A comprehensive phylogenetic analysis of Rieske and Rieske-type iron-sulfur proteins

Rieske Oxygenases and Other Ferredoxin-Dependent Enzymes: Electron Transfer Principles and Catalytic Capabilities

Enzymes that depend on sophisticated electron transfer via ferredoxins (Fds) exhibit outstanding catalytic capabilities, but despite decades of research, many of them are still not well understood or exploited for synthetic applications. This review aims to provide a general overview of the most important Fd-dependent enzymes and the electron transfer processes involved. While several examples are discussed, we focus in particular on the family of Rieske non-heme iron-dependent oxygenases (ROs). In addition to illustrating their electron transfer principles and catalytic potential, the current state of knowledge on structure-function relationships and the mode of interaction between the redox partner proteins is reviewed. Moreover, we highlight several key catalyzed transformations, but also take a deeper dive into their engineerability for biocatalytic applications. The overall findings from these case studies highlight the catalytic capabilities of these biocatalysts and could stimulate future interest in developing additional Fd-dependent enzyme classes for synthetic applications.

Characterisation of an unusual cysteine pair in the Rieske carnitine monooxygenase CntA catalytic site

Rieske monooxygenases undertake complex catalysis integral to marine, terrestrial and human gut-ecosystems. Group-I to -IV Rieske monooxygenases accept aromatic substrates and have well-characterised catalytic mechanisms. Nascent to our understanding are Group-V members catalysing the oxidation/breakdown of quaternary ammonium substrates. Phylogenetic analysis of Group V highlights a cysteine residue-pair adjacent to the mononuclear Fe active site with no established role. Following our elucidation of the carnitine monooxygenase CntA structure, we probed the function of the cysteine pair Cys206/Cys209. Utilising biochemical and biophysical techniques, we found the cysteine residues do not play a structural role nor influence the electron transfer pathway, but rather are used in a nonstoichiometric role to ensure the catalytic iron centre remains in an Fe(II) state.

A review on enzyme complexes of electron transport chain from Mycobacterium tuberculosis as promising drug targets

Energy metabolism is a universal process occurring in all life forms. In Mycobacterium tuberculosis (Mtb), energy production is carried out in two possible ways, oxidative phosphorylation (OxPhos) and substrate-level phosphorylation. Mtb is an obligate aerobic bacterium, making it dependent on OxPhos for ATP synthesis and growth. Mtb inhabits varied micro-niches during the infection cycle, outside and within the host cells, which alters its primary metabolic pathways during the pathogenesis. In this review, we discuss cellular respiration in the context of the mechanism and structural importance of the proteins and enzyme complexes involved. These protein-protein complexes have been proven to be essential for Mtb virulence as they aid the bacteria's survival during aerobic and hypoxic conditions. ATP synthase, a crucial component of the electron transport chain, has been in the limelight, as a prominent drug target against tuberculosis. Likewise, in this review, we have explored other protein-protein complexes of the OxPhos pathway, their functional essentiality, and their mechanism in Mtb's diverse lifecycle. The review summarises crucial target proteins and reported inhibitors of the electron transport chain pathway of Mtb.

Estructura y función de las oxigenasas tipo Rieske/mononuclear

Las oxigenasas Rieske/mononuclear son un grupo de metaloenzimas que catalizan la oxidación de una variedad de compuestos, destaca su participación en la degradación de compuestos xenobióticos contaminantes; estas enzimas también participan en la biosíntesis de algunos compuestos de interés comercial. Poseen una amplia especificidad por el sustrato, convirtiéndolas en un grupo de enzimas con un alto potencial de aplicación en procesos biotecnológicos que hasta el momento no ha sido explotado. La presente revisión aborda aspectos generales acerca de la función y estructura de este importante grupo de enzimas. 

Putative Iron-Sulfur Proteins Are Required for Hydrogen Consumption and Enhance Survival of Mycobacteria

Aerobic soil bacteria persist by scavenging molecular hydrogen (H2) from the atmosphere. This key process is the primary sink in the biogeochemical hydrogen cycle and supports the productivity of oligotrophic ecosystems. In Mycobacterium smegmatis, atmospheric H2 oxidation is catalyzed by two phylogenetically distinct [NiFe]-hydrogenases, Huc (group 2a) and Hhy (group 1h). However, it is currently unresolved how these enzymes transfer electrons derived from H2 oxidation into the aerobic respiratory chain. In this work, we used genetic approaches to confirm that two putative iron-sulfur cluster proteins encoded on the hydrogenase structural operons, HucE and HhyE, are required for H2 consumption in M. smegmatis. Sequence analysis show that these proteins, while homologous, fall into distinct phylogenetic clades and have distinct metal-binding motifs. H2 oxidation was reduced when the genes encoding these proteins were deleted individually and was eliminated when they were deleted in combination. In turn, the growth yield and long-term survival of these deletion strains was modestly but significantly reduced compared to the parent strain. In both biochemical and phenotypic assays, the mutant strains lacking the putative iron-sulfur proteins phenocopied those of hydrogenase structural subunit mutants. We hypothesize that these proteins mediate electron transfer between the catalytic subunits of the hydrogenases and the menaquinone pool of the M. smegmatis respiratory chain; however, other roles (e.g., in maturation) are also plausible and further work is required to resolve their role. The conserved nature of these proteins within most Hhy- or Huc-encoding organisms suggests that these proteins are important determinants of atmospheric H2 oxidation.

Bona fide choline monoxygenases evolved in Amaranthaceae plants from oxygenases of unknown function: Evidence from phylogenetics, homology modeling and docking studies

Few land plants can synthesize and accumulate the osmoprotectant glycine betaine (GB) even though this metabolic trait has major adaptive importance given the prevalence of drought, hypersaline soils or cold. GB is synthesized from choline in two reactions catalyzed by choline monooxygenases (CMOs) and enzymes of the family 10 of aldehyde dehydrogenases (ALDH10s) that gained betaine aldehyde dehydrogenase activity (BADH). Homolog genes encoding CMO and ALDH10 enzymes are present in all known land plant genomes, but since GB-non-accumulators plants lack the BADH-type ALDH10 isozyme, they would be expected to also lack the CMO activity to avoid accumulation of the toxic betaine aldehyde. To explore CMOs substrate specificity, we performed amino acid sequence alignments, phylogenetic analysis, homology modeling and docking simulations. We found that plant CMOs form a monophyletic subfamily within the Rieske/mononuclear non-heme oxygenases family with two clades: CMO1 and CMO2, the latter diverging from CMO1 after gene duplication. CMO1 enzymes are present in all plants; CMO2s only in the Amaranthaceae high-GB-accumulators plants. CMO2s, and particularly their mononuclear non-heme iron domain where the active site is located, evolved at a faster rate than CMO1s, which suggests positive selection. The homology model and docking simulations of the spinach CMO2 enzyme showed at the active site three aromatic residues forming a box with which the trimethylammonium group of choline could interact through cation-π interactions, and a glutamate, which also may interact with the trimethylammonium group through a charge-charge interaction. The aromatic box and the carboxylate have been shown to be critical for choline binding in other proteins. Interestingly, these residues are conserved in CMO2 proteins but not in CMO1 proteins, where two of these aromatic residues are leucine and the glutamate is asparagine. These findings reinforce our proposal that the CMO1s physiological substrate is not choline but a still unknown metabolite.

