Control of the redox potential of Pseudomonas aeruginosa cytochrome c551 through the Fe-Met coordination bond strength and pKa of a buried heme propionic acid side chain

Pseudomonas aeruginosa cytochrome c(551) and a series of its mutants exhibiting various thermostabilities have been studied by paramagnetic H-1 NMR and cyclic voltammetry in an effort to elucidate the molecular mechanisms responsible for control of the redox potentials (E degrees ') of the proteins. The study revealed that the E degrees ' value of the protein is regulated by two molecular mechanisms operating independently of each other. One is based on the Fe-Met coordination bond strength in the protein, which is determined by the amino acid side chain packing in the protein, and the other on the pK(a) of the heme 17-propionic acid side chain, which is affected by the electrostatic environment. The former mechanism alters the magnitude of the E degrees ' value throughout the entire pH range, and the latter regulates the pK values reflected by the pH profile of the E degrees ' value. These findings provide novel insights into functional regulation of the protein, which could be utilized for tuning the E degrees ' value of the protein by means of protein engineering.