Aberrant expression of cuproptosis­related gene LIPT1 is associated with metabolic dysregulation of fatty acid and prognosis in hepatocellular carcinoma.

PURPOSE: Lipoyltransferase 1 (LIPT1) has been recently identified as a cuproptosis­related gene. As a key enzyme of lipoic acid metabolism, LIPT1 has been revealed to play important roles in hereditary diseases involved with lipoic acid biosynthesis defects, while its roles in hepatocellular carcinoma (HCC) remain to be elucidated. Hence, we aimed to explore the roles and mechanisms of LIPT1 in HCC progression. METHODS: The expression of LIPT1 in HCC tissues and its clinical significance for HCC were evaluated by bioinformatic analysis and in our patient cohort. The influences of LIPT1 on the growth, migration, and lipid metabolism of HCC cells were assessed in vitro. The underlying mechanisms were explored using gene set enrichment analysis (GSEA) and molecular experiments. RESULTS: LIPT1 expression was significantly elevated in HCC tissues compared to the normal tissues, and such upregulation was associated with more malignant pathological features and poor prognosis of patients with HCC. LIPT1 silencing significantly inhibited cell proliferation, migration, and lipid content. GSEA revealed that LIPT1 upregulation was significantly associated with various cancer-associated signaling pathways, including the PI3K-AKT signaling pathway and the Wnt/ß-catenin pathway. Further molecular experiments indicated that LIPT1 silencing repressed the expression of peroxisome proliferator-activated receptor gamma (PPARγ) and inactivated the AKT/GSK-3ß/ß-catenin signaling axis. CONCLUSIONS: Upregulation of LIPT1 is involved in metabolic dysregulation of fatty acid and poor prognosis of HCC patients, which suggests that LIPT1 plays an important role in reprogramming lipid metabolism and could act as a potential prognostic marker and therapeutic target for HCC.

Identification of a novel lipoic acid biosynthesis pathway reveals the complex evolution of lipoate assembly in prokaryotes.

Lipoic acid is an essential biomolecule found in all domains of life and is involved in central carbon metabolism and dissimilatory sulfur oxidation. The machineries for lipoate assembly in mitochondria and chloroplasts of higher eukaryotes, as well as in the apicoplasts of some protozoa, are all of prokaryotic origin. Here, we provide experimental evidence for a novel lipoate assembly pathway in bacteria based on a sLpl(AB) lipoate:protein ligase, which attaches octanoate or lipoate to apo-proteins, and 2 radical SAM proteins, LipS1 and LipS2, which work together as lipoyl synthase and insert 2 sulfur atoms. Extensive homology searches combined with genomic context analyses allowed us to precisely distinguish between the new and established pathways and map them on the tree of life. This not only revealed a much wider distribution of lipoate biogenesis systems than expected, in particular, the novel sLpl(AB)-LipS1/S2 pathway, and indicated a highly modular nature of the enzymes involved, with unforeseen combinations, but also provided a new framework for the evolution of lipoate assembly. Our results show that dedicated machineries for both de novo lipoate biogenesis and scavenging from the environment were implemented early in evolution and that their distribution in the 2 prokaryotic domains was shaped by a complex network of horizontal gene transfers, acquisition of additional genes, fusions, and losses. Our large-scale phylogenetic analyses identify the bipartite archaeal LplAB ligase as the ancestor of the bacterial sLpl(AB) proteins, which were obtained by horizontal gene transfer. LipS1/S2 have a more complex evolutionary history with multiple of such events but probably also originated in the domain archaea.

Genome-Wide Survey and Analysis of Microsatellites in Waterlily, and Potential for Polymorphic Marker Development.