Engineered 3-Ketosteroid 9α-Hydroxylases in Mycobacterium neoaurum: an Efficient Platform for Production of Steroid Drugs

3-Ketosteroid 9α-hydroxylase (Ksh) consists of a terminal oxygenase (KshA) and a ferredoxin reductase and is indispensable in the cleavage of steroid nucleus in microorganisms. The activities of Kshs are crucial factors in determining the yield and distribution of products in the biotechnological transformation of sterols in industrial applications. In this study, two KshA homologues, KshA1_N and KshA2_N, were characterized and further engineered in a sterol-digesting strain, Mycobacterium neoaurum ATCC 25795, to construct androstenone-producing strains. kshA1 _N is a member of the gene cluster encoding sterol catabolism enzymes, and its transcription exhibited a 4.7-fold increase under cholesterol induction. Furthermore, null mutation of kshA1 _N led to the stable accumulation of androst-4-ene-3,17-dione (AD) and androst-1,4-diene-3,17-dione (ADD). We determined kshA2 _N to be a redundant form of kshA1 _N Through a combined modification of kshA1 _N, kshA2 _N, and other key genes involved in the metabolism of sterols, we constructed a high-yield ADD-producing strain that could produce 9.36 g liter^-1 ADD from the transformation of 20 g liter^-1 phytosterols in 168 h. Moreover, we improved a previously established 9α-hydroxy-AD-producing strain via the overexpression of a mutant KshA1_N that had enhanced Ksh activity. Genetic engineering allowed the new strain to produce 11.7 g liter^-1 9α-hydroxy-4-androstene-3,17-dione (9-OHAD) from the transformation of 20.0 g liter^-1 phytosterol in 120 h.IMPORTANCE Steroidal drugs are widely used for anti-inflammation, anti-tumor action, endocrine regulation, and fertility management, among other uses. The two main starting materials for the industrial synthesis of steroid drugs are phytosterol and diosgenin. The phytosterol processing is carried out by microbial transformation, which is thought to be superior to the diosgenin processing by chemical conversions, given its simple and environmentally friendly process. However, diosgenin has long been used as the primary starting material instead of phytosterol. This is in response to challenges in developing efficient microbial strains for industrial phytosterol transformation, which stem from complex metabolic processes that feature many currently unclear details. In this study, we identified two oxygenase homologues of 3-ketosteroid-9α-hydroxylase, KshA1_N and KshA2_N, in M. neoaurum and demonstrated their crucial role in determining the yield and variety of products from phytosterol transformation. This work has practical value in developing industrial strains for phytosterol biotransformation.

Patterns of Ligands Coordinated to Metallocofactors Extracted from the Protein Data Bank

A new R tool is described that rapidly identifies, ranks, and clusters sequence patterns coordinated to metallocofactors. This tool, PdPDB, fills a void because, unlike currently available tools, PdPDB searches through sequences with metal coordination as the primary determinant and can identify patterns consisting of amino acids, nucleotides, and small molecule ligands at once. PdPDB was tested by analyzing structures that coordinate Fe^2+/3+, [2Fe-2S], [4Fe-4S], Zn²⁺, and Mg²⁺ cofactors. PdPDB confirmed previously identified sequence motifs and revealed which residues are enriched (e.g., glycine) and are under-represented (e.g., glutamine) near ligands to metal centers. The data show the similarities and differences between different metal-binding sites. The patterns that coordinate metallocofactors vary, depending upon whether the metal ions play a structural or catalytic role, with catalytic metal centers exhibiting partial coordination by small molecule ligands. PdPDB 2.0.1 is freely available as a CRAN package.

Twin-Arginine Protein Translocation

Twin-arginine protein translocation systems (Tat) translocate fully folded and co-factor-containing proteins across biological membranes. In this review, we focus on the Tat pathway of Gram-positive bacteria. The minimal Tat pathway is composed of two components, namely a TatA and TatC pair, which are often complemented with additional TatA-like proteins. We provide overviews of our current understanding of Tat pathway composition and mechanistic aspects related to Tat-dependent cargo protein translocation. This includes Tat pathway flexibility, requirements for the correct folding and incorporation of co-factors in cargo proteins and the functions of known cargo proteins. Tat pathways of several Gram-positive bacteria are discussed in detail, with emphasis on the Tat pathway of Bacillus subtilis. We discuss both shared and unique features of the different Gram-positive bacterial Tat pathways. Lastly, we highlight topics for future research on Tat, including the development of this protein transport pathway for the biotechnological secretion of high-value proteins and its potential applicability as an antimicrobial drug target in pathogens.

Characterization of quorum sensing-controlled transcriptional regulator MarR and Rieske (2Fe-2S) cluster-containing protein (Orf5), which are involved in resistance to environmental stresses in Pseudomonas syringae pv. tabaci 6605

Pseudomonas syringae pv. tabaci 6605 (Pta6605) produces acyl homoserine lactones (AHLs), quorum sensing (QS) molecules that are indispensable for virulence in host tobacco infection. Genome-wide transcriptional profiling of several QS-defective mutants revealed that the expression of the genes encoding the MarR family transcriptional regulator (MarR) and a Rieske 2Fe-2S cluster-containing protein (Orf5) located adjacent to psyI, a gene encoding AHL synthetase, are significantly repressed. Exogenous application of AHL recovered the expression of both marR and orf5 genes in the ΔpsyI mutant, indicating that AHL positively regulates the expression of these genes. To investigate the role of these genes in the virulence of Pta6605, ΔmarR and Δorf5 mutants were generated. Both mutants showed decreased swimming and swarming motilities, decreased survival ability under oxidative and nitrosative stresses and, consequently, reduced virulence on host tobacco plants. Transmission electron micrographs showed that the structure of the cell membranes of ΔmarR and Δorf5 mutants was severely damaged. Furthermore, not only the ratio of dead cells, but also the amount of flagella, extracellular DNA and protein released into the culture supernatant, was significantly increased in both mutants, indicating that the disruption of marR and orf5 genes might induce structural changes in the membrane and cell lysis. Because both mutants showed partly similar expression profiles, both gene products might be involved in the same regulatory cascades that are required for QS-dependent survival under environmentally stressed conditions.

