Engineering a novel iron-sulfur cluster into the catalytic subunit of Escherichia coli dimethyl-sulfoxide reductase

The Two Sets of DMSO Respiratory Systems of Shewanella piezotolerans WP3 Are Involved in Deep Sea Environmental Adaptation

Dimethyl sulfoxide (DMSO) is an abundant methylated sulfur compound in deep sea ecosystems. However, the mechanism underlying DMSO-induced reduction in benthic microorganisms is unknown. Shewanella piezotolerans WP3, which was isolated from a west Pacific deep sea sediment, can utilize DMSO as the terminal electron acceptor. In this study, two putative dms gene clusters [type I (dmsEFA1B1G1H1) and type II (dmsA2B2G2H2)] were identified in the WP3 genome. Genetic and physiological analyses demonstrated that both dms gene clusters were functional and the transcription of both gene clusters was affected by changes in pressure and temperature. Notably, the type I system is essential for WP3 to thrive under in situ conditions (4°C/20 MPa), whereas the type II system is more important under high pressure or low temperature conditions (20°C/20 MPa, 4°C/0.1 MPa). Additionally, DMSO-dependent growth conferred by the presence of both dms gene clusters was higher than growth conferred by either of the dms gene clusters alone. These data collectively suggest that the possession of two sets of DMSO respiratory systems is an adaptive strategy for WP3 survival in deep sea environments. We propose, for the first time, that deep sea microorganisms might be involved in global DMSO/DMS cycling.

Biosynthesis and Insertion of the Molybdenum Cofactor

The transition element molybdenum (Mo) is of primordial importance for biological systems as it is required by enzymes catalyzing key reactions in global carbon, sulfur, and nitrogen metabolism. In order to gain biological activity, Mo has to be complexed by a special cofactor. With the exception of bacterial nitrogenase, all Mo-dependent enzymes contain a unique pyranopterin-based cofactor coordinating a Mo atom at their catalytic site. Various types of reactions are catalyzed by Mo enzymes in prokaryotes, including oxygen atom transfer, sulfur or proton transfer, hydroxylation, or even nonredox ones. Mo enzymes are widespread in prokaryotes, and many of them were likely present in LUCA. To date, more than 50-mostly bacterial-Mo enzymes are described in nature. In a few eubacteria and in many archaea, Mo is replaced by tungsten bound to the same unique pyranopterin. How Moco is synthesized in bacteria is reviewed as well as the way until its insertion into apo-Mo-enzymes.

S- and N-Oxide Reductases

Escherichia coli is a versatile facultative anaerobe that can respire on a number of terminal electron acceptors, including oxygen, fumarate, nitrate, and S- and N-oxides. Anaerobic respiration using S- and N-oxides is accomplished by enzymatic reduction of these substrates by dimethyl sulfoxide reductase (DmsABC) and trimethylamine N-oxide reductase (TorCA). Both DmsABC and TorCA are membrane-associated redox enzymes that couple the oxidation of menaquinol to the reduction of S- and N-oxides in the periplasm. DmsABC is membrane bound and is composed of a membrane-extrinsic dimer with a 90.4-kDa catalytic subunit (DmsA) and a 23.1-kDa electron transfer subunit (DmsB). These subunits face the periplasm and are held to the membrane by a 30.8-kDa membrane anchor subunit (DmsC). The enzyme provides the scaffold for an electron transfer relay composed of a quinol binding site, five [4Fe-4S] clusters, and a molybdo-bis(molybdopterin guanine dinucleotide) (present nomenclature: Mo-bis-pyranopterin) (Mo-bisMGD) cofactor. TorCA is composed of a soluble periplasmic subunit (TorA, 92.5 kDa) containing a Mo-bis-MGD. TorA is coupled to the quinone pool via a pentaheme c subunit (TorC, 40.4 kDa) in the membrane. Both DmsABC and TorCA require system-specific chaperones (DmsD or TorD) for assembly, cofactor insertion, and/or targeting to the Tat translocon. In this chapter, we discuss the complex regulation of the dmsABC and torCAD operons, the poorly understood paralogues, and what is known about the assembly and translocation to the periplasmic space by the Tat translocon.