Waterlily (Nymphaeaceae), a diploid dicotyledon, is an ornamental aquatic plant. In 2020, the complete draft genome for the blue-petal waterlily (Nymphaea colorata) was made available in GenBank. To date, the genome-wide mining of microsatellites or simple sequence repeats (SSRs) in waterlily is still absent. In the present study, we investigated the characteristics of genome-wide microsatellites for N. colorata and developed polymorphic SSR markers across tropical and hardy waterlilies. A total of 238,816 SSRs were identified in 14 N. colorata chromosomes with an average density of 662.60 SSRs per Mb, and the largest number of SSRs were present on chromosome 1 (n = 30,426, 705.94 SSRs per Mb). The dinucleotide was the most common type, and AT-rich repeats prevail in the N. colorata genome. The SSR occurrence frequencies decreased as the number of motif repeats increased. Among 2442 protein-coding region SSRs, trinucleotides, accounting for 63.84%, were the most abundant. Gene ontology terms for signal transduction (e.g., GO: 0045859 and GO: 0019887) and the lipoic acid metabolism (ko00785,) were overrepresented in GO and KEGG enrichment analysis, respectively. In addition, 107,152 primer pairs were identified, and 13 novel polymorphism SSR markers were employed to distinguish among nine waterlily cultivars, of which Ny-5.2 and Ny-10.1 were the most informative SSR loci. This study contributes the first detailed characterization of SSRs in N. colorata genomes and delivers 13 novel polymorphism markers, which are useful for the molecular breeding strategies, genetic diversity and population structure analysis of waterlily.

Defining Caenorhabditis elegans as a model system to investigate lipoic acid metabolism.

Lipoic acid (LA) is a sulfur-containing cofactor that covalently binds to a variety of cognate enzymes that are essential for redox reactions in all three domains of life. Inherited mutations in the enzymes that make LA, namely lipoyl synthase, octanoyltransferase, and amidotransferase, result in devastating human metabolic disorders. Unfortunately, because many aspects of this essential pathway are still obscure, available treatments only serve to alleviate symptoms. We envisioned that the development of an organismal model system might provide new opportunities to interrogate LA biochemistry, biology, and physiology. Here we report our investigations on three Caenorhabditis elegans orthologous proteins involved in this post-translational modification. We established that M01F1.3 is a lipoyl synthase, ZC410.7 an octanoyltransferase, and C45G3.3 an amidotransferase. Worms subjected to RNAi against M01F1.3 and ZC410.7 manifest larval arrest in the second generation. The arrest was not rescued by LA supplementation, indicating that endogenous synthesis of LA is essential for C. elegans development. Expression of the enzymes M01F1.3, ZC410.7, and C45G3.3 completely rescue bacterial or yeast mutants affected in different steps of the lipoylation pathway, indicating functional overlap. Thus, we demonstrate that, similarly to humans, C. elegans is able to synthesize LA de novo via a lipoyl-relay pathway, and suggest that this nematode could be a valuable model to dissect the role of protein mislipoylation and to develop new therapies.

Mutational studies on Leishmania donovani dihydrolipoamide dehydrogenase (LdBPK291950.1) indicates that the enzyme may not be classical class-I pyridine nucleotide-disulfide oxidoreductase.

We report biochemical studies on two Cys residues mutation (Cys15Thr, Cys38Gly) nearest to the active site and three other amino acid substitution mutations expected to be the part of active site of LdDLDH_Variant1. Our biochemical studies show that the replacement of Cys15 increases the Km for dihydrolipoamide (DLD) substrate by five folds and NAD+ by three fold indicating that this mutation affects the binding of DLD and NAD+ significantly. Cys38 was also mutated to 'Gly' which resulted in nine fold greater Km for NAD+ without affecting Km for DLD. However, even after these mutations (Cys15Thr and Cys38Gly), reduced enzyme activity suggests that both the 'Cys' residues are not involved in disulfide bond formation but affect the binding of substrates. The data hints towards the possibility of a different catalytic mechanism from the classical class I - pyridine nucleotide-disulfide oxidoreductase. Remaining other mutated residues Ala48Ile, Asp49Gly, and Ala54Ile showed an increase in two to three-folds Km value for NAD+, which means these residues are important for the binding of NAD+ to the enzyme. However, Ala48Ile and Asp49Gly mutations showed a decrease of Km for DLD. Apart from the mutational studies, localization of LdDLDH_Variant2 of LdDLDH was also analyzed.

