Evidence that ubiquinone is a required intermediate for rhodoquinone biosynthesis in Rhodospirillum rubrum

Rhodoquinone carries electrons in the mammalian electron transport chain

Ubiquinone (UQ), the only known electron carrier in the mammalian electron transport chain (ETC), preferentially delivers electrons to the terminal electron acceptor oxygen (O2). In hypoxia, ubiquinol (UQH2) diverts these electrons onto fumarate instead. Here, we identify rhodoquinone (RQ), an electron carrier detected in mitochondria purified from certain mouse and human tissues that preferentially delivers electrons to fumarate through the reversal of succinate dehydrogenase, independent of environmental O2 levels. The RQ/fumarate ETC is strictly present in vivo and is undetectable in cultured mammalian cells. Using genetic and pharmacologic tools that reprogram the ETC from the UQ/O2 to the RQ/fumarate pathway, we establish that these distinct ETCs support unique programs of mitochondrial function and that RQ confers protection upon hypoxia exposure in vitro and in vivo. Thus, in discovering the presence of RQ in mammals, we unveil a tractable therapeutic strategy that exploits flexibility in the ETC to ameliorate hypoxia-related conditions.

Review of cancer cell volatile organic compounds: their metabolism and evolution

Cancer is ranked as the top cause of premature mortality. Volatile organic compounds (VOCs) are produced from catalytic peroxidation by reactive oxygen species (ROS) and have become a highly attractive non-invasive cancer screening approach. For future clinical applications, however, the correlation between cancer hallmarks and cancer-specific VOCs requires further study. This review discusses and compares cellular metabolism, signal transduction as well as mitochondrial metabolite translocation in view of cancer evolution and the basic biology of VOCs production. Certain cancerous characteristics as well as the origin of the ROS removal system date back to procaryotes and early eukaryotes and share commonalities with non-cancerous proliferative cells. This calls for future studies on metabolic cross talks and regulation of the VOCs production pathway.

Structural diversity and modularity of photosynthetic RC-LH1 complexes

Bacterial photosynthesis is essential for sustaining life on Earth as it aids in carbon assimilation, atmospheric composition, and ecosystem maintenance. Many bacteria utilize anoxygenic photosynthesis to convert sunlight into chemical energy while producing organic matter. The core machinery of anoxygenic photosynthesis performed by purple photosynthetic bacteria and Chloroflexales is the reaction center-light-harvesting 1 (RC-LH1) pigment-protein supercomplex. In this review, we discuss recent structural studies of RC-LH1 core complexes based on the advancement in structural biology techniques. These studies have provided fundamental insights into the assembly mechanisms, structural variations, and modularity of RC-LH1 complexes across different bacterial species, highlighting their functional adaptability. Understanding the natural architectures of RC-LH1 complexes will facilitate the design and engineering of artificial photosynthetic systems, which can enhance photosynthetic efficiency and potentially find applications in sustainable energy production and carbon capture.

S-Adenosylmethionine: more than just a methyl donor

Covering: from 2000 up to the very early part of 2023S-Adenosyl-L-methionine (SAM) is a naturally occurring trialkyl sulfonium molecule that is typically associated with biological methyltransfer reactions. However, SAM is also known to donate methylene, aminocarboxypropyl, adenosyl and amino moieties during natural product biosynthetic reactions. The reaction scope is further expanded as SAM itself can be modified prior to the group transfer such that a SAM-derived carboxymethyl or aminopropyl moiety can also be transferred. Moreover, the sulfonium cation in SAM has itself been found to be critical for several other enzymatic transformations. Thus, while many SAM-dependent enzymes are characterized by a methyltransferase fold, not all of them are necessarily methyltransferases. Furthermore, other SAM-dependent enzymes do not possess such a structural feature suggesting diversification along different evolutionary lineages. Despite the biological versatility of SAM, it nevertheless parallels the chemistry of sulfonium compounds used in organic synthesis. The question thus becomes how enzymes catalyze distinct transformations via subtle differences in their active sites. This review summarizes recent advances in the discovery of novel SAM utilizing enzymes that rely on Lewis acid/base chemistry as opposed to radical mechanisms of catalysis. The examples are categorized based on the presence of a methyltransferase fold and the role played by SAM within the context of known sulfonium chemistry.

