1,721,055 research outputs found
Terminal oxidases, flavodiiron proteins and nitric oxide: insights into reaction mechanism and biological roles
Full text linkHow bacteria breathe in hydrogen sulfide-rich environments
No full textHydrogen sulfide (H2S) is now universally recognized as an endogenous signalling molecule
playing a central role in human physiology. This gas, although it controls a number of
physiological processes at low (submicromolar) concentrations, is toxic at high concentrations as
it blocks cell respiration by potently inhibiting cytochrome c oxidase, the terminal enzyme of the
mitochondrial respiratory chain. In a recent study on the model micro-organism Escherichia coli,
it was shown that the bacterial respiratory oxidase cytochrome bd is resistant to H2S inhibition,
thus enabling bacterial O2 respiration and growth in the presence of sulfide. This may be relevant
because many microbes are H2S producers and some of them live in sulfide-rich environments,
such as the human gut and other natural habitats. The potential impact of this finding in different
areas (environment, life evolution and human health) is discussed
The terminal oxidase cytochrome bd-I confers carbon monoxide resistance to Escherichia coli cells
Full text linkCarbon monoxide (CO) plays a multifaceted role in the physiology of organisms, from poison to signaling molecule. Heme proteins, including terminal oxidases, are plausible CO targets. Three quinol oxidases terminate the branched aerobic respiratory chain of Escherichia coli. These are the heme-copper cytochrome bo3 and two copper-lacking bd-type cytochromes, bd-I and bd-II. All three enzymes generate a proton motive force during the four-electron oxygen reduction reaction that is used for ATP production. The bd-type oxidases also contribute to mechanisms of bacterial defense against various types of stresses. Here we report that in E. coli cells, at the enzyme concentrations tested, cytochrome bd-I is much more resistant to inhibition by CO than cytochrome bd-II and cytochrome bo3. The apparent half-maximal inhibitory concentration values, IC50, for inhibition of O2 consumption of the membrane-bound bd-II and bo3 oxidases by CO at-150 & mu;M O2 were estimated to be 187.1 & PLUSMN; 11.1 and 183.3 & PLUSMN; 13.5 & mu;M CO, respectively. Under the same conditions, the maximum inhibition observed with the membrane-bound cytochrome bd-I was 20 & PLUSMN; 10% at-200 & mu;M CO
Bioenergetics and Reactive Nitrogen Species in Bacteria
Full text linkThe production of reactive nitrogen species (RNS) by the innate immune system is part of the host’s defense against invading pathogenic bacteria. In this review, we summarize recent studies on the molecular basis of the effects of nitric oxide and peroxynitrite on microbial respiration and energy conservation. We discuss possible molecular mechanisms underlying RNS resistance in bacteria mediated by unique respiratory oxygen reductases, the mycobacterial bcc-aa(3) supercomplex, and bd-type cytochromes. A complete picture of the impact of RNS on microbial bioenergetics is not yet available. However, this research area is developing very rapidly, and the knowledge gained should help us develop new methods of treating infectious diseases
Cytochrome bd oxidase and bacterial tolerance to oxidative and nitrosative stress
No full textCytochrome bd is a prokaryotic respiratory quinol:O2 oxidoreductase, phylogenetically unrelated to the extensively studied heme-copper oxidases (HCOs). The enzyme contributes to energy conservation by generating a proton motive force, though working with a lower energetic efficiency as compared to HCOs. Relevant to patho-physiology, members of the bd-family were shown to promote virulence in some pathogenic bacteria, which makes these enzymes of interest also as potential drug targets. Beyond its role in cell bioenergetics, cytochrome bd accomplishes several additional physiological functions, being apparently implicated in the response of the bacterial cell to a number of stress conditions. Compelling experimental evidence suggests that the enzyme enhances bacterial tolerance to oxidative and nitrosative stress conditions, owing to its unusually high nitric oxide (NO) dissociation rate and a notable catalase activity; the latter has been recently documented in one of the two bd-type oxidases of Escherichia coli. Current knowledge on cytochrome bd and its reactivity with O2, NO and H2O2 is summarized in this review in the light of the hypothesis that the preferential (over HCOs) expression of cytochrome bd in pathogenic bacteria may represent a strategy to evade the host immune attack based on production of NO and reactive oxygen species (ROS)
Reaction of nitric oxide with the oxidized di-heme and heme-copper oxygen-reducing centers of terminal oxidases: Different reaction pathways and end-products.
