REDOX PROPERTIES OF THE BASIC BLUE PROTEIN (PLANTACYANIN) FROM SPINACH

The reduction potential of spinach plantacyanin was determined through direct electrochemistry as a function of temperature and pH. This spe cies shows a higher reduction potential than the homologous cucumber b asic protein (CBP) (E-0 = +345 mV vs. +304 mV for CBP, in 0.1 M NaCl, pH 7, T = 25 degrees C), which turns out to be primarily the result of a more negative reduction enthalpy. Like CBP, spinach plantacyanin un dergoes a positive entropy change upon reduction, at variance with mos t cupredoxins. Both species show a low-pH increase in E-0, indicative of protonation and detachment from the Cu(I) center of a histidine lig and. However, the pK(a) value for the spinach protein is sensibly high er (5.7 vs, about 3.5 for CBP). It is concluded that the copper site d iffers to some extent in the two species, although the main coordinati on features are likely conserved. The differences likely involve solva tion properties, and, possibly, protein sequence in the metal domain. (C) 1998 Elsevier Science Inc. All rights reserved.