The prokaryotic Mo/W-bisPGD enzymes family: a catalytic workhorse in bioenergetic

Over the past two decades, prominent importance of molybdenum-containing enzymes in prokaryotes has been put forward by studies originating from different fields. Proteomic or bioinformatic studies underpinned that the list of molybdenum-containing enzymes is far from being complete with to date, more than fifty different enzymes involved in the biogeochemical nitrogen, carbon and sulfur cycles. In particular, the vast majority of prokaryotic molybdenum-containing enzymes belong to the so-called dimethylsulfoxide reductase family. Despite its extraordinary diversity, this family is characterized by the presence of a Mo/W-bis(pyranopterin guanosine dinucleotide) cofactor at the active site. This review highlights what has been learned about the properties of the catalytic site, the modular variation of the structural organization of these enzymes, and their interplay with the isoprenoid quinones. In the last part, this review provides an integrated view of how these enzymes contribute to the bioenergetics of prokaryotes. This article is part of a Special Issue entitled: Metals in Bioenergetics and Biomimetics Systems.

Iron-sulfur cluster engineering provides insight into the evolution of substrate specificity among sulfonucleotide reductases

Assimilatory sulfate reduction supplies prototrophic organisms with reduced sulfur that is required for the biosynthesis of all sulfur-containing metabolites, including cysteine and methionine. The reduction of sulfate requires its activation via an ATP-dependent activation to form adenosine-5'-phosphosulfate (APS). Depending on the species, APS can be reduced directly to sulfite by APS reductase (APR) or undergo a second phosphorylation to yield 3'-phosphoadenosine-5'-phosphosulfate (PAPS), the substrate for PAPS reductase (PAPR). These essential enzymes have no human homologue, rendering them attractive targets for the development of novel antibacterial drugs. APR and PAPR share sequence and structure homology as well as a common catalytic mechanism, but the enzymes are distinguished by two features, namely, the amino acid sequence of the phosphate-binding loop (P-loop) and an iron-sulfur cofactor in APRs. On the basis of the crystal structures of APR and PAPR, two P-loop residues are proposed to determine substrate specificity; however, this hypothesis has not been tested. In contrast to this prevailing view, we report here that the P-loop motif has a modest effect on substrate discrimination. Instead, by means of metalloprotein engineering, spectroscopic, and kinetic analyses, we demonstrate that the iron-sulfur cluster cofactor enhances APS reduction by nearly 1000-fold, thereby playing a pivotal role in substrate specificity and catalysis. These findings offer new insights into the evolution of this enzyme family and extend the known functions of protein-bound iron-sulfur clusters.

Genome sequence of Desulfitobacterium hafniense DCB-2, a Gram-positive anaerobe capable of dehalogenation and metal reduction

Background

The genome of the Gram-positive, metal-reducing, dehalorespiring Desulfitobacterium hafniense DCB-2 was sequenced in order to gain insights into its metabolic capacities, adaptive physiology, and regulatory machineries, and to compare with that of Desulfitobacterium hafniense Y51, the phylogenetically closest strain among the species with a sequenced genome.

Results

The genome of Desulfitobacterium hafniense DCB-2 is composed of a 5,279,134-bp circular chromosome with 5,042 predicted genes. Genome content and parallel physiological studies support the cell's ability to fix N₂ and CO₂, form spores and biofilms, reduce metals, and use a variety of electron acceptors in respiration, including halogenated organic compounds. The genome contained seven reductive dehalogenase genes and four nitrogenase gene homologs but lacked the Nar respiratory nitrate reductase system. The D. hafniense DCB-2 genome contained genes for 43 RNA polymerase sigma factors including 27 sigma-24 subunits, 59 two-component signal transduction systems, and about 730 transporter proteins. In addition, it contained genes for 53 molybdopterin-binding oxidoreductases, 19 flavoprotein paralogs of the fumarate reductase, and many other FAD/FMN-binding oxidoreductases, proving the cell's versatility in both adaptive and reductive capacities. Together with the ability to form spores, the presence of the CO₂-fixing Wood-Ljungdahl pathway and the genes associated with oxygen tolerance add flexibility to the cell's options for survival under stress.

Conclusions

D. hafniense DCB-2's genome contains genes consistent with its abilities for dehalogenation, metal reduction, N₂ and CO₂ fixation, anaerobic respiration, oxygen tolerance, spore formation, and biofilm formation which make this organism a potential candidate for bioremediation at contaminated sites.