Co-factor insertion and disulfide bond requirements for twin-arginine translocase-dependent export of the Bacillus subtilis Rieske protein QcrA

The twin-arginine translocation (Tat) pathway can transport folded and co-factor-containing cargo proteins over bacterial cytoplasmic membranes. Functional Tat machinery components, a folded state of the cargo protein and correct co-factor insertion in the cargo protein are generally considered as prerequisites for successful translocation. The present studies were aimed at a dissection of these requirements with regard to the Rieske iron-sulfur protein QcrA of Bacillus subtilis. Notably, QcrA is a component of the cytochrome bc1 complex, which is conserved from bacteria to man. Single amino acid substitutions were introduced into the Rieske domain of QcrA to prevent either co-factor binding or disulfide bond formation. Both types of mutations precluded QcrA translocation. Importantly, a proofreading hierarchy was uncovered, where a QcrA mutant defective in disulfide bonding was quickly degraded, whereas mutant QcrA proteins defective in co-factor binding accumulated in the cytoplasm and membrane. Altogether, these are the first studies on Tat-dependent protein translocation where both oxidative folding and co-factor attachment have been addressed in a single native molecule.

Carnitine metabolism to trimethylamine by an unusual Rieske-type oxygenase from human microbiota

Dietary intake of L-carnitine can promote cardiovascular diseases in humans through microbial production of trimethylamine (TMA) and its subsequent oxidation to trimethylamine N-oxide by hepatic flavin-containing monooxygenases. Although our microbiota are responsible for TMA formation from carnitine, the underpinning molecular and biochemical mechanisms remain unclear. In this study, using bioinformatics approaches, we first identified a two-component Rieske-type oxygenase/reductase (CntAB) and associated gene cluster proposed to be involved in carnitine metabolism in representative genomes of the human microbiota. CntA belongs to a group of previously uncharacterized Rieske-type proteins and has an unusual "bridging" glutamate but not the aspartate residue, which is believed to facilitate intersubunit electron transfer between the Rieske center and the catalytic mononuclear iron center. Using Acinetobacter baumannii as the model, we then demonstrate that cntAB is essential in carnitine degradation to TMA. Heterologous overexpression of cntAB enables Escherichia coli to produce TMA, confirming that these genes are sufficient in TMA formation. Site-directed mutagenesis experiments have confirmed that this unusual "bridging glutamate" residue in CntA is essential in catalysis and neither mutant (E205D, E205A) is able to produce TMA. Taken together, the data in our study reveal the molecular and biochemical mechanisms underpinning carnitine metabolism to TMA in human microbiota and assign the role of this novel group of Rieske-type proteins in microbial carnitine metabolism.

The Tat system of Gram-positive bacteria

The twin-arginine protein translocation (Tat) system has a unique ability to translocate folded and co-factor-containing proteins across lipid bilayers. The Tat pathway is present in bacteria, archaea and in the thylakoid membranes of chloroplasts and, depending on the organism and environmental conditions, it can be deemed important for cell survival, virulence or bioproduction. This review provides an overview of the current understanding of the Tat system with specific focus on Gram-positive bacteria. The 'universal minimal Tat system' is composed of a TatA and a TatC protein. However, this pathway is more commonly composed of two TatA-like proteins and one TatC protein. Often the TatA-like proteins have diverged to have two different functions and, in this case, the second TatA-like protein is usually referred to as TatB. The correct folding and/or incorporation of co-factors are requirements for translocation, and the known quality control mechanisms are examined in this review. A number of examples of crosstalk between the Tat system and other protein transport systems, such as the Sec-YidC translocon and signal peptidases or sheddases are also discussed. Further, an overview of specific Gram-positive bacterial Tat systems found in monoderm and diderm species is detailed. Altogether, this review highlights the unique features of Gram-positive bacterial Tat systems and pinpoints key questions that remain to be addressed in future research. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Update on the biochemistry of chlorophyll breakdown

In land plants, chlorophyll is broken down to colorless linear tetrapyrroles in a highly conserved multi-step pathway. The pathway is termed the 'PAO pathway', because the opening of the chlorine macrocycle present in chlorophyll catalyzed by pheophorbide a oxygenase (PAO), the key enzyme of the pathway, provides the characteristic structural basis found in all further downstream chlorophyll breakdown products. To date, most of the biochemical steps of the PAO pathway have been elucidated and genes encoding many of the chlorophyll catabolic enzymes been identified. This review summarizes the current knowledge on the biochemistry of the PAO pathway and provides insight into recent progress made in the field that indicates that the pathway is more complex than thought in the past.

Genome of a SAR116 bacteriophage shows the prevalence of this phage type in the oceans

The abundance, genetic diversity, and crucial ecological and evolutionary roles of marine phages have prompted a large number of metagenomic studies. However, obtaining a thorough understanding of marine phages has been hampered by the low number of phage isolates infecting major bacterial groups other than cyanophages and pelagiphages. Therefore, there is an urgent requirement for the isolation of phages that infect abundant marine bacterial groups. In this study, we isolated and characterized HMO-2011, a phage infecting a bacterium of the SAR116 clade, one of the most abundant marine bacterial lineages. HMO-2011, which infects "Candidatus Puniceispirillum marinum" strain IMCC1322, has an ~55-kb dsDNA genome that harbors many genes with novel features rarely found in cultured organisms, including genes encoding a DNA polymerase with a partial DnaJ central domain and an atypical methanesulfonate monooxygenase. Furthermore, homologs of nearly all HMO-2011 genes were predominantly found in marine metagenomes rather than cultured organisms, suggesting the novelty of HMO-2011 and the prevalence of this phage type in the oceans. A significant number of the viral metagenome sequences obtained from the ocean surface were best assigned to the HMO-2011 genome. The number of reads assigned to HMO-2011 accounted for 10.3%-25.3% of the total reads assigned to viruses in seven viromes from the Pacific and Indian Oceans, making the HMO-2011 genome the most or second-most frequently assigned viral genome. Given its ability to infect the abundant SAR116 clade and its widespread distribution, Puniceispirillum phage HMO-2011 could be an important resource for marine virus research.

