Electron transfer from cytochrome c to cupredoxins

Electron transfer (ET) through and between proteins is a fundamental biological process. The activation energy for an ET reaction depends upon the Gibbs energy change upon ET (DeltaG(0)) and the reorganization energy. Here, we characterized ET from Pseudomonas aeruginosa cytochrome c(551) (PA) and its designed mutants to cupredoxins, Silene pratensis plastocyanin (PC) and Acidithiobacillus ferrooxidans rusticyanin (RC), through measurement of pseudo-first-order ET rate constants (k(obs)). The influence of the DeltaG (0) value for ET from PA to PC or RC on the k(obs) value was examined using a series of designed PA proteins exhibiting a variety of E (m) values, which afford the DeltaG (0) variation range of 58-399 meV. The plots of the k(obs) values obtained against the DeltaG(0) values for both PA-PC and PA-RC redox pairs could be fitted well with a single Marcus equation. We have shown that the ET activity of cytochrome c can be controlled by tuning the E(m) value of the protein through the substitution of amino acid residues located in hydrophobic-core regions relatively far from the redox center. These findings provide novel insights into the molecular design of cytochrome c, which could be utilized for controlling its ET activity by means of protein engineering.