Redox properties of the basic blue protein (plantacyanin) from spinach

Abstract

The reduction potential of spinach plantacyanin was determined through direct electrochemistry as a function of temperature and pH. This species shows a higher reduction potential than the homologous cucumber basic protein (CBP) ( E 0=+345 mV vs. +304 mV for CBP, in 0.1 M NaCl, pH 7, T=25°C), which turns out to be primarily the result of a more negative reduction enthalpy. Like CBP, spinach plantacyanin undergoes a positive entropy change upon reduction, at variance with most cupredoxins. Both species show a low-pH increase in E 0, indicative of protonation and detachment from the Cu(I) center of a histidine ligand. However, the p K a value for the spinach protein is sensibly higher (5.7 vs. about 3.5 for CBP). It is concluded that the copper site differs to some extent in the two species, although the main coordination features are likely conserved. The differences likely involve solvation properties, and, possibly, protein sequence in the metal domain.