Uncompetitive substrate inhibition and noncompetitive inhibition by 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole (UHDBT) and 2-n-nonyl-4-hydroxyquinoline-N-oxide (NQNO) is observed for the cytochrome bo3 complex: implications for a Q(H2)-loop proton translocation mechanism

Synthesis and Biological Screening of New Lawson Derivatives as Selective Substrate-Based Inhibitors of Cytochrome bo3 Ubiquinol Oxidase from Escherichia coli

The respiratory chain of Escherichia coli contains two different types of terminal oxidase that are differentially regulated as a response to changing environmental conditions. These oxidoreductases catalyze the reduction of molecular oxygen to water and contribute to the proton motive force. The cytochrome bo3 oxidase (cyt bo3 ) acts as the primary terminal oxidase under atmospheric oxygen levels, whereas the bd-type oxidase is most abundant under microaerobic conditions. In E. coli, both types of respiratory terminal oxidase (HCO and bd-type) use ubiquinol-8 as electron donor. Here, we assess the inhibitory potential of newly designed and synthesized 3-alkylated Lawson derivatives through L-proline-catalyzed three-component reductive alkylation (TCRA). The inhibitory effects of these Lawson derivatives on the terminal oxidases of E. coli (cyt bo3 and cyt bd-I) were tested potentiometrically. Four compounds were able to reduce the oxidoreductase activity of cyt bo3 by more than 50 % without affecting the cyt bd-I activity. Moreover, two inhibitors for both cyt bo3 and cyt bd-I oxidase could be identified. Based on molecular-docking simulations, we propose binding modes of the new Lawson inhibitors. The molecular fragment benzyl enhances the inhibitory potential and selectivity for cyt bo3 , whereas heterocycles reduce this effect. This work extends the library of 3-alkylated Lawson derivatives as selective inhibitors for respiratory oxidases and provides molecular probes for detailed investigations of the mechanisms of respiratory-chain enzymes of E. coli.

Location of the Substrate Binding Site of the Cytochrome bo3 Ubiquinol Oxidase from Escherichia coli

Cytochrome bo₃ is a respiratory proton-pumping oxygen reductase that is a member of the heme-copper superfamily that utilizes ubiquinol-8 (Q₈H₂) as a substrate. The current consensus model has Q₈H₂ oxidized at a low affinity site (Q_L), passing electrons to a tightly bound quinone cofactor at a high affinity site (Q_H site) that stabilizes the one-electron reduced ubisemiquinone, facilitating the transfer of electrons to the redox active metal centers where O₂ is reduced to water. The current work shows that the Q₈ bound to the Q_H site is more dynamic than previously thought. In addition, mutations of residues at the Q_H site that do not abolish activity have been re-examined and shown to have properties expected of mutations at the substrate binding site (Q_L): an increase in the K_M of the substrate ubiquinol-1 (up to 4-fold) and an increase in the apparent K_i of the inhibitor HQNO (up to 8-fold). The data suggest that there is only one binding site for ubiquinol in cyt bo₃ and that site corresponds to the Q_H site.

Searching for the low affinity ubiquinone binding site in cytochrome bo3 from Escherichia coli

The cytochrome bo₃ ubiquinol oxidase is one of three respiratory oxygen reductases in the aerobic respiratory chain of Escherichia coli. The generally accepted model of catalysis assumes that cyt bo₃ contains two distinct ubiquinol binding sites: (i) a low affinity (Q_L) site which is the traditional substrate binding site; and (ii) a high affinity (Q_H) site where a "permanently" bound quinone acts as a cofactor, taking two electrons from the substrate quinol and passing them one-by-one to the heme b component of the enzyme which, in turn, transfers them to the heme o₃/Cu_B active site. Whereas the residues at the Q_H site are well defined, the location of the Q_L site remains unknown. The published X-ray structure does not contain quinone, and substantial amounts of the protein are missing as well. A recent bioinformatics study by Bossis et al. [Biochem J. (2014) 461, 305-314] identified a sequence motif G¹⁶³EFX₃GWX₂Y¹⁷³ as the likely Q_L site in the family of related quinol oxidases. In the current work, this was tested by site-directed mutagenesis. The results show that these residues are not important for catalytic function and do not define the Q_L substrate binding site.

