Thermodynamic Redox Behavior of the Heme Centers in A-Type Heme-Copper Oxygen Reductases: Comparison between the Two Subfamilies

Abstract

The study of the thermodynamic redox behavior of the hemes from two members of the A family of heme-copper oxygen reductases, Paracoccus denitrificans aa 3 (A1 subfamily) and Rhodothermus marinus caa 3 (A2 subfamily) enzymes, is presented. At different pH values, midpoint reduction potentials and interaction potentials were obtained in the framework of a pairwise model for two interacting redox centers. In both enzymes, the hemes have different reduction potentials. For the A1-type enzyme, it was shown that heme a has a pH-dependent midpoint reduction potential, whereas that of heme a 3 is pH independent. For the A2-type enzyme the opposite was observed. The midpoint reduction potential of heme c from subunit II of the caa 3 enzyme was determined by fitting the data with a single-electron Nernst curve, and it was shown to be pH dependent. The results presented here for these A-type enzymes are compared with those previously obtained for representative members of the B and C families.