Cytochrome aa3 from Paracoccus denitrificans and cytochrome ba3 from Thermus thermophilus, two distinct members of the heme–copper oxidase superfamily, were immobilized on electrodes modified with gold nanoparticles. This procedure allowed us to achieve direct electron transfer between the enzyme and the gold nanoparticles and to obtain evidence for different electrocatalytic properties of the two enzymes. The pH dependence and thermostability reveal that the enzymes are highly adapted to their native environments. These results suggest that evolution resulted in different solutions to the common problem of electron transfer to oxygen.

No abstract

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.

New membrane-protein based electrodes were prepared incorporating cytochrome bo3 from E. coli and gold nanoparticles. Direct electron transfer between the electrode and the immobilized enzymes was achieved, resulting in an electrocatalytic activity in presence of O2. The size of the gold nanoparticles was shown to be important and smaller particles were shown to reduce the overpotential of the process.