Temperature dependence of the formal reduction potential of putidaredoxin

Putidaredoxin (Pdx), a [2Fe-2S] redox protein of size M(r) 11,600, transfers two electrons in two separate steps from the flavin containing putidaredoxin reductase to the heme protein, cytochrome CYP101 in the P450cam catalytic cycle. It has recently come to light, through NMR measurements, that there can be appreciable differences in the Pdx conformational dynamics between its reduced and oxidized states. The redox reaction entropy, deltaS(0')rc = (S(0')Pdx(r)-S(0')Pdx(0)), as determined from measurements of the variation in formal potential with temperature, E0'(T), provides a measure of the strength of this influence on Pdx function. We designed a spectroelectrochemical cell using optically transparent tin oxide electrodes, without fixed or diffusible mediators, to measure E0'(T) over the temperature range 0-40 degrees C. The results indicate that the redox reaction entropy for Pdx is biphasic, decreasing from -213 +/- 27 J mol(-1) K(-1) over 0-27 degrees C, to -582 +/- 150 J mol(-1) K (-1) over 27-40 degrees C. These redox reaction entropy changes are significantly more negative than the changes reported for most cytochromes, although our measurement over the temperature interval 0-27 degrees C is in the range reported for other iron-sulfur proteins. This suggests that Pdx (and other ferredoxins) is a less rigid system than monohemes, and that redox-linked changes in conformation, and/or conformational dynamics, impart to these proteins the ability to interact with a number of redox partners.