A study of cytochrome bo3 in a tethered bilayer lipid membrane

An assay has been developed in which the activity of an ubiquinol oxidase from Escherichia coli, cytochrome bo(3) (cbo(3)), is determined as a function of the hydrophobic substrate ubiquinol-10 (UQ-10) in tethered bilayer lipid membranes (tBLMs). UQ-10 was added in situ, while the enzyme activity and the UQ-10 concentration in the membrane have been determined by cyclic voltammetry. Cbo(3) is inhibited by UQ-10 at concentrations above 5-10 pmol/cm(2), while product inhibition is absent. Cyclic voltammetry has also been used to characterise the effects of three inhibitors; cyanide, inhibiting oxygen reduction; 2-n-Heptyl-4-hydroxyquinoline N-oxide (HQNO), inhibiting the quinone oxidation and Zn(II), thought to block the proton channels required for oxygen reduction and proton pumping activity. The electrochemical behaviour of cbo(3) inhibited with HQNO and Zn(II) is almost identical, suggesting that Zn(II) ions inhibit the enzyme reduction by quinol, rather than oxygen reduction. This suggests that at Zn(II) concentration below 50µM the proton release of cbo(3) is inhibited, but not the proton uptake required to reduce oxygen to water.

The quinone-binding sites of the cytochrome bo3 ubiquinol oxidase from Escherichia coli

Cytochrome bo(3) is the major respiratory oxidase located in the cytoplasmic membrane of Escherichia coli when grown under high oxygen tension. The enzyme catalyzes the 2-electron oxidation of ubiquinol-8 and the 4-electron reduction of dioxygen to water. When solubilized and isolated using dodecylmaltoside, the enzyme contains one equivalent of ubiquinone-8, bound at a high affinity site (Q(H)). The quinone bound at the Q(H) site can form a stable semiquinone, and the amino acid residues which hydrogen bond to the semiquinone have been identified. In the current work, it is shown that the tightly bound ubiquinone-8 at the Q(H) site is not displaced by ubiquinol-1 even during enzyme turnover. Furthermore, the presence of high affinity inhibitors, HQNO and aurachin C1-10, does not displace ubiquinone-8 from the Q(H) site. The data clearly support the existence of a second binding site for ubiquinone, the Q(L) site, which can rapidly exchange with the substrate pool. HQNO is shown to bind to a single site on the enzyme and to prevent formation of the stable ubisemiquinone, though without displacing the bound quinone. Inhibition of the steady state kinetics of the enzyme indicates that aurachin C1-10 may compete for binding with quinol at the Q(L) site while, at the same time, preventing formation of the ubisemiquinone at the Q(H) site. It is suggested that the two quinone binding sites may be adjacent to each other or partially overlap.

Synthesis and antifungal activity of benzo[d]oxazole-4,7-diones

Benzo[d]oxazole-4,7-diones were synthesized and tested for in vitro antifungal activity against fungi. Among them tested, many compounds showed good antifungal activity. The results suggest that benzo[d]oxazole-4,7-diones would be potent antifungal agents.

Electrodes modified with lipid membranes to study quinone oxidoreductases

Quinone oxidoreductases are a class of membrane enzymes that catalyse the oxidation or reduction of membrane-bound quinols/quinones. The conversion of quinone/quinol by these enzymes is difficult to study because of the hydrophobic nature of the enzymes and their substrates. We describe some biochemical properties of quinones and quinone oxidoreductases and then look in more detail at two model membranes that can be used to study quinone oxidoreductases in a native-like membrane environment with their native lipophilic quinone substrates. The results obtained with these model membranes are compared with classical enzyme assays that use water-soluble quinone analogues.

