SYMMETRICAL REARRANGEMENT OF THE CATION-BINDING SITES OF PARVALBUMIN UPON CA2+/MG2+ EXCHANGE - A STUDY BY H-1 2D NMR

Two forms of parvalbumin, i.e., the fully Ca-loaded form PaCa2 and the fully Mg-loaded form PaMg2, are investigated by 2D H-1 NMR in solution. A detailed analysis of the resonances, which belong to residues involved in direct coordination of Ca2+ and Mg2+, establishes that the sixth ligand, a highly conserved Glu residue at the relative position 12 in both cation-binding sites CD and EF, undergoes a conformational rearrangement through a 120-degrees rotation of its side chain about the C(alpha)-C(beta) bond with PaMg2 adopting the less energetically favored g- conformation, as inferred from scalar coupling constants and dipole-dipole contacts measured on the COSY and NOESY spectra, respectively. Similarly, chemical shift effects, which selectively involve NH and C(alpha)H resonances (as well as side-chain resonances) in both CD and EF sites, point to a symmetrical behavior of both cation-binding sites upon Ca2+/Mg2+ exchange.