Structural and Functional Relevance of the Conserved Residue V13 in the Triheme Cytochrome PpcA from Geobacter sulfurreducens

The triheme cytochrome PpcA from Geobacter sulfurreducens is highly abundant under several growth conditions and is important for extracellular electron transfer. PpcA plays a central role in transferring electrons resulting from the cytoplasmic oxidation of carbon compounds to the cell exterior. This cytochrome is designed to couple electron and proton transfer at physiological pH, a process achieved via the selection of dominant microstates during the redox cycle of the protein, which are ultimately regulated by a well-established order of oxidation of the heme groups. The three hemes are covered only by a polypeptide chain of 71 residues and are located in the small hydrophobic core of the protein. In this work, we used NMR and X-ray crystallography to investigate the structural and functional role of a conserved valine residue (V13) located within van der Waals contact of hemes III and IV. The residue was replaced by alanine (V13A), isoleucine (V13I), serine (V13S), and threonine (V13T) to probe the e...