Unravelling the lipoyl-relay of exogenous lipoate utilization in Bacillus subtilis.

Lipoate is an essential cofactor for key enzymes of oxidative and one-carbon metabolism. It is covalently attached to E2 subunits of dehydrogenase complexes and GcvH, the H subunit of the glycine cleavage system. Bacillus subtilis possess two protein lipoylation pathways: biosynthesis and scavenging. The former requires octanoylation of GcvH, insertion of sulfur atoms and amidotransfer of the lipoate to E2s, catalyzed by LipL. Lipoate scavenging is mediated by a lipoyl protein ligase (LplJ) that catalyzes a classical two-step ATP-dependent reaction. Although these pathways were thought to be redundant, a ∆lipL mutant, in which the endogenous lipoylation pathway of E2 subunits is blocked, showed growth defects in minimal media even when supplemented with lipoate and despite the presence of a functional LplJ. In this study, we demonstrate that LipL is essential to modify E2 subunits of branched chain ketoacid and pyruvate dehydrogenases during lipoate scavenging. The crucial role of LipL during lipoate utilization relies on the strict substrate specificity of LplJ, determined by charge complementarity between the ligase and the lipoylable subunits. This new lipoyl-relay required for lipoate scavenging highlights the relevance of the amidotransferase as a valid target for the design of new antimicrobial agents among Gram-positive pathogens.

Protein moonlighting elucidates the essential human pathway catalyzing lipoic acid assembly on its cognate enzymes.

The lack of attachment of lipoic acid to its cognate enzyme proteins results in devastating human metabolic disorders. These mitochondrial disorders are evident soon after birth and generally result in early death. The mutations causing specific defects in lipoyl assembly map in three genes, LIAS, LIPT1, and LIPT2 Although physiological roles have been proposed for the encoded proteins, only the LIPT1 protein had been studied at the enzyme level. LIPT1 was reported to catalyze only the second partial reaction of the classical lipoate ligase mechanism. We report that the physiologically relevant LIPT1 enzyme activity is transfer of lipoyl moieties from the H protein of the glycine cleavage system to the E2 subunits of the 2-oxoacid dehydrogenases required for respiration (e.g., pyruvate dehydrogenase) and amino acid degradation. We also report that LIPT2 encodes an octanoyl transferase that initiates lipoyl group assembly. The human pathway is now biochemically defined.

Integrated transcriptomic and proteomic analyses reveal ɑ-lipoic acid-regulated cell proliferation via Grb2-mediated signalling in hepatic cancer cells.

Hepatocellular carcinoma is the most frequent primary liver cancer worldwide. The use of antioxidants as cancer prevention and treatment agents has become a focus of research in recent years due to their limited adverse effects. Alpha lipoic acid (ɑ-LA) is synthesized in the liver and is considered a naturally occurring antioxidant. In this study, a total of 4446 differentially expressed genes (2097 down-regulated and 2349 up-regulated) were identified via RNA-Seq in HepG2 cells after exposure to α-LA for 24 hrs. Moreover, GO and KEGG pathway analyses showed that cancer-relevant cell membrane proteins were significantly affected. An interaction network analysis predicted that Grb2 might mediate the key target pathways activated by exposure to ɑ-LA. Verification of the RNA-Seq and iTRAQ results confirmed that Grb2 mediated the ɑ-LA-induced inhibition of cell proliferation in vitro. Furthermore, the analysis of human hepatocellular carcinoma specimens obtained from the GEO database showed that the expression of EGFR and Met correlated with that of Grb2. These findings provide a novel mechanism through which ɑ-LA regulates cell proliferation via the down-regulation of growth factor-stimulated Grb2 signalling.