Microbial rhodoquinone biosynthesis proceeds via an atypical RquA-catalyzed amino transfer from S-adenosyl-L-methionine to ubiquinone

Rhodoquinone (RQ) is a close analogue of ubiquinone (UQ) that confers diverse bacterial and eukaryotic taxa the ability to utilize fumarate as an electron acceptor in hypoxic conditions. The RquA protein, identified in a Rhodospirillum rubrum RQ-deficient mutant, has been shown to be required for RQ biosynthesis in bacteria. In this report, we demonstrate that RquA, homologous to SAM-dependent methyltransferases, is necessary and sufficient to catalyze RQ biosynthesis from UQ in vitro. Remarkably, we show that RquA uses SAM as the amino group donor in a substitution reaction that converts UQ to RQ. In contrast to known aminotransferases, RquA does not use pyridoxal 5'-phosphate (PLP) as a coenzyme, but requires the presence of Mn²⁺ as a cofactor. As these findings reveal, RquA provides an example of a non-canonical SAM-dependent enzyme that does not catalyze methyl transfer, instead it uses SAM in an atypical amino transfer mechanism.

Transcriptome Profiling Revealed Multiple rquA Genes in the Species of Spirostomum (Protozoa: Ciliophora: Heterotrichea)

Adaptation to life at different oxygen tensions plays a role in protozoan ecology and controls the distribution of different species in anoxic habitats. The ciliate genus Spirostomum inhabiting fresh or low salinity water globally where these species are considered as bioindicators. Under anaerobic or low oxygen conditions, the rhodoquinol-dependent pathway has been reported in the species from the class Heterotrichea. With the help of RNA sequencing (RNAseq) data, Spirostomum spp., are suitable for deep molecular investigations on rquA for rhodoquinone (RQ) biosynthesis. In this study, Spirostomum ambiguum, Spirostomum subtilis, and Spirostomum teres collected from densely vegetated freshwater habitat in Fuzhou, China, explored the evidence of rquA. Based on transcriptome analysis, two to three RquA proteins were identified in S. ambiguum, S. teres, and S. subtilis, respectively. The presence of a key Motif-I of RquA and mitochondrial targeting signals (MTS), also confirmed the identity of these as RquA. Furthermore, Spirostomum RquA proteins could be sorted into two groups based on their conserved amino acid (CAA) residues. Phylogenetic analysis also exhibited RquA division into two subclades contained RquA1 and RquA2/RquA3 and supports two to three paralogs of rquA genes in the genomes Spirostomum spp. Additional transcriptomes and genomes analysis of Blepharisma spp., and Stentor spp., respectively, also revealed at least two paralogs of rquA in members of the class Heterotrichea. The present study provides evidence for the presence of RquA and rhodoquinol dependent fumarate reduction pathway in Spirostomum species potentially use to respire in the oxygen-depleted habitats and two to three diverse rquA genes.

Alternative splicing of coq-2 controls the levels of rhodoquinone in animals

Parasitic helminths use two benzoquinones as electron carriers in the electron transport chain. In normoxia, they use ubiquinone (UQ), but in anaerobic conditions inside the host, they require rhodoquinone (RQ) and greatly increase RQ levels. We previously showed the switch from UQ to RQ synthesis is driven by a change of substrates by the polyprenyltransferase COQ-2 (Del Borrello et al., 2019; Roberts Buceta et al., 2019); however, the mechanism of substrate selection is not known. Here, we show helminths synthesize two coq-2 splice forms, coq-2a and coq-2e, and the coq-2e-specific exon is only found in species that synthesize RQ. We show that in Caenorhabditis elegans COQ-2e is required for efficient RQ synthesis and survival in cyanide. Importantly, parasites switch from COQ-2a to COQ-2e as they transit into anaerobic environments. We conclude helminths switch from UQ to RQ synthesis principally via changes in the alternative splicing of coq-2.

Adaptive evolution reveals a tradeoff between growth rate and oxidative stress during naphthoquinone-based aerobic respiration

Evolution fine-tunes biological pathways to achieve a robust cellular physiology. Two and a half billion years ago, rapidly rising levels of oxygen as a byproduct of blooming cyanobacterial photosynthesis resulted in a redox upshift in microbial energetics. The appearance of higher-redox-potential respiratory quinone, ubiquinone (UQ), is believed to be an adaptive response to this environmental transition. However, the majority of bacterial species are still dependent on the ancient respiratory quinone, naphthoquinone (NQ). Gammaproteobacteria can biosynthesize both of these respiratory quinones, where UQ has been associated with aerobic lifestyle and NQ with anaerobic lifestyle. We engineered an obligate NQ-dependent γ-proteobacterium, Escherichia coli ΔubiC, and performed adaptive laboratory evolution to understand the selection against the use of NQ in an oxic environment and also the adaptation required to support the NQ-driven aerobic electron transport chain. A comparative systems-level analysis of pre- and postevolved NQ-dependent strains revealed a clear shift from fermentative to oxidative metabolism enabled by higher periplasmic superoxide defense. This metabolic shift was driven by the concerted activity of 3 transcriptional regulators (PdhR, RpoS, and Fur). Analysis of these findings using a genome-scale model suggested that resource allocation to reactive oxygen species (ROS) mitigation results in lower growth rates. These results provide a direct elucidation of a resource allocation tradeoff between growth rate and ROS mitigation costs associated with NQ usage under oxygen-replete condition.

