Proton and electrical charge translocation by cytochrome-c oxidase

Effect of Membrane Environment on the Ligand-Binding Properties of the Terminal Oxidase Cytochrome bd-I from Escherichia coli

Cytochrome bd-I is a terminal oxidase of the Escherichia coli respiratory chain. This integral membrane protein contains three redox-active prosthetic groups (hemes b₅₅₈, b₅₉₅, and d) and couples the electron transfer from quinol to molecular oxygen to the generation of proton motive force, as one of its important physiological functions. The study was aimed at examining the effect of the membrane environment on the ligand-binding properties of cytochrome bd-I by absorption spectroscopy. The membrane environment was found to modulate the ligand-binding characteristics of the hemoprotein in both oxidized and reduced states. Absorption changes upon the addition of exogenous ligands, such as cyanide or carbon monoxide (CO), to the detergent-solubilized enzyme were much more significant and heterogeneous than those observed with the membrane-bound enzyme. In the native membranes, both cyanide and CO interacted mainly with heme d. An additional ligand-binding site (heme b₅₅₈) appeared in the isolated enzyme, as was evidenced by more pronounced changes in the absorption in the Soret band. This additional reactivity could also be detected after treatment of E. coli membranes with a detergent. The observed effect did not result from the enzyme denaturation, since reconstitution of the isolated enzyme into azolectin liposomes restored the ligand-binding pattern close to that observed for the intact membranes.

ROS Defense Systems and Terminal Oxidases in Bacteria

Reactive oxygen species (ROS) comprise the superoxide anion (O₂^•−), hydrogen peroxide (H₂O₂), hydroxyl radical (^•OH), and singlet oxygen (¹O₂). ROS can damage a variety of macromolecules, including DNA, RNA, proteins, and lipids, and compromise cell viability. To prevent or reduce ROS-induced oxidative stress, bacteria utilize different ROS defense mechanisms, of which ROS scavenging enzymes, such as superoxide dismutases, catalases, and peroxidases, are the best characterized. Recently, evidence has been accumulating that some of the terminal oxidases in bacterial respiratory chains may also play a protective role against ROS. The present review covers this role of terminal oxidases in light of recent findings.

Terminal Oxidase Cytochrome bd Protects Bacteria Against Hydrogen Sulfide Toxicity

Hydrogen sulfide (H2S) is often called the third gasotransmitter (after nitric oxide and carbon monoxide), or endogenous gaseous signaling molecule. This compound plays important roles in organisms from different taxonomic groups, from bacteria to animals and humans. In mammalian cells, H2S has a cytoprotective effect at nanomolar concentrations, but becomes cytotoxic at higher concentrations. The primary target of H2S is mitochondria. At submicromolar concentrations, H2S inhibits mitochondrial heme-copper cytochrome c oxidase, thereby blocking aerobic respiration and oxidative phosphorylation and eventually leading to cell death. Since the concentration of H2S in the gut is extremely high, the question arises - how can gut bacteria maintain the functioning of their oxygen-dependent respiratory electron transport chains under such conditions? This review provides an answer to this question and discusses the key role of non-canonical bd-type terminal oxidases of the enterobacterium Escherichia coli, a component of the gut microbiota, in maintaining aerobic respiration and growth in the presence of toxic concentrations of H2S in the light of recent experimental data.

Proton pumping by cytochrome oxidase: progress, problems and postulates

The current status of our knowledge about the mechanism of proton pumping by cytochrome oxidase is discussed. Significant progress has resulted from the study of site-directed mutants within the proton-conducting pathways of the bacterial oxidases. There appear to be two channels to facilitate proton translocation within the enzyme and they are important at different parts of the catalytic cycle. The use of hydrogen peroxide as an alternative substrate provides a very useful experimental tool to explore the enzymology of this system, and insights gained from this approach are described. Proton transfer is coupled to and appears to regulate the rate of electron transfer steps during turnover. It is proposed that the initial step in the reaction involves a proton transfer to the active site that is important to convert metal-ligated hydroxide to water, which can more rapidly dissociate from the metals and allow the reaction with dioxygen which, we propose, can bind the one-electron reduced heme-copper center. Coordinated movement of protons and electrons over both short and long distances within the enzyme appear to be important at different parts of the catalytic cycle. During the initial reduction of dioxygen, direct hydrogen transfer to form a tyrosyl radical at the active site seems likely. Subsequent steps can be effectively blocked by mutation of a residue at the surface of the protein, apparently preventing the entry of protons.