Is the redox state of the ci heme of the cytochrome b6f complex dependent on the occupation and structure of the Qi site and vice versa?

Oxidoreductases of the cytochrome bc(1)/b(6)f family transfer electrons from a liposoluble quinol to a soluble acceptor protein and contribute to the formation of a transmembrane electrochemical potential. The crystal structure of cyt b(6)f has revealed the presence in the Q(i) site of an atypical c-type heme, heme c(i). Surprisingly, the protein does not provide any axial ligand to the iron of this heme, and its surrounding structure suggests it can be accessed by exogenous ligand. In this work we describe a mutagenesis approach aimed at characterizing the c(i) heme and its interaction with the Q(i) site environment. We engineered a mutant of Chlamydomonas reinhardtii in which Phe(40) from subunit IV was substituted by a tyrosine. This results in a dramatic slowing down of the reoxidation of the b hemes under single flash excitation, suggesting hindered accessibility of the heme to its quinone substrate. This modified accessibility likely originates from the ligation of the heme iron by the phenol(ate) side chain introduced by the mutation. Indeed, it also results in a marked downshift of the c(i) heme midpoint potential (from +100 mV to -200 mV at pH 7). Yet the overall turnover rate of the mutant cytochrome b(6)f complex under continuous illumination was found similar to the wild type one, both in vitro and in vivo. We propose that, in the mutant, a change in the ligation state of the heme upon its reduction could act as a redox switch that would control the accessibility of the substrate to the heme and trigger the catalysis.

Identification of the nitrogen donor hydrogen bonded with the semiquinone at the Q(H) site of the cytochrome bo3 from Escherichia coli

The selective (15)N isotope labeling was used for the identification of the nitrogen involved in a hydrogen bond formation with the semiquinone in the high-affinity Q(H) site in the cytochrome bo(3) ubiquinol oxidase. This nitrogen produces dominating contribution to X-Band (14)N ESEEM spectra. The 2D ESEEM (HYSCORE) experiments with the Q(H) site SQ in the series of selectively (15)N labeled bo(3) oxidase proteins have directly identified the N(epsilon) of R71 as an H-bond donor. In addition, selective (15)N labeling has allowed us for the first time to determine weak hyperfine couplings with the side-chain nitrogens from all residues around the SQ. Those are reflecting a distribution of the unpaired spin density over the protein in the SQ state of the quinone processing site.

Characterization of Mutants That Change the Hydrogen Bonding of the Semiquinone Radical at the QH Site of the Cytochrome bo3 from Escherichia coli

The cytochrome bo3 ubiquinol oxidase catalyzes the two-electron oxidation of ubiquinol in the cytoplasmic membrane of Escherichia coli, and reduces O2 to water. This enzyme has a high affinity quinone binding site (QH), and the quinone bound to this site acts as a cofactor, necessary for rapid electron transfer from substrate ubiquinol, which binds at a separate site (QL), to heme b. Previous pulsed EPR studies have shown that a semiquinone at the QH site formed during the catalytic cycle is a neutral species, with two strong hydrogen bonds to Asp-75 and either Arg-71 or Gln-101. In the current work, pulsed EPR studies have been extended to two mutants at the QH site. The D75E mutation has little influence on the catalytic activity, and the pattern of hydrogen bonding is similar to the wild type. In contrast, the D75H mutant is virtually inactive. Pulsed EPR revealed significant structural changes in this mutant. The hydrogen bond to Arg-71 or Gln-101 that is present in both the wild type and D75E mutant oxidases is missing in the D75H mutant. Instead, the D75H has a single, strong hydrogen bond to a histidine, likely His-75. The D75H mutant stabilizes an anionic form of the semiquinone as a result of the altered hydrogen bond network. Either the redistribution of charge density in the semiquinone species, or the altered hydrogen bonding network is responsible for the loss of catalytic function.

