Redox Properties of Thermus thermophilus ba3: Different Electron-Proton Coupling in Oxygen Reductases?

Abstract

A comprehensive study of the thermodynamic redox behavior of the hemes of the ba 3 enzyme from Thermus thermophilus, a B-type heme-copper oxygen reductase, is presented. This enzyme, in contrast to those having a single type of heme, allows the B- and A-type hemes to be monitored separately by visible spectroscopy and the reduction potential of each heme to be determined unequivocally. The relative order of the midpoint reduction potentials of each center changed in the pH range from 6 to 8.4, and both hemes present a significant redox-Bohr effect. For instance, at pH 7, the midpoint reduction potentials of the hemes B and A 3 are 213 mV and 285 mV, respectively, whereas at pH 8.4, the order is reversed: 246 mV for heme B and 199 mV for heme A 3. The existence of redox anticooperativity was established by introducing a redox interaction parameter in a model of pairwise interacting redox centers.