The unusual redox properties of C-type oxidases.

Cytochrome cbb (also known as C-type) oxidases belong to the family of heme-copper terminal oxidases which couple at the end of the respiratory chain the reduction of molecular oxygen into water and the pumping of protons across the membrane. They are expressed most often at low pressure of O and they exhibit a low homology of sequence with the cytochrome aa (A-type) oxidases found in mitochondria. Their binuclear active site comprises a high-spin heme b associated with a Cu center. The protein also contains one low-spin heme b and 3 hemes c. We address here the redox properties of cbb oxidases from three organisms, Rhodobacter sphaeroides, Vibrio cholerae and Pseudomonas stutzeri by means of electrochemical and spectroscopic techniques. We show that the redox potential of the heme b exhibits a relatively low midpoint potential, as in related cytochrome c-dependent nitric oxide reductases. Potential implications for the coupled electron transfer and proton uptake mechanism of C-type oxidases are discussed.