SAD phasing towards structure determination of a thermostable Rieske ferredoxin with a novel stabilizing disulfide bridge

Rieske proteins and Rieske ferredoxins are ubiquitous electron-transfer metalloproteins that are characterized by a [2Fe-2S] cluster coordinated by pairs of cysteine and histidine residues. The thermoacidophilic archaeon Acidianus ambivalens contains a Rieske ferredoxin termed RFd2, which has an hitherto unknown additional region of 40-44 residues at the C-terminus with a Cx3C motif that introduces a novel disulfide bond within the Rieske fold. RFd2 was crystallized with the aim of determining its three-dimensional structure in order to understand the contribution of this as yet unique disulfide bridge to the function and stability of RFd2. RFd2 crystals were successively improved, increasing their diffraction to 1.9 Å resolution. Molecular replacement did not solve the RFd2 structure, but a highly multiple in-house diffraction data set collected at the Cu Kα edge led to solution of the phase problem.

Isolation and functional gene analyses of aromatic-hydrocarbon-degrading bacteria from a polychlorinated-dioxin-dechlorinating process

Aerobic aromatic-hydrocarbon-degrading bacteria from a semi-anaerobic microbial microcosm that exhibited apparent complete dechlorination of polychlorinated dibenzo-p-dioxins/dibenzofurans (PCDD/Fs) were isolated through enrichment and plating culture procedures with dibenzofuran as the model substrate. By 16S rRNA gene sequence comparisons, these dibenzofuran-degrading isolates were identified as being members of the phyla Actinobacteria, Firmicutes, and Proteobacteria, among which those of the genera Paenibacillus and Rhizobium were most abundant. All of the isolates utilized naphthalene as the sole carbon and energy source and degraded dibenzofuran metabolically or co-metabolically; however, they hardly attacked monochlorinated dibenzofuran and dibenzo-p-dioxin. By PCR cloning and sequencing, genes predicted to encode aromatic-ring-hydroxylating dioxygenase (AhDO) were detected in all test isolates. Real-time quantitative PCR assays with specific primer sets detected approximately 10⁵ copies of the AhDO large subunit genes g⁻¹ wet wt in the microcosm from which the isolates were obtained. This order of the copy number corresponded to approximately 1% of the 16S rRNA gene copies from "Dehalococcoides" and its relatives present as potent dechlorinators. These results suggest that aerobic AhDO-containing bacteria co-exist and play a role in the oxidative degradation of less chlorinated and completely dechlorinated products in the PCDD/F-dechlorinating process, thereby achieving the apparent complete dechlorination of PCDD/Fs.

An insight into the origin and functional evolution of bacterial aromatic ring-hydroxylating oxygenases

Bacterial aromatic ring-hydroxylating oxygenases (RHOs) are multicomponent enzyme systems which have potential utility in bioremediation of aromatic compounds in the environment. To cope with the enormous diversity of aromatic compounds in the environment, this enzyme family has evolved remarkably exhibiting broad substrate specificity. RHOs are multicomponent enzymes comprising of a homo- or hetero-multimeric terminal oxygenase and one or more electron transport (ET) protein(s). The present study attempts in depicting the evolutionary scenarios that might have occurred during the evolution of RHOs, by analyzing a set of available sequences including those obtained from complete genomes. A modified classification scheme identifying four new RHO types has been suggested on the basis of their evolutionary and functional behaviours, in relation to structural configuration of substrates and preferred oxygenation site(s). The present scheme emphasizes on the fact that the phylogenetic affiliation of RHOs is distributed among four distinct 'Similarity classes', independent of the constituent ET components. Similar combination of RHO components that was previously considered to be equivalent and classified together [Kweon et al., BMC Biochemistry 9, 11 (2008)] were found here in distinct similarity classes indicating the role of substrate-binding terminal oxygenase in guiding the evolution of RHOs irrespective of the nature of constituent ET components. Finally, a model for evolution of the multicomponent RHO enzyme system has been proposed, beginning from genesis of the terminal oxygenase components followed by recruitment of constituent ET components, finally evolving into various 'extant' RHO types.

Quaternary ammonium oxidative demethylation: X-ray crystallographic, resonance Raman, and UV-visible spectroscopic analysis of a Rieske-type demethylase

Herein, the structure resulting from in situ turnover in a chemically challenging quaternary ammonium oxidative demethylation reaction was captured via crystallographic analysis and analyzed via single-crystal spectroscopy. Crystal structures were determined for the Rieske-type monooxygenase, stachydrine demethylase, in the unliganded state (at 1.6 Å resolution) and in the product complex (at 2.2 Å resolution). The ligand complex was obtained from enzyme aerobically cocrystallized with the substrate stachydrine (N,N-dimethylproline). The ligand electron density in the complex was interpreted as proline, generated within the active site at 100 K by the absorption of X-ray photon energy and two consecutive demethylation cycles. The oxidation state of the Rieske iron-sulfur cluster was characterized by UV-visible spectroscopy throughout X-ray data collection in conjunction with resonance Raman spectra collected before and after diffraction data. Shifts in the absorption band wavelength and intensity as a function of absorbed X-ray dose demonstrated that the Rieske center was reduced by solvated electrons generated by X-ray photons; the kinetics of the reduction process differed dramatically for the liganded complex compared to unliganded demethylase, which may correspond to the observed turnover in the crystal.

The conserved Rieske oxygenase DAF-36/Neverland is a novel cholesterol-metabolizing enzyme

Steroid hormones play essential roles in a wide variety of biological processes in multicellular organisms. The principal steroid hormones in nematodes and arthropods are dafachronic acids and ecdysteroids, respectively, both of which are synthesized from cholesterol as an indispensable precursor. The first critical catalytic step in the biosynthesis of these ecdysozoan steroids is the conversion of cholesterol to 7-dehydrocholesterol. However, the enzymes responsible for cholesterol 7,8-dehydrogenation remain unclear at the molecular level. Here we report that the Rieske oxygenase DAF-36/Neverland (Nvd) is a cholesterol 7,8-dehydrogenase. The daf-36/nvd genes are evolutionarily conserved, not only in nematodes and insects but also in deuterostome species that do not produce dafachronic acids or ecdysteroids, including the sea urchin Hemicentrotus pulcherrimus, the sea squirt Ciona intestinalis, the fish Danio rerio, and the frog Xenopus laevis. An in vitro enzymatic assay system reveals that all DAF-36/Nvd proteins cloned so far have the ability to convert cholesterol to 7-dehydrocholesterol. Moreover, the lethality of loss of nvd function in the fruit fly Drosophila melanogaster is rescued by the expression of daf-36/nvd genes from the nematode Caenorhabditis elegans, the insect Bombyx mori, or the vertebrates D. rerio and X. laevis. These data suggest that daf-36/nvd genes are functionally orthologous across the bilaterian phylogeny. We propose that the daf-36/nvd family of proteins is a novel conserved player in cholesterol metabolism across the animal phyla.

