on proteins under physiologically relevant conditions is largely unknown. We have previously shown that low levels of lead increased the amount of osteocalcin bound to hydroxyapatite (BBA 1535:153). This suggested that lead induced a more compact structure in the protein. We have determined the 3D structure of Pb2+-osteocalcin (49 amino acids), a bone protein from a target tissue, using 1H 2D NMR techniques. Lead, at a stoichiometry of only 1:1, induced a similar fold in the protein as that induced by Ca2+ at a stoichiometry of 3:1. The structure consisted of an unstructured N-terminus and an ordered C-terminal consisting of a hydrophobic core (residues 16–49). The genetic algorithm–molecular dynamics simulation predicted the lead ion was coordinated by the Gla 24 and Gla 21 residues. It is proposed that mineral binding occurs via uncoordinated Gla oxygen ions binding to calcium in hydroxyapatite. A comparison of Pb2+- and Ca2+-osteocalcin suggests Pb2+, at a lower stoichiometry, may induce similar conformational changes in proteins and subsequent molecular processes normally controlled by calcium alone. This may contribute to a molecular mechanism of lead toxicity for calcium binding proteins. Lead exposure may alter the amount of mineral bound osteocalcin and contribute to abnormal bone remodeling.