The ci/bH moiety in the b6f complex studied by EPR: a pair of strongly interacting hemes

X-band EPR features in the region of 90-150 mT have previously been attributed to heme ci of the b6 complex [Zhang H, Primak A, Bowman MK, Kramer DM, Cramer WA (2004) Biochemistry 43:16329-16336] and interpreted as arising from a high-spin species. However, the complexity of the observed spectrum is rather untypical for high-spin hemes. In this work, we show that addition of the inhibitor 2-n-nonyl-4-hydroxyquinoline N-oxide largely simplifies heme ci's EPR properties. The spectrum in the presence of 2-n-nonyl-4-hydroxyquinoline N-oxide is demonstrated to be caused by a simple S = 5/2, rhombic species split by magnetic dipolar interaction (A(xx )= 7.5 mT) with neighboring heme bH. The large spacing of lines in the uninhibited system, by contrast, cannot be rationalized solely on the basis of magnetic dipolar coupling but is likely to encompass strong contributions from exchange interactions. The role of the H2O/OH- molecule bridging heme ci's Fe atom and heme bH's propionate side chain in mediating these interactions is discussed.

Synthesis and antifungal activity of 1H-indole-4,7-diones

1H-Indole-4,7-diones were synthesized and tested for in vitro antifungal activity against fungi. The synthesized 1H-indole-4,7-diones generally showed good antifungal activity against Candida krusei, Cryptococcus neoformans, and Aspergillus niger. The results suggest that 1H-indole-4,7-diones would be potent antifungal agents.

Characterization of the exchangeable protons in the immediate vicinity of the semiquinone radical at the QH site of the cytochrome bo3 from Escherichia coli

The cytochrome bo3 ubiquinol oxidase from Escherichia coli resides in the bacterial cytoplasmic membrane and catalyzes the two-electron oxidation of ubiquinol-8 and four-electron reduction of O2 to water. The one-electron reduced semiquinone forms transiently during the reaction, and the enzyme has been demonstrated to stabilize the semiquinone. Two-dimensional electron spin echo envelope modulation has been applied to explore the exchangeable protons involved in hydrogen bonding to the semiquinone by substitution of 1H2O by 2H2O. Three exchangeable protons possessing different isotropic and anisotropic hyperfine couplings were identified. The strength of the hyperfine interaction with one proton suggests a significant covalent O-H binding of carbonyl oxygen O1 that is a characteristic of a neutral radical, an assignment that is also supported by the unusually large hyperfine coupling to the methyl protons. The second proton with a large anisotropic coupling also forms a strong hydrogen bond with a carbonyl oxygen. This second hydrogen bond, which has a significant out-of-plane character, is from an NH2 or NH nitrogen, probably from an arginine (Arg-71) known to be in the quinone binding site. Assignment of the third exchangeable proton with smaller anisotropic coupling is more ambiguous, but it is clearly not involved in a direct hydrogen bond with either of the carbonyl oxygens. The results support a model that the semiquinone is bound to the protein in a very asymmetric manner by two strong hydrogen bonds from Asp-75 and Arg-71 to the O1 carbonyl, while the O4 carbonyl is not hydrogen-bonded to the protein.

Regulation of Melanin Synthesis by Selenium-Containing Carbohydrates

This study reports depigmenting potency of selenium-containing carbohydrates, which would be based upon the finding of direct inhibition to mushroom tyrosinase. Two selenoglycosiede, SG-3 (bis(2,3,4-tri-O-acetyl-beta-D-arabinopyranosyl) selenide) and SG-8 (4'-methylbenzoyl 2,3,4,6-tetra-O-acetyl-D-selenomanopyranoside) among eleven selenium-containing compounds examined, were discovered to be effective depigmenting compounds on a mushroom tyrosinase inhibitory assay. SG-3 exhibited a competitive inhibition effect that was similar to kojic acid, well-known tyrosinase inhibitor. At 100 microM and 150 microM, SG-8 had an uncompetitive inhibitory effect that was higher than kojic acid. A study of a melan-a cell originated-tyrosinase inhibition assay showed that SG-8 had a lower inhibitory effect than kojic acid. SG-3 showed a similar inhibition effect to kojic acid on the melan-a cell-originated tyrosinase inhibitory assay. SG-8 showed dose-dependently cytotoxicity in a study of inhibition melanin synthesis by melan-a cells. Most melan-a cells did not survive after being treated with 20 microM of SG-8. At 10 microM, SG-3 inhibited melanin synthesis in the melan-a cells, and the effect was similar to phenylthiourea, which is a well-known inhibitor of melanin synthesis. Therefore, SG-3 is a new candidate for depigmenting reagents.