The small subunit AroB of arsenite oxidase: lessons on the [2Fe-2S] Rieske protein superfamily

Here, we describe the characterization of the [2Fe-2S] clusters of arsenite oxidases from Rhizobium sp. NT-26 and Ralstonia sp. 22. Both reduced Rieske proteins feature EPR signals similar to their homologs from Rieske-cyt b complexes, with g values at 2.027, 1.88, and 1.77. Redox titrations in a range of pH values showed that both [2Fe-2S] centers have constant E(m) values up to pH 8 at approximately +210 mV. Above this pH value, the E(m) values of both centers are pH-dependent, similar to what is observed for the Rieske-cyt b complexes. The redox properties of these two proteins, together with the low E(m) value (+160 mV) of the Alcaligenes faecalis arsenite oxidase Rieske (confirmed herein), are in line with the structural determinants observed in the primary sequences, which have previously been deduced from the study of Rieske-cyt b complexes. Since the published E(m) value of the Chloroflexus aurantiacus Rieske (+100 mV) is in conflict with this sequence analysis, we re-analyzed membrane samples of this organism and obtain a new value (+200 mV). Arsenite oxidase activity was affected by quinols and quinol analogs, which is similar to what is found with the Rieske-cyt b complexes. Together, these results show that the Rieske protein of arsenite oxidase shares numerous properties with its counterpart in the Rieske-cyt b complex. However, two cysteine residues, strictly conserved in the Rieske-cyt b-Rieske and considered to be crucial for its function, are not conserved in the arsenite oxidase counterpart. We discuss the role of these residues.

A profile of ring-hydroxylating oxygenases that degrade aromatic pollutants

Numerous aromatic compounds are pollutants to which exposure exists or is possible, and are of concern because they are mutagenic, carcinogenic, or display other toxic characteristics. Depending on the types of dioxygenation reactions of which microorganisms are capable, they utilize ring-hydroxylating oxygenases (RHOs) to initiate the degradation and detoxification of such aromatic compound pollutants. Gene families encoding for RHOs appear to be most common in bacteria. Oxygenases are important in degrading both natural and synthetic aromatic compounds and are particularly important for their role in degrading toxic pollutants; for this reason, it is useful for environmental scientists and others to understand more of their characteristics and capabilities. It is the purpose of this review to address RHOs and to describe much of their known character, starting with a review as to how RHOs are classified. A comprehensive phylogenetic analysis has revealed that all RHOs are, in some measure, related, presumably by divergent evolution from a common ancestor, and this is reflected in how they are classified. After we describe RHO classification schemes, we address the relationship between RHO structure and function. Structural differences affect substrate specificity and product formation. In the alpha subunit of the known terminal oxygenase of RHOs, there is a catalytic domain with a mononuclear iron center that serves as a substrate-binding site and a Rieske domain that retains a [2Fe-2S] cluster that acts as an entity of electron transfer for the mononuclear iron center. Oxygen activation and substrate dihydroxylation occurring at the catalytic domain are dependent on the binding of substrate at the active site and the redox state of the Rieske center. The electron transfer from NADH to the catalytic pocket of RHO and catalyzing mechanism of RHOs is depicted in our review and is based on the results of recent studies. Electron transfer involving the RHO system typically involves four steps: NADH-ferredoxin reductase receives two electrons from NADH; ferredoxin binds with NADH-ferredoxin reductase and accepts electron from it; the reduced ferredoxin dissociates from NADH-ferredoxin reductase and shuttles the electron to the Rieske domain of the terminal oxygenase; the Rieske cluster donates electrons to O2 through the mononuclear iron. On the basis of crystal structure studies, it has been proposed that the broad specificity of the RHOs results from the large size and specific topology of its hydrophobic substrate-binding pocket. Several amino acids that determine the substrate specificity and enantioselectivity of RHOs have been identified through sequence comparison and site-directed mutagenesis at the active site. Exploiting the crystal structure data and the available active site information, engineered RHO enzymes have been and can be designed to improve their capacity to degrade environmental pollutants. Such attempts to enhance degradation capabilities of RHOs have been made. Dioxygenases have been modified to improve the degradation capacities toward PCBs, PAHs, dioxins, and some other aromatic hydrocarbons. We hope that the results of this review and future research on enhancing RHOs will promote their expanded usage and effectiveness for successfully degrading environmental aromatic pollutants.

Role of a novel disulfide bridge within the all-beta fold of soluble Rieske proteins

Rieske proteins and Rieske ferredoxins are present in the three domains of life and are involved in a variety of cellular processes. Despite their functional diversity, these small Fe-S proteins contain a highly conserved all-beta fold, which harbors a [2Fe-2S] Rieske center. We have identified a novel subtype of Rieske ferredoxins present in hyperthermophilic archaea, in which a two-cysteine conserved SKTPCX((2-3))C motif is found at the C-terminus. We establish that in the Acidianus ambivalens representative, Rieske ferredoxin 2 (RFd2), these cysteines form a novel disulfide bond within the Rieske fold, which can be selectively broken under mild reducing conditions insufficient to reduce the [2Fe-2S] cluster or affect the secondary structure of the protein, as shown by visible circular dichroism, absorption, and attenuated total reflection Fourier transform IR spectroscopies. RFd2 presents all the EPR, visible absorption, and visible circular dichroism spectroscopic features of the [2Fe-2S] Rieske center. The cluster has a redox potential of +48 mV (25 degrees C and pH 7) and a pK (a) of 10.1 +/- 0.2. These shift to +77 mV and 8.9 +/- 0.3, respectively, upon reduction of the disulfide. RFd2 has a melting temperature near the boiling point of water (T(m) = 99 degrees C, pH 7.0), but it becomes destabilized upon disulfide reduction (DeltaT(m) = -9 degrees C, DeltaC(m) = -0.7 M guanidinium hydrochloride). This example illustrates how the incorporation of an additional structural element such as a disulfide bond in a highly conserved fold such as that of the Rieske domain may fine-tune the protein for a particular function or for increased stability.