Development and retention of a primordial moonlighting pathway of protein modification in the absence of selection presents a puzzle.

Lipoic acid is synthesized by a remarkably atypical pathway in which the cofactor is assembled on its cognate proteins. An octanoyl moiety diverted from fatty acid synthesis is covalently attached to the acceptor protein, and sulfur insertion at carbons 6 and 8 of the octanoyl moiety form the lipoyl cofactor. Covalent attachment of this cofactor is required for function of several central metabolism enzymes, including the glycine cleavage H protein (GcvH). In Bacillus subtilis, GcvH is the sole substrate for lipoate assembly. Hence lipoic acid-requiring 2-oxoacid dehydrogenase (OADH) proteins acquire the cofactor only by transfer from lipoylated GcvH. Lipoyl transfer has been argued to be the primordial pathway of OADH lipoylation. The Escherichia coli pathway where lipoate is directly assembled on both its GcvH and OADH proteins, is proposed to have arisen later. Because roughly 3 billion years separate the divergence of these bacteria, it is surprising that E. coli GcvH functionally substitutes for the B. subtilis protein in lipoyl transfer. Known and putative GcvHs from other bacteria and eukaryotes also substitute for B. subtilis GcvH in OADH modification. Because glycine cleavage is the primary GcvH role in ancestral bacteria that lack OADH enzymes, lipoyl transfer is a "moonlighting" function: that is, development of a new function while retaining the original function. This moonlighting has been conserved in the absence of selection by some, but not all, GcvH proteins. Moreover, Aquifex aeolicus encodes five putative GcvHs, two of which have the moonlighting function, whereas others function only in glycine cleavage.

Impact of mutations within the [Fe-S] cluster or the lipoic acid biosynthesis pathways on mitochondrial protein expression profiles in fibroblasts from patients.

Lipoic acid (LA) is the cofactor of the E2 subunit of mitochondrial ketoacid dehydrogenases and plays a major role in oxidative decarboxylation. De novo LA biosynthesis is dependent on LIAS activity together with LIPT1 and LIPT2. LIAS is an iron­sulfur (Fe-S) cluster-containing mitochondrial protein, like mitochondrial aconitase (mt-aco) and some subunits of respiratory chain (RC) complexes I, II and III. All of them harbor at least one [Fe-S] cluster and their activity is dependent on the mitochondrial [Fe-S] cluster (ISC) assembly machinery. Disorders in the ISC machinery affect numerous Fe-S proteins and lead to a heterogeneous group of diseases with a wide variety of clinical symptoms and combined enzymatic defects. Here, we present the biochemical profiles of several key mitochondrial [Fe-S]-containing proteins in fibroblasts from 13 patients carrying mutations in genes encoding proteins involved in either the lipoic acid (LIPT1 and LIPT2) or mitochondrial ISC biogenesis (FDX1L, ISCA2, IBA57, NFU1, BOLA3) pathway. Ten of them are new patients described for the first time. We confirm that the fibroblast is a good cellular model to study these deficiencies, except for patients presenting mutations in FDX1L and a muscular clinical phenotype. We find that oxidative phosphorylation can be affected by LA defects in LIPT1 and LIPT2 patients due to excessive oxidative stress or to another mechanism connecting LA and respiratory chain activity. We confirm that NFU1, BOLA3, ISCA2 and IBA57 operate in the maturation of [4Fe-4S] clusters and not in [2Fe-2S] protein maturation. Our work suggests a functional difference between IBA57 and other proteins involved in maturation of [Fe-S] proteins. IBA57 seems to require BOLA3, NFU1 and ISCA2 for its stability and NFU1 requires BOLA3. Finally, our study establishes different biochemical profiles for patients according to their mutated protein.

Staphylococcus aureus SufT: an essential iron-sulphur cluster assembly factor in cells experiencing a high-demand for lipoic acid.