Prenylquinones in Human Parasitic Protozoa: Biosynthesis, Physiological Functions, and Potential as Chemotherapeutic Targets

Human parasitic protozoa cause a large number of diseases worldwide and, for some of these diseases, there are no effective treatments to date, and drug resistance has been observed. For these reasons, the discovery of new etiological treatments is necessary. In this sense, parasitic metabolic pathways that are absent in vertebrate hosts would be interesting research candidates for the identification of new drug targets. Most likely due to the protozoa variability, uncertain phylogenetic origin, endosymbiotic events, and evolutionary pressure for adaptation to adverse environments, a surprising variety of prenylquinones can be found within these organisms. These compounds are involved in essential metabolic reactions in organisms, for example, prevention of lipoperoxidation, participation in the mitochondrial respiratory chain or as enzymatic cofactors. This review will describe several prenylquinones that have been previously characterized in human pathogenic protozoa. Among all existing prenylquinones, this review is focused on ubiquinone, menaquinone, tocopherols, chlorobiumquinone, and thermoplasmaquinone. This review will also discuss the biosynthesis of prenylquinones, starting from the isoprenic side chains to the aromatic head group precursors. The isoprenic side chain biosynthesis maybe come from mevalonate or non-mevalonate pathways as well as leucine dependent pathways for isoprenoid biosynthesis. Finally, the isoprenic chains elongation and prenylquinone aromatic precursors origins from amino acid degradation or the shikimate pathway is reviewed. The phylogenetic distribution and what is known about the biological functions of these compounds among species will be described, as will the therapeutic strategies associated with prenylquinone metabolism in protozoan parasites.

The kynurenine pathway is essential for rhodoquinone biosynthesis in Caenorhabditis elegans

A key metabolic adaptation of some species that face hypoxia as part of their life cycle involves an alternative electron transport chain in which rhodoquinone (RQ) is required for fumarate reduction and ATP production. RQ biosynthesis in bacteria and protists requires ubiquinone (Q) as a precursor. In contrast, Q is not a precursor for RQ biosynthesis in animals such as parasitic helminths, and most details of this pathway have remained elusive. Here, we used Caenorhabditis elegans as a model animal to elucidate key steps in RQ biosynthesis. Using RNAi and a series of C. elegans mutants, we found that arylamine metabolites from the kynurenine pathway are essential precursors for RQ biosynthesis de novo Deletion of kynu-1, encoding a kynureninase that converts l-kynurenine (KYN) to anthranilic acid (AA) and 3-hydroxykynurenine (3HKYN) to 3-hydroxyanthranilic acid (3HAA), completely abolished RQ biosynthesis but did not affect Q levels. Deletion of kmo-1, which encodes a kynurenine 3-monooxygenase that converts KYN to 3HKYN, drastically reduced RQ but not Q levels. Knockdown of the Q biosynthetic genes coq-5 and coq-6 affected both Q and RQ levels, indicating that both biosynthetic pathways share common enzymes. Our study reveals that two pathways for RQ biosynthesis have independently evolved. Unlike in bacteria, where amination is the last step in RQ biosynthesis, in worms the pathway begins with the arylamine precursor AA or 3HAA. Because RQ is absent in mammalian hosts of helminths, inhibition of RQ biosynthesis may have potential utility for targeting parasitic infections that cause important neglected tropical diseases.

Rhodoquinone biosynthesis in C. elegans requires precursors generated by the kynurenine pathway

Parasitic helminths infect over a billion humans. To survive in the low oxygen environment of their hosts, these parasites use unusual anaerobic metabolism - this requires rhodoquinone (RQ), an electron carrier that is made by very few animal species. Crucially RQ is not made or used by any parasitic hosts and RQ synthesis is thus an ideal target for anthelmintics. However, little is known about how RQ is made and no drugs are known to block RQ synthesis. C. elegans makes RQ and can use RQ-dependent metabolic pathways - here, we use C. elegans genetics to show that tryptophan degradation via the kynurenine pathway is required to generate the key amine-containing precursors for RQ synthesis. We show that C. elegans requires RQ for survival in hypoxic conditions and, finally, we establish a high throughput assay for drugs that block RQ-dependent metabolism. This may drive the development of a new class of anthelmintic drugs. This study is a key first step in understanding how RQ is made in parasitic helminths.