Correct assembly of iron-sulfur cluster FS0 into Escherichia coli dimethyl sulfoxide reductase (DmsABC) is a prerequisite for molybdenum cofactor insertion

The FS0 [4Fe-4S] cluster of the catalytic subunit (DmsA) of Escherichia coli dimethyl sulfoxide reductase (DmsABC) plays a key role in the electron transfer relay. We have now established an additional role for the cluster in directing molybdenum cofactor assembly during enzyme maturation. EPR spectroscopy indicates that FS0 has a high spin ground state (S = 3/2) in its reduced form, resulting in an EPR spectrum with a peak at g ∼ 5.0. The cluster is predicted to be in close proximity to the molybdo-bis(pyranopterin guanine dinucleotide) (Mo-bisPGD) cofactor, which provides the site of dimethyl sulfoxide reduction. Comparison with nitrate reductase A (NarGHI) indicates that a sequence of residues ((18)CTVNC(22)) plays a role in both FS0 and Mo-bisPGD coordination. A DmsA(ΔN21) mutant prevented Mo-bisPGD binding and resulted in a degenerate [3Fe-4S] cluster form of FS0 being assembled. DmsA belongs to the Type II subclass of Mo-bisPGD-containing catalytic subunits that is distinguished from the Type I subclass by having three rather than two residues between the first two Cys residues coordinating FS0 and a conserved Arg residue rather than a Lys residue following the fourth cluster coordinating Cys. We introduced a Type I Cys group into DmsA in two stages. We changed its sequence from (18)C(A)TVNC(B)GSRC(C)P(27) to (18)C(A)TYC(B)GVGC(C)G(26) (similar to that of formate dehydrogenase (FdnG)) and demonstrated that this eliminated both Mo-bisPGD binding and EPR-detectable FS0. We then combined this change with a DmsA(R61K) mutation and demonstrated that this additional change partially rescued Mo-bisPGD insertion.

Protein crystallography reveals a role for the FS0 cluster of Escherichia coli nitrate reductase A (NarGHI) in enzyme maturation

We have used site-directed mutagenesis, EPR spectroscopy, redox potentiometry, and protein crystallography to monitor assembly of the FS0 [4Fe-4S] cluster and molybdo-bis(pyranopterin guanine dinucleotide) cofactor (Mo-bisPGD) of the Escherichia coli nitrate reductase A (NarGHI) catalytic subunit (NarG). Cys and Ser mutants of NarG-His(49) both lack catalytic activity, with only the former assembling FS0 and Mo-bisPGD. Importantly, both prosthetic groups are absent in the NarG-H49S mutant. EPR spectroscopy of the Cys mutant reveals that the E(m) value of the FS0 cluster is decreased by at least 500 mV, preventing its participation in electron transfer to the Mo-bisPGD cofactor. To demonstrate that decreasing the FS0 cluster E(m) results in decreased enzyme activity, we mutated a critical Arg residue (NarG-Arg(94)) in the vicinity of FS0 to a Ser residue. In this case, the E(m) of FS0 is decreased by 115 mV, with a concomitant decrease in enzyme turnover to approximately 30% of the wild type. Analysis of the structure of the NarG-H49S mutant reveals two important aspects of NarGHI maturation: (i) apomolybdo-NarGHI is able to bind GDP moieties at their respective P and Q sites in the absence of the Mo-bisPGD cofactor, and (ii) a critical segment of residues in NarG, (49)HGVNCTG(55), must be correctly positioned to ensure holoenzyme maturation.

Investigation of the redox centres of periplasmic selenate reductase from Thauera selenatis by EPR spectroscopy