Spectral and redox characterization of the heme ci of the cytochrome b6f complex

Absorption spectra of the purified cytochrome b(6)f complex from Chlamydomonas reinhardtii were monitored as a function of the redox potential. Four spectral and redox components were identified: in addition to heme f and the two b hemes, the fourth component must be the new heme c(i) (also denoted x) recently discovered in the crystallographic structures. This heme is covalently attached to the protein, but has no amino acid axial ligand. It is located in the plastoquinone-reducing site Q(i) in the immediate vicinity of a b heme. Each heme titrated as a one-electron Nernst curve, with midpoint potentials at pH 7.0 of -130 mV and -35 mV (hemes b), +100 mV (heme c(i)), and +355 mV (heme f). The reduced minus oxidized spectrum of heme c(i) consists of a broad absorption increase centered approximately 425 nm. Its potential has a dependence of -60 mV/pH unit, implying that the reduced form binds one proton in the pH 6-9 range. The Q(i) site inhibitor 2-n-nonyl-4-hydroxyquinoline N-oxide, a semiquinone analogue, induces a shift of this potential by about -225 mV. The spectrum of c(i) matches the absorption changes previously observed in vivo for an unknown redox center denoted "G." The data are discussed with respect to the effect of the membrane potential on the electron transfer equilibrium between G and heme b(H) found in earlier experiments.

Synthesis and antifungal activity of 5-arylamino-4,7-dioxobenzo[b]thiophenes

5-Arylamino-4,7-dioxobenzo[b]thiophenes 3-6 were synthesized and tested for in vitro antifungal activity against Candida and Aspergillus species. 5-Arylamino-6-chloro-2-(methoxycarbonyl)-4,7-dioxobenzo[b]thiophenes 5 showed, in general, more potent antifungal activity against Candida species than the other 4,7-dioxobenzo[b]thiophenes 3, 4 and 6. The results suggest that 5-arylamino-4,7-dioxobenzo[b]thiophenes would be potent antifungal agents.

Synthesis and antifungal activity of noble 5-arylamino- and 6-arylthio-4,7-dioxobenzoselenazoles

5-Arylamino- and 6-arylthio-4,7-dioxobenzoselenazoles 4 and 5 were synthesized and tested for in vitro antifungal activity against Candida and Aspergillus species. 5-Arylamino-4,7-dioxobenzoselenazoles 4 showed, in general, more potent antifungal activity than 6-arylthio-4,7-dioxobenzoselenazoles 5. The results suggest that 5-arylamino-4,7-dioxobenzoselenazoles 4 would be potent antifungal agents.

Synthesis and antifungal activity of 6-arylthio-/6-arylamino-4,7-dioxobenzothiazoles

6-Arylthio-/6-arylamino-4,7-dioxobenzothiazoles were synthesized and tested for in vitro antifungal activity against Candida species and Aspergillus niger. 6-Arylamino-4,7-dioxobenzothiazoles 5 and 6 showed, in general, more potent antifungal activity than 6-arylthio-4,7-dioxobenzothiazoles 3 and 4. The 6-arylamino-substituted compounds 5 and 6 exhibited the greatest activity. In contrast, 6-arylthio-, 2-/5-methyl- or 5-methoxy-moieties of compounds 3-4 did not improve their antifungal activity significantly. The results of this study suggest that 6-arylamino-4,7-dioxobenzothiazoles would be potent antifungal agents.

