Thermodynamic Characterization of Triheme Cytochrome PpcA from Geobacter sulfurreducens:  Evidence for a Role Played in e^-/H⁺ Energy Transduction^†,‡

The facultative aerobic bacterium Geobacter sulfurreducens produces a small periplasmic c-type triheme cytochrome with 71 residues (PpcA) under anaerobic growth conditions, which is involved in the iron respiration. The thermodynamic properties of the PpcA redox centers and of a protonatable center were determined using NMR and visible spectroscopy techniques. The redox centers have negative and different reduction potentials (−162, −143, and −133 mV for heme I, III, and IV, respectively, for the fully reduced and protonated protein), which are modulated by redox interactions among the hemes (covering a range from 10 to 36 mV) and by redox−Bohr interactions (up to −62 mV) between the hemes and a protonatable center located in the proximity of heme IV. All the interactions between the four centers are dominated by electrostatic effects. The microscopic reduction potential of heme III is the one most affected by the oxidation of the other hemes, whereas heme IV is the most affected by the protonation state of the molecule. The thermodynamic properties of PpcA showed that pH strongly modulates the redox behavior of the individual heme groups. A preferred electron transfer pathway at physiologic pH is defined, showing that PpcA has the necessary thermodynamic properties to perform e^-/H⁺ energy transduction, contributing to a H⁺ electrochemical potential gradient across the periplasmic membrane that drives ATP synthesis. PpcA is 46% identical in sequence to and shares a high degree of structural similarity with a periplasmic triheme cytochrome c₇ isolated from Desulfuromonas acetoxidans, a bacterium closely related to the Geobacteracea family. However, the results obtained for PpcA are quite different from those published for D. acetoxidans c₇, and the physiological consequences of these differences are discussed.