Gene structure of Drosophila diaphorase-1: diversity of transcripts in adult males and females, in different organs and at different stages of development

The gene EG:22E5.5 or CG4199 (accession number O77266, Q9W529) from Berkeley Drosophila Genome Project (BDGP) was found using the partial amino acid sequences of three tryptic peptides obtained from purified Drosophila virilis diaphorase-1. This gene is located on the X chromosome at position 2C9-2C10. The structure of the gene reveals three exons and two long introns. Using BDGP, we found six transcripts in this gene. The difference between these transcripts is in their 5' ends; the 3' ends of the six transcripts are identical. Thirty-four ESTs from different cDNA libraries were found, most of them from Schneider L2 cell culture (SH) cDNA library. The transcripts are represented at very low level in the cells of different organs and at different stages of Drosophila development. Using RT-PCR, we obtained five of these transcripts in cDNA samples from female adult flies. However, we could not find any of them in cDNA samples from male adult flies. Moreover, we obtained only the third transcript (CG4199-RC) in the sample of testis from adult flies and the fourth transcript (CG4199-RD) in an embryo sample. None of the other five transcripts were found in the samples of different organs and in the samples obtained at different stages of Drosophila development.

X-ray structure of a soluble Rieske-type ferredoxin from Mus musculus

The X-ray crystal structure of a soluble Rieske ferredoxin from M. musculus was solved at 2.07 Å resolution, revealing an iron–sulfur cluster-binding domain with similar architecture to the Rieske-type domains of bacterial aromatic dioxygenases. The ferredoxin was also shown to be capable of accepting electrons from both eukaryotic and prokaryotic oxidoreductases.

The 2.07 Å resolution X-ray crystal structure of a soluble Rieske-type ferredoxin from Mus musculus encoded by the gene Mm.266515 is reported. Although they are present as covalent domains in eukaryotic membrane oxidase complexes, soluble Rieske-type ferredoxins have not previously been observed in eukaryotes. The overall structure of the mouse Rieske-type ferredoxin is typical of this class of iron–sulfur proteins and consists of a larger partial β-barrel domain and a smaller domain containing Cys57, His59, Cys80 and His83 that binds the [2Fe–2S] cluster. The S atoms of the cluster are hydrogen-bonded by six backbone amide N atoms in a pattern typical of membrane-bound high-potential eukaryotic respiratory Rieske ferredoxins. However, phylogenetic analysis suggested that the mouse Rieske-type ferredoxin was more closely related to bacterial Rieske-type ferredoxins. Correspondingly, the structure revealed an extended loop most similar to that seen in Rieske-type ferredoxin subunits of bacterial aromatic dioxygenases, including the positioning of an aromatic side chain (Tyr85) between this loop and the [2Fe–2S] cluster. The mouse Rieske-type ferredoxin was shown to be capable of accepting electrons from both eukaryotic and prokaryotic oxidoreductases, although it was unable to serve as an electron donor for a bacterial monooxygenase complex. The human homolog of mouse Rieske-type ferredoxin was also cloned and purified. It behaved identically to mouse Rieske-type ferredoxin in all biochemical characterizations but did not crystallize. Based on its high sequence identity, the structure of the human homolog is likely to be modeled well by the mouse Rieske-type ferredoxin structure.

Characterization of the recombinant Rieske [2Fe-2S] proteins HcaC and YeaW from E. coli

Three genes within the genome of E. coli K12 are predicted to encode proteins containing the typical Rieske iron-sulfur cluster-binding motifs. Two of these, hcaC and yeaW, were overexpressed in E. coli BL21 and Tuner (DE3) pLacI. The recombinant proteins were purified and analyzed by UV/Vis- and EPR-spectroscopy. HcaC and YeaW display the typical redox-dependent UV/Vis-spectra of iron-sulfur proteins. The EPR spectrum of reduced HcaC shows characteristic g-values of a Rieske cluster whereas the g-values for YeaW are close to the upper limit for this type of iron-sulfur cluster. Both iron-sulfur clusters could be reduced by dithionite, but not by ascorbate, confirming their classification as low-potential Rieske proteins as derived from the amino acid sequences. A phylogenetic analysis of the two proteins reveals that HcaC clearly segregates with the Rieske ferredoxins of class IIB oxygenases whereas the classification of YeaW remains doubtful.

N-glycolylneuraminic acid deficiency in mice: implications for human biology and evolution

Humans and chimpanzees share >99% identity in most proteins. One rare difference is a human-specific inactivating deletion in the CMAH gene, which determines biosynthesis of the sialic acid N-glycolylneuraminic acid (Neu5Gc). Since Neu5Gc is prominent on most chimpanzee cell surfaces, this mutation could have affected multiple systems. However, Neu5Gc is found in human cancers and fetuses and in trace amounts in normal human tissues, suggesting an alternate biosynthetic pathway. We inactivated the mouse Cmah gene and studied the in vivo consequences. There was no evidence for an alternate pathway in normal, fetal, or malignant tissue. Rather, null fetuses accumulated Neu5Gc from heterozygous mothers and dietary Neu5Gc was incorporated into oncogene-induced tumors. As with humans, there were accumulation of the precursor N-acetylneuraminic acid and increases in sialic acid O acetylation. Null mice showed other abnormalities reminiscent of the human condition. Adult mice showed a diminished acoustic startle response and required higher acoustic stimuli to increase responses above the baseline level. In this regard, histological abnormalities of the inner ear occurred in older mice, which had impaired hearing. Adult animals also showed delayed skin wound healing. Loss of Neu5Gc in hominid ancestors approximately 2 to 3 million years ago likely had immediate and long-term consequences for human biology.

The Rieske protein from Paracoccus denitrificans is inserted into the cytoplasmic membrane by the twin-arginine translocase

The Rieske [2Fe-2S] protein (ISP) is an essential subunit of cytochrome bc(1) complexes in mitochondrial and bacterial respiratory chains. Based on the presence of two consecutive arginines, it was argued that the ISP of Paracoccus denitrificans, a Gram-negative soil bacterium, is inserted into the cytoplasmic membrane via the twin-arginine translocation (Tat) pathway. Here, we provide experimental evidence that membrane integration of the bacterial ISP indeed relies on the Tat translocon. We show that targeting of the ISP depends on the twin-arginine motif. A strict requirement is established particularly for the second arginine residue (R16); conservative replacement of the first arginine (R15K) still permits substantial ISP transport. Comparative sequence analysis reveals characteristics common to Tat signal peptides in several bacterial ISPs; however, there are distinctive features relating to the fact that the presumed ISP Tat signal simultaneously serves as a membrane anchor. These differences include an elevated hydrophobicity of the h-region compared with generic Tat signals and the absence of an otherwise well-conserved '+5'-consensus motif lysine residue. Substitution of the +5 lysine (Y20K) compromises ISP export and/or cytochrome bc(1) stability to some extent and points to a specific role for this deviation from the canonical Tat motif. EPR spectroscopy confirms cytosolic insertion of the [2Fe-2S] cofactor. Mutation of an essential cofactor binding residue (C152S) decreases the ISP membrane levels, possibly indicating that cofactor insertion is a prerequisite for efficient translocation along the Tat pathway.