Synthesis and antifungal activity of 2,5-disubstituted-6-arylamino-4,7-benzimidazolediones

2,5-Disubstituted-6-arylamino-4,7-benzimidazolediones were synthesized and tested for in vitro antifungal activity against pathogenic fungi. Among them, 6-arylamino-5-chloro-2-(2-pyridyl)-4,7-benzimidazolediones exhibited potent antifungal activity against Candida species and Aspergillus niger.

Evidence that a type-2 NADH:quinone oxidoreductase mediates electron transfer to particulate methane monooxygenase in methylococcus capsulatus

NADH readily provides reducing equivalents to membrane-bound methane monooxygenase (pMMO) from Methylococcus capsulatus (Bath) in isolated membrane fractions, but detergent solubilization disrupts this electron-transfer process. Addition of exogenous quinones (especially decyl-plastoquinone and duroquinone) restores the NADH-dependent pMMO activity. Results of inhibitor and substrate dependence of this activity indicate the presence of only a type-2 NADH:quinone oxidoreductase (NDH-2). A 100-fold purification of the NDH-2 was achieved using lauryl-maltoside solubilization followed by ion exchange, hydrophobic-interaction, and gel-filtration chromatography. The purified NDH-2 has a subunit molecular weight of 36 kDa and exists as a monomer in solution. UV-visible and fluorescence spectroscopy identified flavin adenine dinucleotide (FAD) as a cofactor present in stoichiometric amounts. NADH served as the source of electrons, whereas NADPH could not. The purified NDH-2 enzyme reduced coenzyme Q(0), duroquinone, and menaquinone at high rates, whereas the decyl analogs of ubiquinone and plastoquinone were reduced at approximately 100-fold lower rates. Rotenone and flavone did not inhibit the NDH-2, whereas amytal caused partial inhibition but only at high concentrations.

Synthesis and antifungal activities of 5/6-arylamino-4,7-dioxobenzothiazoles

5/6-Arylamino-4,7-dioxobenzothiazoles were synthesized and tested for in vitro antifungal activities against pathogenic fungi. Most of the tested 4,7-dioxobenzothiazoles exhibited potent antifungal activities against Candida species and Aspergillus niger.

Reaction of Escherichia coli cytochrome bo(3) and mitochondrial cytochrome bc(1) with a photoreleasable decylubiquinol

In order to probe the reaction chemistry of respiratory quinol-oxidizing enzymes on a rapid time scale, a photoreleasable quinol substrate was synthesized by coupling decylubiquinol with the water-soluble protecting group 3',5'-bis(carboxymethoxy)benzoin (BCMB) through a carbonate linkage. The resulting compound, DQ-BCMB, was highly soluble in aqueous detergent solution, and showed no reactivity with quinol-oxidizing enzymes prior to photolysis. Upon photolysis in acetonitrile, 5, 7-bis(carboxymethoxy)-2-phenylbenzofuran, carbon dioxide, and decylubiquinol were formed. In aqueous media, free 3', 5'-bis(carboxymethoxy)benzoin was also produced. Photolysis of DQ-BCMB with a 308 nm excimer laser led to the release of the BCMB group in less than 10(-6) s. Decylubiquinol was released in the form of a carbonate monoester, which decarboxylated with an observed first-order rate constant of 195-990 s(-1), depending on the reaction medium. Yields of decylubiquinol as high as 35 microM per laser pulse were attained readily. In the presence of Escherichia coli cytochrome bo(3), photolysis of DQ-BCMB led to the oxidation of quinol by the enzyme with a rate that was limited by the rate of the decylubiquinol release. Mitochondrial cytochrome bc(1) reacted with photoreleased decylubiquinol with distinct kinetic phases corresponding to rapid b heme reduction and somewhat slower c heme reduction. Oxidation of photoreleased ubiquinol by this enzyme showed saturation kinetics with a K(m) of 3.6 microM and a k(cat) of 210 s(-1). The saturation behavior was a result of decylubiquinol being released as a carbonate monoester during the photolysis of DQ-BCMB and interacting with cytochrome bc(1) before decarboxylation of this intermediate yielded free decylubiquinol. The reaction of cytochrome bc(1) and photoreleased decylubiquinol in the presence of antimycin A led to monophasic b heme reduction, but also yielded slower quinol oxidation kinetics. The discrimination of kinetic phases in the reaction of cytochrome bc(1) with ubiquinol substrates has provided a means of exploring the bifurcation of electron transfer that is central to the operation of the Q-cycle in this enzyme.