Cytochrome c oxidase: catalytic cycle and mechanisms of proton pumping--a discussion

Cytochrome c oxidase catalyzes the reduction of molecular oxygen to water, a process in which four electrons, four protons, and one molecule of oxygen are consumed. The reaction is coupled to the pumping of four additional protons across the membrane. According to the currently accepted concept, the pumping of all four protons occurs after the binding of oxygen to the reduced enzyme and is exclusively coupled to the last two electron transfer steps. A careful analysis of the existing data shows that there is no experimental evidence for this paradigm. It is more likely that only three protons are pumped during the second half of the catalytic cycle of cytochrome c oxidase after the reaction with oxygen. In this article a variant of a recent mechanistic model of proton pumping by electrostatic repulsion is discussed. It is based on the electroneutrality principle in a way that in the catalytic cycle each electron transfer to the membrane-embedded electron acceptors is charge-compensated by uptake of one proton. The mechanism takes into account the findings with mutant cytochrome c oxidases and explains the results of many recent experiments, including the effects of hydrogen peroxide.

ATP-regulation of cytochrome oxidase in yeast mitochondria: role of subunit VIa

The role of the nuclear-encoded subunit VIa in the regulation of cytochrome oxidase by ATP was investigated in isolated yeast mitochondria. As the subunit VIa-null strain possesses a fully active and assembled cytochrome oxidase, multiple ATP-regulating sites were characterized with respect to their location and their kinetic effect: (a) intra-mitochondrial ATP inhibited the complex IV activity of the null strain, whereas the prevailing effect of ATP on the wild-type strain, at low ionic strength, was activation on the cytosolic side of complex IV, mediated by subunit VIa. However, at physiological ionic strength (i.e. approximately 200 mM), activation by ATP was absent but inhibition was not impaired; (b) in ethanol-respiring mitochondria, when the electron flux was modulated using a protonophoric uncoupler, the redox state of aa3 cytochromes varied with respect to activation (wild-type) or inhibition (null-mutant) of the cytochrome oxidase by ATP; (c) consequently, the control coefficient of cytochrome oxidase on respiratory flux, decreased (wild-type) or increased (null-mutant) in the presence of ATP; (d) considering electron transport from cytochrome c to oxygen, the response of cytochrome oxidase to its thermodynamic driving force was increased by ATP for the wild-type but not for the mutant subunit. Taken together, these findings indicate that at physiological concentration, ATP regulates yeast cytochrome oxidase via subunit-mediated interactions on both sides of the inner membrane, thus subtly tuning the thermodynamic and kinetic control of respiration. This study opens up new prospects for understanding the feedback regulation of the respiratory chain by ATP.

The generation of proton electrochemical potential gradient by cytochrome c oxidase

Cytochrome c oxidase, the terminal oxidase of mitochondria and some bacteria, catalyzes the four electron reduction of oxygen, and generates a proton electrochemical potential gradient (Delta microH). The recently determined structures of the bacterial and the bovine enzymes, together with studies of site directed mutants of a bacterial cytochrome c oxidase and a closely related ubiquinol oxidase, have greatly advanced our understanding of the mechanism by which oxygen reduction is coupled to the generation of Delta microH. Two different mechanisms contribute to the generation of Delta microH: protons that are consumed by the reduction of oxygen, are taken exclusively from the mitochondrial matrix ('consumed' protons), while other protons are translocated by the enzyme across the membrane ('pumped' protons). It is suggested that both proton consumption and proton pumping are driven by the electrostatic charging of the enzyme reaction center by the reducing electrons. Proton consumption is suggested to result from the electrostatically driven ejection of hydroxyls into the matrix that is catalyzed by a tyrosine residue in the reaction center. Proton pumping is suggested to result from the electrostatically driven translocation of a glutamate residue near the reaction center, and is assisted by secondary acceptors that release the translocated protons.

Proton pumping by cytochrome c oxidase is coupled to peroxidase half of its catalytic cycle

The four-electron reaction cycle of cytochrome oxidase is comprised of an eu-oxidase phase in which the enzyme receives the first two electrons and reduces oxygen to bound peroxide and a peroxidase phase in which the peroxy state formed in the eu-oxidase half of the cycle is reduced by the 3rd and 4th electrons to the ferryl-oxo state and oxidized form, respectively. Here we show that the ferrocyanide-peroxidase activity of cytochrome c oxidase incorporated in phospholipid vesicles is coupled to proton pumping. The H+/e- ratio for the ferrocyanide-peroxidase partial reaction is twice higher than for the overall ferrocyanide-oxidase activity and is close to 2. These results show that proton pumping by COX is confined to the peroxidase part of the enzyme catalytic cycle (transfer of the 3rd and 4th electron) whereas the eu-oxidase part (transfer of the first two electrons) may not be proton pumping.