Recombinant RquA catalyzes the in vivo conversion of ubiquinone to rhodoquinone in Escherichia coli and Saccharomyces cerevisiae

Terpenoid quinones are liposoluble redox-active compounds that serve as essential electron carriers and antioxidants. One such quinone, rhodoquinone (RQ), couples the respiratory electron transfer chain to the reduction of fumarate to facilitate anaerobic respiration. This mechanism allows RQ-synthesizing organisms to operate their respiratory chain using fumarate as a final electron acceptor. RQ biosynthesis is restricted to a handful of prokaryotic and eukaryotic organisms, and details of this biosynthetic pathway remain enigmatic. One gene, rquA, was discovered to be required for RQ biosynthesis in Rhodospirillum rubrum. However, the function of the gene product, RquA, has remained unclear. Here, using reverse genetics approaches, we demonstrate that RquA converts ubiquinone to RQ directly. We also demonstrate the first in vivo synthetic production of RQ in Escherichia coli and Saccharomyces cerevisiae, two organisms that do not natively produce RQ. These findings help clarify the complete RQ biosynthetic pathway in species which contain RquA homologs.

Investigation of candidate genes involved in the rhodoquinone biosynthetic pathway in Rhodospirillum rubrum

The lipophilic electron-transport cofactor rhodoquinone (RQ) facilitates anaerobic metabolism in a variety of bacteria and selected eukaryotic organisms in hypoxic environments. We have shown that an intact rquA gene in Rhodospirillum rubrum is required for RQ production and efficient growth of the bacterium under anoxic conditions. While the explicit details of RQ biosynthesis have yet to be fully delineated, ubiquinone (Q) is a required precursor to RQ in R. rubrum, and the RquA gene product is homologous to a class I methyltransferase. In order to identify any additional requirements for RQ biosynthesis or factors influencing RQ production in R. rubrum, we performed transcriptome analysis to identify differentially expressed genes in anoxic, illuminated R. rubrum cultures, compared with those aerobically grown in the dark. To further select target genes, we employed a bioinformatics approach to assess the likelihood that a given differentially expressed gene under anoxic conditions may also have a direct role in RQ production or regulation of its levels in vivo. Having thus compiled a list of candidate genes, nine were chosen for further study by generation of knockout strains. RQ and Q levels were quantified using liquid chromatography-mass spectrometry, and rquA gene expression was measured using the real-time quantitative polymerase chain reaction. In one case, Q and RQ levels were decreased relative to wild type; in another case, the opposite effect was observed. These results comport with the crucial roles of rquA and Q in RQ biosynthesis, and reveal the existence of potential modulators of RQ levels in R. rubrum.

Synthesis of 11 C-labeled ubiquinone and ubiquinol via Pd0 -mediated rapid C-[11 C]methylation using [11 C]methyl iodide and 39-demethyl-39-(pinacolboryl)ubiquinone

To enable positron emission tomography (PET) imaging of the in vivo kinetics of ubiquinone and ubiquinol, which is referred to as coenzyme Q₁₀, their ¹¹C‐radiolabeled counterparts were synthesized herein. ¹¹C‐Labeled ubiquinone [¹¹C]‐1 was realized by Pd‐mediated rapid C‐[¹¹C]methylation of [¹¹C]CH₃I with 39‐demethyl‐39‐(pinacolboryl)ubiquinone, prepared by Ru‐catalyzed olefin metathesis of unradiolabeled ubiquinone with 2‐(pinacolboryl)propene. Subsequent reduction of [¹¹C]‐1 using Na₂S₂O₄ yielded ¹¹C‐labeled ubiquinol [¹¹C]‐2. The synthesis time and [¹¹C]CH₃I‐based radiochemical yield of [¹¹C]‐1 were within 36 minutes and up to 53%, while those of [¹¹C]‐2 were within 38 minutes and up to 39%, respectively. After radiopharmaceutical formulation, the qualities of [¹¹C]‐1 and [¹¹C]‐2 were confirmed to be applicable for animal PET studies. The analytical values of [¹¹C]‐1 and [¹¹C]‐2 are as follows: radioactivity of up to 3.5 and 1.4 GBq, molar activity of 21 to 78 and 48 to 76 GBq/μmol, radiochemical purity of greater than 99% and greater than 95%, and chemical purity of greater than 99% and 77%, respectively. The concept behind this radiolabeling procedure is that unradiolabeled natural ubiquinone can be converted to ¹¹C‐radiolabeled ubiquinone and ubiquinol via a pinacolborane‐substituted ubiquinone derivative. Each PET probe was used for molecular imaging using rats to investigate the in vivo kinetics and biodistribution of the coenzyme Q₁₀.