Redox-linked transient deprotonation at the binuclear site in the aa(3)-type quinol oxidase from Acidianus ambivalens: implications for proton translocation

The hyperthermophilic archaeon Acidianus ambivalens expresses a membrane-bound aa(3)-type quinol oxidase, when grown aerobically, that we have studied by resonance Raman spectroscopy. The purified aa(3) oxidase, which does not contain bound quinol, undergoes a reversible slow conformational change at heme a(3) upon reduction, as indicated by a change in the frequency of its heme formyl stretching mode, from 1,660 cm(-1) to 1,667 cm(-1). In contrast, upon reduction of the integral membrane enzyme or the purified enzyme preincubated with decylubiquinol, this mode appears at 1,667 cm(-1) much more rapidly, suggesting a role of the bound quinol in controlling the redox-linked conformational changes. The shift of the formyl mode to higher frequency is attributed to a loss of hydrogen bonding that is associated with a group having a pKa of approximately 3.8. Based on these observations, a crucial element for proton translocation involving a redox-linked conformational change near the heme a(3) formyl group is postulated.

Thermodynamics of electron transfer in Escherichia coli cytochrome bo3

The proton translocation mechanism of the Escherichia coli cytochrome bo3 complex is intimately tied to the electron transfers within the enzyme. Herein we evaluate two models of proton translocation in this enzyme, a cytochrome c oxidase-type ion-pump and a Q-cycle mechanism, on the basis of the thermodynamics of electron transfer. We conclude that from a thermodynamic standpoint, a Q-cycle is the more favorable mechanism for proton translocation and is likely occurring in the enzyme.

Hydroxylated naphthoquinones as substrates for Escherichia coli anaerobic reductases

We have used two hydroxylated naphthoquinol menaquinol analogues, reduced plumbagin (PBH2, 5-hydroxy-2-methyl-1,4-naphthoquinol) and reduced lapachol [LPCH2, 2-hydroxy-3-(3-methyl-2-butenyl)-1, 4-naphthoquinol], as substrates for Escherichia coli anaerobic reductases. These compounds have optical, solubility and redox properties that make them suitable for use in studies of the enzymology of menaquinol oxidation. Oxidized plumbagin and oxidized lapachol have well resolved absorbances at 419 nm (epsilon=3.95 mM-1. cm-1) and 481 nm (epsilon=2.66 mM-1.cm-1) respectively (in Mops/KOH buffer, pH 7.0). PBH2 is a good substrate for nitrate reductase A (Km=282+/-28 microM, kcat=120+/-6 s-1) and fumarate reductase (Km=155+/-24 microM, kcat=30+/-2 s-1), but not for DMSO reductase. LPCH2 is a good substrate for nitrate reductase A (Km=57+/-35 microM, kcat=68+/-13 s-1), fumarate reductase (Km=85+/-27 microM, kcat=74+/-6 s-1) and DMSO reductase (Km=238+/-30 microM, kcat=191+/-21 s-1). The sensitivity of enzymic LPCH2 and PBH2 oxidation to 2-n-heptyl-4-hydroxyquinoline N-oxide inhibition is consistent with their oxidation occurring at sites of physiological quinol binding.