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.