The unusual redox properties of C-type oxidases

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.

Role of the tightly bound quinone for the oxygen reaction of cytochrome bo3 oxidase from Escherichia coli

“…In the presence of O 2 , a sigmoidal curve with an onset at +0.05 V and half‐wave potential value of −0.19 V is observed. This behavior has been observed with other heme‐copper oxygen reductases, including cytochrome aa 3 from Paracoccus denitrificans , cytochrome ba 3 oxidase from Thermus thermophilus and cytochrome cbb 3 oxidases from Pseudomonas stutzeri , Rhodobacter sphaeroides and Vibrio cholerae and it is typical for the electrocatalytic reduction of O 2 . The limiting current at −0.4 V increases as a function of the electrode rotation speed, and follows a Koutecky–Levich relationship (see Fig.…”

The electron distribution in the “activated” state of cytochrome c oxidase

“…We refer to “apparent” redox potentials because the analysis reflects only the distribution of a single electron after 500 ms in the five-electron system (among five sites). Because interactions between the redox sites are not considered (see 35 , 36 ), the values differ from those obtained in redox titrations, which typically yield a higher midpoint potential for the catalytic site heme (here heme a 3 ) than for the intermediate electron acceptor (here heme a ) 35 , 36 , although in many bacterial oxidases the difference between the midpoint potentials is reversed 37 . Furthermore, because a single electron equilibrates among the five redox sites, the situation differs from a redox titration 38 .…”

Heme redox potentials hold the key to reactivity differences between nitric oxide reductase and heme-copper oxidase

“…These results also explain the selective NO reduction cross-reactivity of HCOs: HCOs with low heme E°′ values such as caa 3 oxidase (E°′ = 180 mV), ba 3 oxidase (E°′ = 199 mV) (39) from Thermus thermophilus, bo 3 oxidase from E. coli (E°′ = 160-200 mV) (40), and cbb 3 oxidase (E°′ = −59 mV) from Pseudomonas stutzeri perform NOR reactivity (41). Specifically, the cbb 3 oxidases are C-type HCOs that exhibit unusually low heme redox potential (approximately −50 mV) due to Hbond donation by their proximal His to nearby carboxylate like the Asp-His-Fe triad of peroxidases (42). These C-type HCOs exhibit the highest NO reduction cross-reactivity, consistent with the conclusions of this study that low values of heme E°′ are preferred for NO reduction reactivity.…”