H-1 and O-17 relaxometric investigations of the binding of Mn(II) ion to human serum albumin

Mn(II) ion binds to human serum albumin (HSA) through one strong binding site (K-A = 7.9 x 10(3) m(-1)) and several weaker binding sites. Through competition assays it has been shown that the strong binding site lies in a region which has been previously identified as the Cd(II)-Zn(II) site. The binding of Mn(II) to HSA is strongly pH dependent (pK(a) approximate to 6.7), consistent with the involvement of histidine residues in the coordination of the metal ion. The analysis of H-1 T-1 as function of magnetic field strength and of O-17 T-2 as function of temperature allowed us to determine the various parameters involved in the relaxation process of the paramagnetic Mn(II)-HSA adduct. The Mn(II) ion bound to HSA maintains three or four coordinated water molecules whose residence lifetime is ca 20 ns at 298 K. Interestingly, a good fitting of the NMRD profile was possible only by assuming that the protons of the coordinated water molecules experience two motional regimes, i.e. a fast local motion (probably the rotation around their coordination axis) and the slower overall motion of the whole protein.