Solution structure of apo CopZ from Bacillus subtilis: Further analysis of the changes associated with the presence of copper

The solution structure of apo CopZ from Bacillus subtilis has been determined with the aim of investigating the changes in the hydrophobic interactions around the M-X-C-X-X-C copper(I) binding motif upon metal binding. The methionine of this motif (Met 11 in CopZ) points toward the solvent in apo CopZ, whereas its sulfur atom is close to the metal ion in the metal-loaded protein, though probably not at binding distance. This change is associated with the weakening of the interaction between Leu 37 and Cys 16, present in the apo form, and the formation of an interaction between Met 11 and Tyr 65. Loops 1, 3, and 5 are affected by metal binding. Comparison with the structure of other homologous proteins confirms that often metal binding affects a hydrophobic patch around the metal site, possibly for optimizing and tuning the hydrophobic interactions with the partners. It is also shown that copper(I) exchanges among apo CopZ molecules in slow exchange on the NMR time scale, whereas it is known that such exchange between partner molecules (i.e., metallochaperones and metal pumps) is fast.