Crystal structure of the ferredoxin component of carbazole 1,9a-dioxygenase of Pseudomonas resinovorans strain CA10, a novel Rieske non-heme iron oxygenase system

The carbazole 1,9a-dioxygenase (CARDO) system of Pseudomonas resinovorans strain CA10 catalyzes the dioxygenation of carbazole; the 9aC carbon bonds to a nitrogen atom and its adjacent 1C carbon as the initial reaction in the mineralization pathway. The CARDO system is composed of ferredoxin reductase (CarAd), ferredoxin (CarAc), and terminal oxygenase (CarAa). CarAc acts as a mediator in the electron transfer from CarAd to CarAa. To understand the structural basis of the protein-protein interactions during electron transport in the CARDO system, the crystal structure of CarAc was determined at 1.9 A resolution by molecular replacement using the structure of BphF, the biphenyl 2,3-dioxygenase ferredoxin from Burkholderia cepacia strain LB400 as a search model. CarAc is composed of three beta-sheets, and the structure can be divided into two domains, a cluster-binding domain and a basal domain. The Rieske [2Fe-2S] cluster is located at the tip of the cluster-binding domain, where it is exposed to solvent. While the overall folding of CarAc and BphF is strongly conserved, the properties of their surfaces are very different from each other. The structure of the cluster-binding domain of CarAc is more compact and protruding than that of BphF, and the distribution of electric charge on its molecular surface is very different. Such differences are thought to explain why these ferredoxins can act as electron mediators in respective electron transport chains composed of different-featured components.

Hormonal Control of C. elegans Dauer Formation and Life Span by a Rieske-like Oxygenase

C. elegans diapause, gonadal outgrowth, and life span are regulated by a lipophilic hormone, which serves as a ligand to the nuclear hormone receptor DAF-12. A key step in hormone production is catalyzed by the CYP450 DAF-9, but the extent of the biosynthetic pathway is unknown. Here, we identify a conserved Rieske-like oxygenase, DAF-36, as a component in hormone metabolism. Mutants display larval developmental and adult aging phenotypes, as well as patterns of epistasis similar to that of daf-9. Larval phenotypes are potently reversed by crude lipid extracts, 7-dehydrocholesterol, and a recently identified DAF-12 sterol ligand, suggesting that DAF-36 works early in the hormone biosynthetic pathway. DAF-36 is expressed primarily within the intestine, a major organ of metabolic and endocrine control, distinct from DAF-9. These results imply that C. elegans hormone production has multiple steps and is distributed, and that it may provide one way that tissues register their current physiological state during organismal commitments.

The Rieske Protein: A Case Study on the Pitfalls of Multiple Sequence Alignments and Phylogenetic Reconstruction

Previously published phylogenetic trees reconstructed on "Rieske protein" sequences frequently are at odds with each other, with those of other subunits of the parent enzymes and with small-subunit rRNA trees. These differences are shown to be at least partially if not completely due to problems in the reconstruction procedures. A major source of erroneous Rieske protein trees lies in the presence of a large, poorly conserved domain prone to accommodate very long insertions in well-defined structural hot spots substantially hampering multiple alignments. The remaining smaller domain, in contrast, is too conserved to allow distant phylogenies to be deduced with sufficient confidence. Three-dimensional structures of representatives from this protein family are now available from phylogenetically distant species and from diverse enzymes. Multiple alignments can thus be refined on the basis of these structures. We show that structurally guided alignments of Rieske proteins from Rieske-cytochrome b complexes and arsenite oxidases strongly reduce conflicts between resulting trees and those obtained on their companion enzyme subunits. Further problems encountered during this work, mainly consisting in database errors such as wrong annotations and frameshifts, are described. The obtained results are discussed against the background of hypotheses stipulating pervasive lateral gene transfer in prokaryotes.

Multiple Rieske proteins in prokaryotes: where and why?

Many microbial genomes have been sequenced in the recent years. Multiple genes encoding Rieske iron-sulfur proteins, which are subunits of cytochrome bc-type complexes or oxygenases, have been detected in many pro- and eukaryotic genomes. The diversity of substrates, co-substrates and reactions offers obvious explanations for the diversity of the low potential Rieske proteins associated with oxygenases, but the physiological significance of the multiple genes encoding high potential Rieske proteins associated with the cytochrome bc-type complexes remains elusive. For some organisms, investigations into the function of the later group of genes have been initiated. Here, we summarize recent finding on the characteristics and physiological functions of multiple high potential Rieske proteins in prokaryotes. We suggest that the existence of multiple high potential Rieske proteins in prokaryotes could be one way of allowing an organism to adapt their electron transfer chains to changing environmental conditions.

Multiple Rieske genes in prokaryotes: exchangeable Rieske subunits in the cytochrome bc-complex of Rubrivivax gelatinosus

Bacterial cytochrome bc1-complex encoded by the petABC operon consists of three subunits, the Rieske iron-sulphur protein, the b-type cytochrome, and the c1-type cytochrome. Disruption of the petA gene of Rubrivivax gelatinosus is not lethal under photosynthetic growth conditions. However, deletion of both petA and petB results in a photosynthesis-deficient strain, suggesting the presence of a second gene encoding a Rieske protein and rescuing a functional cytochrome bc1-complex in the PETA1 mutant. The corresponding petA2 gene was identified and the PETA2 mutant could also grow under photosynthetic conditions. The double mutant PETA12, however, was unable to grow photosynthetically. The presence of a photo-induced cyclic electron transfer was tested by monitoring the kinetics of cytochrome photo-oxidation on intact cells; the data confirm the capacity of petA2 to replace petA1 in the bc1-complex to support photosynthesis. Soluble forms of both PetA1 and PetA2 Rieske proteins were purified from Escherichia coli and found to contain correctly inserted [2Fe-2S] clusters. Electron paramagnetic resonance (EPR) spectroscopy and midpoint potential measurements showed typical [2Fe-2S] signals and E(m) values of +275 mV for both Rieske proteins. The high amino acid sequence similarity and the obtained midpoint potential values argue for a functional role of these proteins in the cytochrome bc1-complex. The presence of duplicated Rieske genes is not restricted to R. gelatinosus. Phylogenetic trees of Rieske genes from Rubrivivax and other proteobacteria as well as from cyanobacteria were reconstructed. On the basis of the phylogenetic analyses, differing evolutionary origins of duplicated Rieske genes in proteo- and cyanobacteria are proposed.

