pH-Dependent Coordination of Pb2+ to Metallothionein2: Structures and Insight into Lead Detoxification

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 Pb7–MT2(I) and Pb7–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 Pb2+ 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 Pb7–MT2(I) the Pb2+ coordination is in the trigonal pyramidal Pb–S3 mode, whereas the Pb7–MT2(II) complex contains mixed trigonal pyramidal Pb–S3, distorted trigonal pyramidal Pb–S2O1, and distorted quadrilateral pyramidal Pb–S3O1 modes. The O-donor ligand in Pb7–MT2(II) was identified as the carboxyl groups of the aspartic acid residues at positions 2 and 56. Our studies also revealed that Pb7–MT2(II) has a greater acid tolerance and coordination stability than Pb7–MT2(I), thereby retaining the Pb2+ coordination at acidic pH. The higher flexibility of Pb7–MT2(II) renders it more accessible to lysosomal proteolysis than Pb7–MT2(I). Similar spectral features were observed in the coordination of Pb2+ by human apo-MT2, suggesting a commonality among mammalian MT2s in the Pb2+ coordination chemistry.
The pH-dependent structures of two different Pb7−MT2 complexes were revealed. The chemical stabilities and structural flexibility of these two complexes in proteolytic processing were investigated to gain insight into the lead detoxification process involving MTs. The similar structural, chemical, and biological properties between rabbit liver Pb7−MT2(II) and human Pb7−MT2(II) suggest a commonality in the Pb2+ coordination chemistry among mammalian MT2s.