Microbial eukaryotes have adapted to hypoxia by horizontal acquisitions of a gene involved in rhodoquinone biosynthesis

Under hypoxic conditions, some organisms use an electron transport chain consisting of only complex I and II (CII) to generate the proton gradient essential for ATP production. In these cases, CII functions as a fumarate reductase that accepts electrons from a low electron potential quinol, rhodoquinol (RQ). To clarify the origins of RQ-mediated fumarate reduction in eukaryotes, we investigated the origin and function of rquA, a gene encoding an RQ biosynthetic enzyme. RquA is very patchily distributed across eukaryotes and bacteria adapted to hypoxia. Phylogenetic analyses suggest lateral gene transfer (LGT) of rquA from bacteria to eukaryotes occurred at least twice and the gene was transferred multiple times amongst protists. We demonstrate that RquA functions in the mitochondrion-related organelles of the anaerobic protist Pygsuia and is correlated with the presence of RQ. These analyses reveal the role of gene transfer in the evolutionary remodeling of mitochondria in adaptation to hypoxia.

The Earliest Stages of Mitochondrial Adaptation to Low Oxygen Revealed in a Novel Rhizarian

Mitochondria exist on a functional and evolutionary continuum that includes anaerobic mitochondrion-related organelles (MROs), such as hydrogenosomes. Hydrogenosomes lack many classical mitochondrial features, including conspicuous cristae, mtDNA, the tricarboxylic acid (TCA) cycle, and ATP synthesis powered by an electron transport chain (ETC); instead, they produce ATP anaerobically, liberating H₂ and CO₂ gas in the process. However, our understanding of the evolutionary transformation from aerobic mitochondria to various MRO types remains incomplete. Here we describe a novel MRO from a cercomonad (Brevimastigomonas motovehiculus n. sp.; Rhizaria). We have sequenced its 30,608-bp mtDNA and characterized organelle function through a combination of transcriptomic, genomic, and cell biological approaches. B. motovehiculus MROs are metabolically versatile, retaining mitochondrial metabolic pathways, such as a TCA cycle and ETC-driven ATP synthesis, but also possessing hydrogenosomal-type pyruvate metabolism and substrate-level phosphorylation. Notably, the B. motovehiculus ETC is degenerate and appears to be losing cytochrome-based electron transport (complexes III and IV). Furthermore, the F₁F_o ATP synthase (complex V) is unique, with the highly conserved Atpα subunit fragmented into four separate pieces. The B. motovehiculus MRO appears to be in the process of losing aerobic metabolic capacities. Our findings shed light on the transition between organelle types, specifically the early stages of mitochondrial adaptation to anaerobiosis.

Powered by light: Phototrophy and photosynthesis in prokaryotes and its evolution

Photosynthesis is a complex metabolic process enabling photosynthetic organisms to use solar energy for the reduction of carbon dioxide into biomass. This ancient pathway has revolutionized life on Earth. The most important event was the development of oxygenic photosynthesis. It had a tremendous impact on the Earth's geochemistry and the evolution of living beings, as the rise of atmospheric molecular oxygen enabled the development of a highly efficient aerobic metabolism, which later led to the evolution of complex multicellular organisms. The mechanism of photosynthesis has been the subject of intensive research and a great body of data has been accumulated. However, the evolution of this process is not fully understood, and the development of photosynthesis in prokaryota in particular remains an unresolved question. This review is devoted to the occurrence and main features of phototrophy and photosynthesis in prokaryotes. Hypotheses concerning the origin and spread of photosynthetic traits in bacteria are also discussed.

Knockdown of the coenzyme Q synthesis gene Smed-dlp1 affects planarian regeneration and tissue homeostasis

The freshwater planarian is a model organism used to study tissue regeneration that occupies an important position among multicellular organisms. Planarian genomic databases have led to the identification of genes that are required for regeneration, with implications for their roles in its underlying mechanism. Coenzyme Q (CoQ) is a fundamental lipophilic molecule that is synthesized and expressed in every cell of every organism. Furthermore, CoQ levels affect development, life span, disease and aging in nematodes and mice. Because CoQ can be ingested in food, it has been used in preventive nutrition. In this study, we investigated the role of CoQ in planarian regeneration. Planarians synthesize both CoQ9 and rhodoquinone 9 (RQ9). Knockdown of Smed-dlp1, a trans-prenyltransferase gene that encodes an enzyme that synthesizes the CoQ side chain, led to a decrease in CoQ9 and RQ9 levels. However, ATP levels did not consistently decrease in these animals. Knockdown animals exhibited tissue regression and curling. The number of mitotic cells decreased in Smed-dlp1 (RNAi) animals. These results suggested a failure in physiological cell turnover and stem cell function. Accordingly, regenerating planarians died from lysis or exhibited delayed regeneration. Interestingly, the observed phenotypes were partially rescued by ingesting food supplemented with α-tocopherol. Taken together, our results suggest that oxidative stress induced by reduced CoQ9 levels affects planarian regeneration and tissue homeostasis.