Recent advances in chlorophyll biosynthesis and breakdown in higher plants

Chlorophyll (Chl) has unique and essential roles in photosynthetic light-harvesting and energy transduction, but its biosynthesis, accumulation and degradation is also associated with chloroplast development, photomorphogenesis and chloroplast-nuclear signaling. Biochemical analyses of the enzymatic steps paved the way to the identification of their encoding genes. Thus, important progress has been made in the recent elucidation of almost all genes involved in Chl biosynthesis and breakdown. In addition, analysis of mutants mainly in Arabidopsis , genetically engineered plants and the application of photo-reactive herbicides contributed to the genetic and regulatory characterization of the formation and breakdown of Chl. This review highlights recent progress in Chl metabolism indicating highly regulated pathways from the synthesis of precursors to Chl and its degradation to intermediates, which are not longer photochemically active.

The Arabidopsis-accelerated cell death gene ACD1 is involved in oxygenation of pheophorbide a: inhibition of the pheophorbide a oxygenase activity does not lead to the "stay-green" phenotype in Arabidopsis

Oxygenation of pheophorbide a is a key step in chlorophyll breakdown. Several biochemical studies have implicated that this step was catalyzed by an iron-containing and ferredoxin-dependent monooxygenase, pheophorbide a oxygenase (PaO). It has been proposed that inhibition of its activity arrests the chlorophyll breakdown and leads to the "stay-green" phenotype. We searched the Arabidopsis genome for a possible PaO-encoding gene and hypothesized that it has homology to known iron-containing Rieske-type monooxygenase sequences. We identified three such open reading frames, Tic55, ACD1 and ACD1-like. We produced transgenic Arabidopsis plants which expressed antisense RNA as a method to inhibit the expression of these genes. The appearance of these antisense plants were indistinguishable from that of the wild type under illumination. However, after they were kept under darkness for 5 d and again illuminated, the leaves of the antisense ACD1 plants (AsACD1) were bleached. Leaves of AsACD1 accumulated 387 nmol (g FW)(-1) pheophorbide a which corresponded to 60% of chlorophyll a degraded. The rate of decrease in chlorophyll a was not influenced in senesced AsACD1 leaves. These results demonstrated that ACD1 is involved in PaO activity, and its inhibition led to photooxidative destruction of the cell instead of the "stay-green" phenotype.

New genes encoding subunits of a cytochrome bc1-analogous complex in the respiratory chain of the hyperthermoacidophilic crenarchaeon Sulfolobus acidocaldarius

The soxL gene from Sulfolobus acidocaldarius (DSM 639) encodes a Rieske iron-sulfur protein. In this study we report the identification of two open reading frames in its downstream region. The first one, named soxN, codes for a membrane protein bearing a resemblance to the b-type cytochromes of the cytochrome bc1 and b6f complexes. The protein is predicted to contain at least 10 transmembrane helices and features the two conserved histidine pairs coordinating the heme groups of these cytochromes. The second open reading frame, named odsN, encodes a soluble protein of unknown function. The genomic region displays a complex transcription pattern. Northern blot and RT-PCR analyses revealed the presence of mono- and bi-cistronic transcripts as well as a tri-cistronic transcript of soxL and cbsAB, encoding the mono-heme cytochrome b558/566. Phylogenetic analyses of the genes of the soxLN pair and of other archaeal gene pairs encoding Rieske iron-sulfur proteins and b-type cytochromes revealed an identical branching patterns for both protein families, suggesting an evolutionary link of these genes provided by the functional interaction of the proteins. On the basis of the findings of this study and the previously studied properties of the soxL and cbsA proteins, we propose the occurrence of a novel cytochrome bc1-analogous complex in the membranes of Sulfolobus, consisting of the cytochrome b homolog soxN, the Rieske protein soxL, the high potential cytochrome cbsA, as well as the non-redox-active subunits cbsB and odsN.

The redox protein construction kit: pre-last universal common ancestor evolution of energy-conserving enzymes

Genome analyses and the resolution of three-dimensional structures have provided evidence in recent years for hitherto unexpected family relationships between redox proteins of very diverse enzymes involved in bioenergetic electron transport. Many of these enzymes appear in fact to be constructed from only a limited set of building blocks. Phylogenetic analysis of selected units from this "redox enzyme construction kit" indicates an origin for several prominent bioenergetic enzymes that is very early, lying before the divergence of Bacteria and Archaea. Possible scenarios for the early evolution of selected complexes are proposed based on the obtained tree topologies.

Isolation and characterization of cytidine-5'-monophosphate-N-acetylneuraminate hydroxylase from the starfish Asterias rubens

The sialic acid N-glycolylneuraminic acid (Neu5Gc) is formed by cytidine-5'-monophosphate-N-acetylneuraminic acid (CMP-Neu5Ac) hydroxylase (EC 1.14.13.45). The enzyme from mammals exhibits several unusual characteristics, raising questions about its evolution. Since echinoderms are the most primitive organisms possessing glycoconjugate-bound Neu5Gc, studies on the hydroxylase from members of this phylum may yield insights into the origin and development of the hydroxylase. Investigations on crude CMP-Neu5Ac hydroxylase in gonads from the starfish Asterias rubens revealed that it shares many properties with its mammalian counterpart. However, the echinoderm hydroxylase also exhibits fundamental differences, particularly its association with a membrane and a requirement for high ionic strength for optimal activity. Here, we describe the isolation of the CMP-Neu5Ac hydroxylase from A. rubens gonads using anion exchange chromatography and chromatography on immobilized cytochrome b(5). The enzyme was enriched 137-fold with a yield of 13%. The preparation exhibited a main polypeptide of 76 kDa, consistent with a cDNA sequence published earlier, and a minor protein of 64 kDa. A kinetic characterization showed that salt activation of this enzyme results from an increase in affinity for CMP-Neu5Ac. Evidence for the formation of a ternary complex of hydroxylase, CMP-Neu5Ac and cytochrome b(5) is also presented. The mechanistic and physiological significance of these results is discussed.