pH-Dependent Coordination of Pb(2+) to Metallothionein2: Structures and Insight into Lead Detoxification

[Image: see text] Lead is a toxic heavy metal whose detoxification in organisms is mainly carried out by its coordination with some metalloproteins such as metallothioneins (MTs). Two Pb–MT complexes, named as Pb(7)–MT2(I) and Pb(7)–MT2(II), form under neutral and weakly acidic conditions, respectively. However, the structures of the two complexes, which are crucial for a better understanding of the detoxification mechanism of Pb–MTs, have not been clearly elucidated. In this Work, coordination of Pb(2+) with rabbit liver apo–MT2, as well as with the two individual domains (apo−αMT2 and apo−βMT2) at different pH, were studied by combined spectroscopic (UV–visible, circular dichroism, and NMR) and computational methods. The results showed that in Pb(7)–MT2(I) the Pb(2+) coordination is in the trigonal pyramidal Pb–S(3) mode, whereas the Pb(7)–MT2(II) complex contains mixed trigonal pyramidal Pb–S(3), distorted trigonal pyramidal Pb–S(2)O(1), and distorted quadrilateral pyramidal Pb–S(3)O(1) modes. The O-donor ligand in Pb(7)–MT2(II) was identified as the carboxyl groups of the aspartic acid residues at positions 2 and 56. Our studies also revealed that Pb(7)–MT2(II) has a greater acid tolerance and coordination stability than Pb(7)–MT2(I), thereby retaining the Pb(2+) coordination at acidic pH. The higher flexibility of Pb(7)–MT2(II) renders it more accessible to lysosomal proteolysis than Pb(7)–MT2(I). Similar spectral features were observed in the coordination of Pb(2+) by human apo-MT2, suggesting a commonality among mammalian MT2s in the Pb(2+) coordination chemistry.