Periplasmic SER (selenate reductase) from Thauera selenatis is classified as a member of the Tat (twin-arginine translocase)-translocated (Type II) molybdoenzymes and comprises three subunits each containing redox cofactors. Variable-temperature X-band EPR spectra of the purified SER complex showed features attributable to centres [3Fe-4S]1+, [4Fe-4S]1+, Mo(V) and haem-b. EPR-monitored redox-potentiometric titration of the SerABC complex (SerA-SerB-SerC, a hetero-trimetric complex of alphabetagamma subunits) revealed that the [3Fe-4S] cluster (FS4, iron-sulfur cluster 4) titrated as n=1 Nernstian component with a midpoint redox potential (E(m)) of +118+/-10 mV for the [3Fe-4S]1+/0 couple. A [4Fe-4S]1+ cluster EPR signal developed over a range of potentials between 300 and -200 mV and was best fitted to two sequential Nernstian n=1 curves with midpoint redox potentials of +183+/-10 mV (FS1) and -51+/-10 mV (FS3) for the two [4Fe-4S]1+/2+ cluster couples. Upon further reduction, the observed signal intensity of the [4Fe-4S]1+ cluster decreases. This change in intensity can again be fitted to an n=1 Nernstian component with a midpoint potential (E(m)) of about -356 mV (FS2). It is considered likely that, at low redox potential (E(m) less than -300 mV), the remaining oxidized cluster is reduced (spin S=1/2) and strongly spin-couples to a neighbouring [4Fe-4S]1+ cluster rendering both centres EPR-silent. The involvement of both [3Fe-4S] and [4Fe-4S] clusters in electron transfer to the active site of the periplasmic SER was demonstrated by the re-oxidation of the clusters under anaerobic selenate turnover conditions. Attempts to detect a high-spin [4Fe-4S] cluster (FS0) in SerA at low temperature (5 K) and high power (100 mW) were unsuccessful. The Mo(V) EPR recorded at 60 K, in samples poised at pH 6.0, displays principal g values of g3 approximately 1.999, g2 approximately 1.996 and g1 approximately 1.965 (g(av) 1.9867). The dominant features at g2 and g3 are not split, but hyperfine splitting is observed in the g1 region of the spectrum and can be best simulated as arising from a single proton with a coupling constant of A1 (1H)=1.014 mT. The presence of the haem-b moiety in SerC was demonstrated by the detection of a signal at g approximately 3.33 and is consistent with haem co-ordinated by methionine and lysine axial ligands. The combined evidence from EPR analysis and sequence alignments supports the assignment of the periplasmic SER as a member of the Type II molybdoenzymes and provides the first spectro-potentiometric insight into an enzyme that catalyses a key reductive reaction in the biogeochemical selenium cycle.

Genomic analysis of anaerobic respiration in the archaeon Halobacterium sp. strain NRC-1: dimethyl sulfoxide and trimethylamine N-oxide as terminal electron acceptors

We have investigated anaerobic respiration of the archaeal model organism Halobacterium sp. strain NRC-1 by using phenotypic and genetic analysis, bioinformatics, and transcriptome analysis. NRC-1 was found to grow on either dimethyl sulfoxide (DMSO) or trimethylamine N-oxide (TMAO) as the sole terminal electron acceptor, with a doubling time of 1 day. An operon, dmsREABCD, encoding a putative regulatory protein, DmsR, a molybdopterin oxidoreductase of the DMSO reductase family (DmsEABC), and a molecular chaperone (DmsD) was identified by bioinformatics and confirmed as a transcriptional unit by reverse transcriptase PCR analysis. dmsR, dmsA, and dmsD in-frame deletion mutants were individually constructed. Phenotypic analysis demonstrated that dmsR, dmsA, and dmsD are required for anaerobic respiration on DMSO and TMAO. The requirement for dmsR, whose predicted product contains a DNA-binding domain similar to that of the Bat family of activators (COG3413), indicated that it functions as an activator. A cysteine-rich domain was found in the dmsR gene, which may be involved in oxygen sensing. Microarray analysis using a whole-genome 60-mer oligonucleotide array showed that the dms operon is induced during anaerobic respiration. Comparison of dmsR+ and DeltadmsR strains by use of microarrays showed that the induction of the dmsEABCD operon is dependent on a functional dmsR gene, consistent with its action as a transcriptional activator. Our results clearly establish the genes required for anaerobic respiration using DMSO and TMAO in an archaeon for the first time.

Architecture of NarGH reveals a structural classification of Mo-bisMGD enzymes

The structure of the catalytic and electron-transfer subunits (NarGH) of the integral membrane protein, respiratory nitrate reductase (Nar) has been determined to 2.0 A resolution revealing the molecular architecture of this Mo-bisMGD (molybdopterin-guanine-dinucleotide) containing enzyme which includes a previously undetected FeS cluster. Nar, together with the related enzyme formate dehydrogenase (Fdh-N), is a key enzyme in the generation of proton motive force across the membrane in Escherichia coli nitrate respiration. A comparative study revealed that Nar and Fdh-N employ different approaches for acquiring substrate, reflecting different catalytic mechanisms. Nar uses a very narrow and nonpolar substrate-conducting cavity with a nonspecific substrate binding site, whereas Fdh-N accommodates a wider, positively charged substrate-conducting cavity with a more specific substrate binding site. The Nar structure also demonstrates the first example of an Asp side chain acting as a Mo ligand providing a structural basis for the classification of Mo-bisMGD enzymes.