The futalosine pathway played an important role in menaquinone biosynthesis during early prokaryote evolution

Menaquinone (MK) is an important component of the electron-transfer system in prokaryotes. One of its precursors, 1,4-dihydroxy-2-naphthoate, can be synthesized from chorismate by the classical MK pathway. Interestingly, in some bacteria, chorismate can also be converted to 1,4-dihydroxy-6-naphthoate by four enzymes encoded by mqnABCD in an alternative futalosine pathway. In this study, six crucial enzymes belonging to these two independent nonhomologous pathways were identified in the predicted proteomes of prokaryotes representing a broad phylogenetic distribution. Although the classical MK pathway was found in 32.1% of the proteomes, more than twice the proportion containing the futalosine pathway, the latter was found in a broader taxonomic range of organisms (18 of 31 phyla). The prokaryotes equipped with the classical MK pathway were almost all aerobic or facultatively anaerobic, but those with the futalosine pathway were not only aerobic or facultatively anaerobic but also anaerobic. Phylogenies of enzymes of the classical MK pathway indicated that its genes in archaea were probably acquired by an ancient horizontal gene transfer from bacterial donors. Therefore, the organization of the futalosine pathway likely predated that of the classical MK pathway in the evolutionary history of prokaryotes.

A SUF Fe-S cluster biogenesis system in the mitochondrion-related organelles of the anaerobic protist Pygsuia

BACKGROUND

Many microbial eukaryotes have evolved anaerobic alternatives to mitochondria known as mitochondrion-related organelles (MROs). Yet, only a few of these have been experimentally investigated. Here we report an RNA-seq-based reconstruction of the MRO proteome of Pygsuia biforma, an anaerobic representative of an unexplored deep-branching eukaryotic lineage.

RESULTS

Pygsuia's MRO has a completely novel suite of functions, defying existing "function-based" organelle classifications. Most notable is the replacement of the mitochondrial iron-sulfur cluster machinery by an archaeal sulfur mobilization (SUF) system acquired via lateral gene transfer (LGT). Using immunolocalization in Pygsuia and heterologous expression in yeast, we show that the SUF system does indeed localize to the MRO. The Pygsuia MRO also possesses a unique assemblage of features, including: cardiolipin, phosphonolipid, amino acid, and fatty acid metabolism; a partial Kreb's cycle; a reduced respiratory chain; and a laterally acquired rhodoquinone (RQ) biosynthesis enzyme. The latter observation suggests that RQ is an electron carrier of a fumarate reductase-type complex II in this MRO.

CONCLUSIONS

The unique functional profile of this MRO underscores the tremendous plasticity of mitochondrial function within eukaryotes and showcases the role of LGT in forging metabolic mosaics of ancestral and newly acquired organellar pathways.

Does menaquinone participate in brain astrocyte electron transport?

Quinone compounds act as membrane resident carriers of electrons between components of the electron transport chain in the periplasmic space of prokaryotes and in the mitochondria of eukaryotes. Vitamin K is a quinone compound in the human body in a storage form as menaquinone (MK); distribution includes regulated amounts in mitochondrial membranes. The human brain, which has low amounts of typical vitamin K dependent function (e.g., gamma carboxylase) has relatively high levels of MK, and different regions of brain have different amounts. Coenzyme Q (Q), is a quinone synthesized de novo, and the levels of synthesis decline with age. The levels of MK are dependent on dietary intake and generally increase with age. MK has a characterized role in the transfer of electrons to fumarate in prokaryotes. A newly recognized fumarate cycle has been identified in brain astrocytes. The MK precursor menadione has been shown to donate electrons directly to mitochondrial complex III.

HYPOTHESIS

Vitamin K compounds function in the electron transport chain of human brain astrocytes.

Mitochondrial respiration without ubiquinone biosynthesis

Ubiquinone (UQ), a.k.a. coenzyme Q, is a redox-active lipid that participates in several cellular processes, in particular mitochondrial electron transport. Primary UQ deficiency is a rare but severely debilitating condition. Mclk1 (a.k.a. Coq7) encodes a conserved mitochondrial enzyme that is necessary for UQ biosynthesis. We engineered conditional Mclk1 knockout models to study pathogenic effects of UQ deficiency and to assess potential therapeutic agents for the treatment of UQ deficiencies. We found that Mclk1 knockout cells are viable in the total absence of UQ. The UQ biosynthetic precursor DMQ9 accumulates in these cells and can sustain mitochondrial respiration, albeit inefficiently. We demonstrated that efficient rescue of the respiratory deficiency in UQ-deficient cells by UQ analogues is side chain length dependent, and that classical UQ analogues with alkyl side chains such as idebenone and decylUQ are inefficient in comparison with analogues with isoprenoid side chains. Furthermore, Vitamin K2, which has an isoprenoid side chain, and has been proposed to be a mitochondrial electron carrier, had no efficacy on UQ-deficient mouse cells. In our model with liver-specific loss of Mclk1, a large depletion of UQ in hepatocytes caused only a mild impairment of respiratory chain function and no gross abnormalities. In conjunction with previous findings, this surprisingly small effect of UQ depletion indicates a nonlinear dependence of mitochondrial respiratory capacity on UQ content. With this model, we also showed that diet-derived UQ10 is able to functionally rescue the electron transport deficit due to severe endogenous UQ deficiency in the liver, an organ capable of absorbing exogenous UQ.

