Redox-Induced Conformational Changes in Plastocyanin:  An Infrared Study

The conformational changes associated with the redox transition of plastocyanin (PC) were investigated by absorption and reaction-induced infrared spectroscopy. In addition to spectral features readily ascribed to β and turn protein secondary structures, the amide I band shows a major component band at 1647 cm-1 in both redox states of the protein. The sensitivity of this component to deuteration and increasing temperature suggests that PC adopts an unusual secondary structure in solution, which differs from those described for other type I copper proteins, such as azurin and halocyanin. The conformations of oxidized and reduced PC are different, as evidenced ( ) by analysis of their amide I band contour and the electrochemically induced oxidized-minus-reduced difference spectrum and ( ) by their different thermal stability. The redox-induced difference spectrum exhibits a number of difference bands within the conformationally sensitive amide I band that could be assigned to peptide CO modes, in light of their small shift upon deuteration, and to signals attributable to side chain vibrational modes of Tyr residues. Lowering the pH to 4.8 induces destabilization of both redox states of the protein, more pronounced for reduced PC, without significantly affecting their secondary structure. Besides the conformational differences obtained at neutral pH, the oxidized-minus-reduced difference spectrum shows two broad and strong negative bands at 1405 and 1571 cm-1, assigned to COO- vibrations, and a broad positive band at 1710 cm-1, attributed to the CO vibration of a COOH group(s). These bands are indicative of a protonation of (an) Asp or Glu side chain(s) upon plastocyanin oxidation at acidic pH.