No full textInhibition of terminal oxidases by nitric oxide (NO) has been extensively investigated as it plays a role in regulation of cellular respiration and pathophysiology. Cytochrome bd is a tri-heme (b(558), b(595), d) bacterial oxidase containing no copper that couples electron transfer from quinol to O(2) (to produce H(2)O) with generation of a transmembrane protonmotive force. In this work, we investigated by stopped-flow absorption spectroscopy the reaction of NO with Escherichia coli cytochrome bd in the fully oxidized (O) state. We show that under anaerobic conditions, the O state of the enzyme binds NO at heme d with second-order rate constant k(on)=1.5+/-0.2x10(2) M(-1) s(-1), yielding a nitrosyl adduct (d(3+)-NO or d(2+)-NO(+)) with characteristic optical features (an absorption increase at 639nm and a red shift of the Soret band). The reaction mechanism is remarkably different from that of O cytochrome c oxidase in which the heme-copper binuclear center reacts with NO approximately three orders of magnitude faster, forming nitrite. The data allow us to conclude that in the reaction of NO with terminal oxidases in the O state, Cu(B) is indispensable for rapid oxidation of NO into nitrite
Enzymatic detoxification of O2 and NO in the human parasite, Giardia intestinalis: A mini review
No full textGiardia intestinalis is the etiological agent of giardiasis, a common human intestinal disease with 280 million cases per year. Giardiasis is typically treated with the broad-range antibiotic metronidazole; however, the emergence of drug-resistant strains calls for the development of new anti-parasitic drugs. Very little is known regarding the molecular mechanisms adopted by Giardia to cope with the oxidative/nitrosative environmental stress, encountered by the parasite during colonization of the human intestine. Giardia is particularly sensitive to oxidative stress, as it lacks some of the most common ROS-detoxifying enzymes and it is endowed with O-2-labile key metabolic enzymes. Surprisingly, it colonizes a fairly aerobic (up to 50 mu M O-2) tract of the human gut (the upper part of the small intestine). Accordingly, survival of the parasite relies on antioxidant systems, though, as yet, the only two H2O-forming and O-2-consuming enzymes described in Giardia are NADH oxidase and flavodiiron protein (FDP). Nitric oxide (NO) is an antimicrobial agent produced, together with ROS, by the host immune system to fight pathogens. In vitro NO-stress has been reported to have cytostatic, rather than cytotoxic, effects on Giardia. This effect leads to the suggestion that Giardia is endowed with defense mechanisms against NO and, very recently, the NO-detoxifying flavohemoglobin from it has been characterized
Catalytic intermediates of cytochrome bd terminal oxidase at steady-state
No full textThe cytochrome bd ubiquinol oxidase from Escherichia coli couples the exergonic reduction of O2 to 2H2O to proton motive force generation by transmembrane charge separation. The oxidase contains hemes b558 and b595 and heme d, where O2 chemistry occurs through sequential formation of a few catalytic intermediates. The steady-state behavior of the isolated cytochrome bd has been examined by stopped-flow multiwavelength absorption spectroscopy. Under turnover conditions sustained by O2 and dithiothreitol-reduced ubiquinone, we found [1] that the mostly populated catalytic intermediates are the ferryl and oxy-ferrous species, with a minor fraction of the enzyme containing ferric heme d and possibly one electron on heme b558. These new findings clearly differ from those obtained with mammalian cytochrome c oxidase [2], in which the oxygen intermediates were not found to be populated at detectable levels under similar conditions. The results are discussed in the light of previously proposed models of the cytochrome bd catalytic cycle
Cytochrome bd-type oxidases and environmental stressors in microbial physiology
No full textCytochrome bd is a tri-haem copper-free terminal oxidase of many respiratory chains of prokaryotes with unique structural and functional characteristics. As the other membrane-bound terminal oxidases, this enzyme couples the four-electron reduction of oxygen to water with the generation of a proton motive force used for ATP synthesis but the molecular mechanism does not include proton pumping. Beyond its bioenergetic role, cytochrome bd is involved in resistance to several stressors and affords protection against oxidative and nitrosative stress. These features agree with its expression in many bacterial pathogens. The importance for bacterial virulence and the absence of eukaryotic homologues make this enzyme an ideal target for new antimicrobial drugs. This review aims to provide an update on the current knowledge about cytochrome bd in light of recent advances in the structural characterisation of this enzyme, focussing on its reactivity with environmental stressors
- …