Molecular genetics of ubiquinone biosynthesis in animals

Ubiquinone (UQ), also known as coenzyme Q (CoQ), is a redox-active lipid present in all cellular membranes where it functions in a variety of cellular processes. The best known functions of UQ are to act as a mobile electron carrier in the mitochondrial respiratory chain and to serve as a lipid soluble antioxidant in cellular membranes. All eukaryotic cells synthesize their own UQ. Most of the current knowledge on the UQ biosynthetic pathway was obtained by studying Escherichia coli and Saccharomyces cerevisiae UQ-deficient mutants. The orthologues of all the genes known from yeast studies to be involved in UQ biosynthesis have subsequently been found in higher organisms. Animal mutants with different genetic defects in UQ biosynthesis display very different phenotypes, despite the fact that in all these mutants the same biosynthetic pathway is affected. This review summarizes the present knowledge of the eukaryotic biosynthesis of UQ, with focus on the biosynthetic genes identified in animals, including Caenorhabditis elegans, rodents, and humans. Moreover, we review the phenotypes of mutants in these genes and discuss the functional consequences of UQ deficiency in general.

Anaerobic energy metabolism in unicellular photosynthetic eukaryotes

Anaerobic metabolic pathways allow unicellular organisms to tolerate or colonize anoxic environments. Over the past ten years, genome sequencing projects have brought a new light on the extent of anaerobic metabolism in eukaryotes. A surprising development has been that free-living unicellular algae capable of photoautotrophic lifestyle are, in terms of their enzymatic repertoire, among the best equipped eukaryotes known when it comes to anaerobic energy metabolism. Some of these algae are marine organisms, common in the oceans, others are more typically soil inhabitants. All these species are important from the ecological (O(2)/CO(2) budget), biotechnological, and evolutionary perspectives. In the unicellular algae surveyed here, mixed-acid type fermentations are widespread while anaerobic respiration, which is more typical of eukaryotic heterotrophs, appears to be rare. The presence of a core anaerobic metabolism among the algae provides insights into its evolutionary origin, which traces to the eukaryote common ancestor. The predicted fermentative enzymes often exhibit an amino acid extension at the N-terminus, suggesting that these proteins might be compartmentalized in the cell, likely in the chloroplast or the mitochondrion. The green algae Chlamydomonas reinhardtii and Chlorella NC64 have the most extended set of fermentative enzymes reported so far. Among the eukaryotes with secondary plastids, the diatom Thalassiosira pseudonana has the most pronounced anaerobic capabilities as yet. From the standpoints of genomic, transcriptomic, and biochemical studies, anaerobic energy metabolism in C. reinhardtii remains the best characterized among photosynthetic protists. This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems.

Biochemistry and evolution of anaerobic energy metabolism in eukaryotes

Major insights into the phylogenetic distribution, biochemistry, and evolutionary significance of organelles involved in ATP synthesis (energy metabolism) in eukaryotes that thrive in anaerobic environments for all or part of their life cycles have accrued in recent years. All known eukaryotic groups possess an organelle of mitochondrial origin, mapping the origin of mitochondria to the eukaryotic common ancestor, and genome sequence data are rapidly accumulating for eukaryotes that possess anaerobic mitochondria, hydrogenosomes, or mitosomes. Here we review the available biochemical data on the enzymes and pathways that eukaryotes use in anaerobic energy metabolism and summarize the metabolic end products that they generate in their anaerobic habitats, focusing on the biochemical roles that their mitochondria play in anaerobic ATP synthesis. We present metabolic maps of compartmentalized energy metabolism for 16 well-studied species. There are currently no enzymes of core anaerobic energy metabolism that are specific to any of the six eukaryotic supergroup lineages; genes present in one supergroup are also found in at least one other supergroup. The gene distribution across lineages thus reflects the presence of anaerobic energy metabolism in the eukaryote common ancestor and differential loss during the specialization of some lineages to oxic niches, just as oxphos capabilities have been differentially lost in specialization to anoxic niches and the parasitic life-style. Some facultative anaerobes have retained both aerobic and anaerobic pathways. Diversified eukaryotic lineages have retained the same enzymes of anaerobic ATP synthesis, in line with geochemical data indicating low environmental oxygen levels while eukaryotes arose and diversified.