A gene cluster for chlorate metabolism in Ideonella dechloratans

Chlorate reductase has been isolated from the chlorate-respiring bacterium Ideonella dechloratans, and the genes encoding the enzyme have been sequenced. The enzyme is composed of three different subunits and contains molybdopterin, iron, probably in iron-sulfur clusters, and heme b. The genes (clr) encoding chlorate reductase are arranged as clrABDC, where clrA, clrB, and clrC encode the subunits and clrD encodes a specific chaperone. Judging from the subunit composition, cofactor content, and sequence comparisons, chlorate reductase belongs to class II of the dimethyl sulfoxide reductase family. The clr genes are preceded by a novel insertion sequence (transposase gene surrounded by inverted repeats), denoted ISIde1. Further upstream, we find the previously characterized gene for chlorite dismutase (cld), oriented in the opposite direction. Chlorate metabolism in I. dechloratans starts with the reduction of chlorate, which is followed by the decomposition of the resulting chlorite to chloride and molecular oxygen. The present work reveals that the genes encoding the enzymes catalyzing both these reactions are in close proximity.

Molecular analysis of dimethyl sulphide dehydrogenase from Rhodovulum sulfidophilum: its place in the dimethyl sulphoxide reductase family of microbial molybdopterin-containing enzymes

Dimethyl sulphide dehydrogenase catalyses the oxidation of dimethyl sulphide to dimethyl sulphoxide (DMSO) during photoautotrophic growth of Rhodovulum sulfidophilum. Dimethyl sulphide dehydrogenase was shown to contain bis(molybdopterin guanine dinucleotide)Mo, the form of the pterin molybdenum cofactor unique to enzymes of the DMSO reductase family. Sequence analysis of the ddh gene cluster showed that the ddhA gene encodes a polypeptide with highest sequence similarity to the molybdopterin-containing subunits of selenate reductase, ethylbenzene dehydrogenase. These polypeptides form a distinct clade within the DMSO reductase family. Further sequence analysis of the ddh gene cluster identified three genes, ddhB, ddhD and ddhC. DdhB showed sequence homology to NarH, suggesting that it contains multiple iron-sulphur clusters. Analysis of the N-terminal signal sequence of DdhA suggests that it is secreted via the Tat secretory system in complex with DdhB, whereas DdhC is probably secreted via a Sec-dependent mechanism. Analysis of a ddhA mutant showed that dimethyl sulphide dehydrogenase was essential for photolithotrophic growth of Rv. sulfidophilum on dimethyl sulphide but not for chemo-trophic growth on the same substrate. Mutational analysis showed that cytochrome c2 mediated photosynthetic electron transfer from dimethyl sulphide dehydrogenase to the photochemical reaction centre, although this cytochrome was not essential for photoheterotrophic growth of the bacterium.

Multiple roles for the twin arginine leader sequence of dimethyl sulfoxide reductase of Escherichia coli

Dimethyl sulfoxide (Me(2)SO) reductase of Escherichia coli is a terminal electron transport chain enzyme that is expressed under anaerobic growth conditions and is required for anaerobic growth with Me(2)SO as the terminal electron acceptor. The trimeric enzyme is composed of a membrane extrinsic catalytic dimer (DmsAB) and a membrane intrinsic anchor (DmsC). The amino terminus of DmsA has a leader sequence with a twin arginine motif that targets DmsAB to the membrane via a novel Sec-independent mechanism termed MTT for membrane targeting and translocation. We demonstrate that the Met-1 present upstream of the twin arginine motif serves as the correct translational start site. The leader is essential for the expression of DmsA, stability of the DmsAB dimer, and membrane targeting of the reductase holoenzyme. Mutation of arginine 17 to aspartate abolished membrane targeting. The reductase was labile in the leader sequence mutants. These mutants failed to support growth on glycerol-Me(2)SO minimal medium. Replacing the DmsA leader with the TorA leader of trimethylamine N-oxide reductase produced a membrane-bound DmsABC with greatly reduced enzyme activity and inefficient anaerobic respiration indicating that the twin arginine leaders may play specific roles in the assembly of redox enzymes.