Identification of a new gene required for the biosynthesis of rhodoquinone in Rhodospirillum rubrum

Rhodoquinone (RQ) is a required cofactor for anaerobic respiration in Rhodospirillum rubrum, and it is also found in several helminth parasites that utilize a fumarate reductase pathway. RQ is an aminoquinone that is structurally similar to ubiquinone (Q), a polyprenylated benzoquinone used in the aerobic respiratory chain. RQ is not found in humans or other mammals, and therefore, the inhibition of its biosynthesis may provide a novel antiparasitic drug target. To identify a gene specifically required for RQ biosynthesis, we determined the complete genome sequence of a mutant strain of R. rubrum (F11), which cannot grow anaerobically and does not synthesize RQ, and compared it with that of a spontaneous revertant (RF111). RF111 can grow anaerobically and has recovered the ability to synthesize RQ. The two strains differ by a single base pair, which causes a nonsense mutation in the putative methyltransferase gene rquA. To test whether this mutation is important for the F11 phenotype, the wild-type rquA gene was cloned into the pRK404E1 vector and conjugated into F11. Complementation of the anaerobic growth defect in F11 was observed, and liquid chromatography-time of flight mass spectrometry (LC-TOF-MS) analysis of lipid extracts confirmed that plasmid-complemented F11 was able to synthesize RQ. To further validate the requirement of rquA for RQ biosynthesis, we generated a deletion mutant from wild-type R. rubrum by the targeted replacement of rquA with a gentamicin resistance cassette. The ΔrquA mutant exhibited the same phenotype as that of F11. These results are significant because rquA is the first gene to be discovered that is required for RQ biosynthesis.

para-Aminobenzoic acid is a precursor in coenzyme Q6 biosynthesis in Saccharomyces cerevisiae

Coenzyme Q (ubiquinone or Q) is a crucial mitochondrial lipid required for respiratory electron transport in eukaryotes. 4-Hydroxybenozoate (4HB) is an aromatic ring precursor that forms the benzoquinone ring of Q and is used extensively to examine Q biosynthesis. However, the direct precursor compounds and enzymatic steps for synthesis of 4HB in yeast are unknown. Here we show that para-aminobenzoic acid (pABA), a well known precursor of folate, also functions as a precursor for Q biosynthesis. A hexaprenylated form of pABA (prenyl-pABA) is normally present in wild-type yeast crude lipid extracts but is absent in yeast abz1 mutants starved for pABA. A stable (13)C(6)-isotope of pABA (p- amino[aromatic-(13)C(6)]benzoic acid ([(13)C(6)]pABA)), is prenylated in either wild-type or abz1 mutant yeast to form prenyl-[(13)C(6)]pABA. We demonstrate by HPLC and mass spectrometry that yeast incubated with either [(13)C(6)]pABA or [(13)C(6)]4HB generate both (13)C(6)-demethoxy-Q (DMQ), a late stage Q biosynthetic intermediate, as well as the final product (13)C(6)-coenzyme Q. Pulse-labeling analyses show that formation of prenyl-pABA occurs within minutes and precedes the synthesis of Q. Yeast utilizing pABA as a ring precursor produce another nitrogen containing intermediate, 4-imino-DMQ(6). This intermediate is produced in small quantities in wild-type yeast cultured in standard media and in abz1 mutants supplemented with pABA. We suggest a mechanism where Schiff base-mediated deimination forms DMQ(6) quinone, thereby eliminating the nitrogen contributed by pABA. This scheme results in the convergence of the 4HB and pABA pathways in eukaryotic Q biosynthesis and has implications regarding the action of pABA-based antifolates.

Occurrence, biosynthesis and function of isoprenoid quinones

Isoprenoid quinones are one of the most important groups of compounds occurring in membranes of living organisms. These compounds are composed of a hydrophilic head group and an apolar isoprenoid side chain, giving the molecules a lipid-soluble character. Isoprenoid quinones function mainly as electron and proton carriers in photosynthetic and respiratory electron transport chains and these compounds show also additional functions, such as antioxidant function. Most of naturally occurring isoprenoid quinones belong to naphthoquinones or evolutionary younger benzoquinones. Among benzoquinones, the most widespread and important are ubiquinones and plastoquinones. Menaquinones, belonging to naphthoquinones, function in respiratory and photosynthetic electron transport chains of bacteria. Phylloquinone K(1), a phytyl naphthoquinone, functions in the photosynthetic electron transport in photosystem I. Ubiquinones participate in respiratory chains of eukaryotic mitochondria and some bacteria. Plastoquinones are components of photosynthetic electron transport chains of cyanobacteria and plant chloroplasts. Biosynthetic pathway of isoprenoid quinones has been described, as well as their additional, recently recognized, diverse functions in bacterial, plant and animal metabolism.