Interactions between the molybdenum cofactor and iron-sulfur clusters of Escherichia coli dimethylsulfoxide reductase

We have used site-directed mutagenesis to study the interactions between the molybdo-bis(molybdopterin guanine dinucleotide) cofactor (Mo-bisMGD) and the other prosthetic groups of Escherichia coli Me2SO reductase (DmsABC). In redox-poised preparations, there is a significant spin-spin interaction between the reduced Em,7 = -120 mV [4Fe-4S] cluster of DmsB and the Mo(V) of the Mo-bisMGD of DmsA. This interaction is significantly modified in a DmsA-C38S mutant that contains a [3Fe-4S] cluster in DmsA, suggesting that the [3Fe-4S] cluster is in close juxtaposition to the vector connecting the Mo(V) and the Em,7 = -120 mV cluster of DmsB. In a DmsA-R77S mutant, the interaction is eliminated, indicating the importance of this residue in defining the interaction pathway. In ferricyanide-oxidized glycerol-inhibited DmsAC38SBC, there is no detectable interaction between the oxidized [3Fe-4S] cluster and the Mo-bisMGD, except for a minor broadening of the Mo(V) spectrum. In a double mutant, DmsAS176ABC102SC, which contains an engineered [3Fe-4S] cluster in DmsB, no significant paramagnetic interaction is detected between the oxidized [3Fe-4S] cluster and the Mo(V). These results have important implications for (i) understanding the magnetic interactions between the Mo(V) and other paramagnetic centers and (ii) delineating the electron transfer pathway from the [4Fe-4S] clusters of DmsB to the Mo-bisMGD of DmsA.

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.

The molybdenum cofactor of Escherichia coli nitrate reductase A (NarGHI). Effect of a mobAB mutation and interactions with [Fe-S] clusters

We have studied the effect of a mobAB mutation and tungstate on molybdo-molybdopterin-guanine dinucleotide (Mo-MGD) insertion into Escherichia coli nitrate reductase (NarGHI). Preparation of fluorescent oxidized derivatives of MGD (Form A and Form B) indicates that in a mobAB mutant there is essentially no detectable cofactor present in either the membrane-bound (NarGHI) or purified soluble (NarGH) forms of the enzyme. Electron paramagnetic resonance characterization of membrane-bound cofactor-deficient NarGHI suggests that it has altered electrochemistry with respect to the dithionite reducibility of the [Fe-S] clusters of NarH. Potentiometric titrations of membrane-bound NarGHI indicate that the NarH [Fe-S] clusters have midpoint potentials at pH 8.0 (Em,8.0 values) of +180 mV ([3Fe-4S] cluster), +130, -55, and -420 mV ([4Fe-4S] clusters) in a wild-type background and +180, +80, -35, and -420 mV in a mobAB mutant background. These data support the following conclusions: (i) a model for Mo-MGD biosynthesis and assembly into NarGHI in which both metal chelation and nucleotide addition to molybdopterin precede cofactor insertion; and (ii) the absence of Mo-MGD significantly affects Em,8.0 of the highest potential [4Fe-4S] cluster.

Consequences of removal of a molybdenum ligand (DmsA-Ser-176) of Escherichia coli dimethyl sulfoxide reductase

We have used site-directed mutagenesis and EPR spectroscopy to examine the consequences of altering the molybdenum ligand in Escherichia coli dimethyl sulfoxide (Me2SO) reductase (DmsABC). Mutagenesis of DmsA-Ser-176 to Ala, Cys, or His abolishes both respiratory growth on Me2SO and in vitro benzyl viologen:Me2SO oxidoreductase activity. EPR spectroscopy reveals changes in the line shape and the gav of the Mo(V) signals of the S176A and S176C enzymes. The midpoint potentials (Em,7) of the Mo(VI)/Mo(V) and Mo(V)/Mo(IV) couples in DmsABC are -15 and -175 mV. The Em,7 of the Mo(V)/Mo(IV) couple in the S176A mutant is 35 mV; however, the Mo(V) species could not be further oxidized with ferricyanide. Titration of the S176C mutant produced several overlapping Mo(V) species occurring at Eh > -150 mV, suggesting heterogeneity in the molybdenum environment. A Mo(V) spectrum was not visible in S176H membranes poised between -435 to 350 mV or oxidized with 200 microM ferricyanide. No differences were detected in the EPR spectra of the reduced [4Fe-4S] clusters of DmsABC and the S176A and S176H mutant enzymes; however, the S176C mutation altered the EPR line shape of one of the reduced [4Fe-4S] clusters.