Staphylococcus aureus SufT is composed solely of the domain of unknown function 59 (DUF59) and has a role in the maturation of iron-sulphur (Fe-S) proteins. We report that SufT is essential for S. aureus when growth is heavily reliant upon lipoamide-utilizing enzymes, but dispensable when this reliance is decreased. LipA requires Fe-S clusters for lipoic acid (LA) synthesis and a ΔsufT strain had phenotypes suggestive of decreased LA production and decreased activities of lipoamide-requiring enzymes. Fermentative growth, a null clpC allele, or decreased flux through the TCA cycle diminished the demand for LA and rendered SufT non-essential. Abundance of the Fe-S cluster carrier Nfu was increased in a ΔclpC strain and a null clpC allele was unable to suppress the LA requirement of a ΔsufT Δnfu strain. Over-expression of nfu suppressed the LA requirement of the ΔsufT strain. We propose a model wherein SufT, and by extension the DUF59, is essential for the maturation of holo-LipA in S. aureus cells experiencing a high demand for lipoamide-dependent enzymes. The findings presented suggest that the demand for products of Fe-S enzymes is a factor governing the usage of one Fe-S cluster assembly factor over another in the maturation of apo-proteins.

Differential diagnosis of lipoic acid synthesis defects.

Lipoic acid (LA) is an essential cofactor required for the activity of five multienzymatic complexes that play a central role in the mitochondrial energy metabolism: four 2-oxoacid dehydrogenase complexes [pyruvate dehydrogenase (PDH), branched-chain ketoacid dehydrogenase (BCKDH), 2-ketoglutarate dehydrogenase (2-KGDH), and 2-oxoadipate dehydrogenase (2-OADH)] and the glycine cleavage system (GCS). LA is synthesized in a complex multistep process that requires appropriate function of the mitochondrial fatty acid synthesis (mtFASII) and the biogenesis of iron-sulphur (Fe-S) clusters. Defects in the biosynthesis of LA have been reported to be associated with multiple and severe defects of the mitochondrial energy metabolism. In recent years, disease-causing mutations in genes encoding for proteins involved in LA metabolism have been reported: NFU1, BOLA3, IBA57, LIAS, GLRX5, LIPT1, ISCA2, and LIPT2. These studies represented important progress in understanding the pathophysiology and molecular bases underlying these disorders. Here we review current knowledge regarding involvement of LA synthesis defects in human diseases with special emphasis on the diagnostic strategies for these disorders. The clinical and biochemical characteristics of patients with LA synthesis defects are discussed and a workup for the differential diagnosis proposed.

Defects in mitochondrial fatty acid synthesis result in failure of multiple aspects of mitochondrial biogenesis in Saccharomyces cerevisiae.

Mitochondrial fatty acid synthesis (mtFAS) shares acetyl-CoA with the Krebs cycle as a common substrate and is required for the production of octanoic acid (C8) precursors of lipoic acid (LA) in mitochondria. MtFAS is a conserved pathway essential for respiration. In a genetic screen in Saccharomyces cerevisiae designed to further elucidate the physiological role of mtFAS, we isolated mutants with defects in mitochondrial post-translational gene expression processes, indicating a novel link to mitochondrial gene expression and respiratory chain biogenesis. In our ensuing analysis, we show that mtFAS, but not lipoylation per se, is required for respiratory competence. We demonstrate that mtFAS is required for mRNA splicing, mitochondrial translation and respiratory complex assembly, and provide evidence that not LA per se, but fatty acids longer than C8 play a role in these processes. We also show that mtFAS- and LA-deficient strains suffer from a mild haem deficiency that may contribute to the respiratory complex assembly defect. Based on our data and previously published information, we propose a model implicating mtFAS as a sensor for mitochondrial acetyl-CoA availability and a co-ordinator of nuclear and mitochondrial gene expression by adapting the mitochondrial compartment to changes in the metabolic status of the cell.

Protein expression profiles in patients carrying NFU1 mutations. Contribution to the pathophysiology of the disease.

Cofactor disorders of mitochondrial energy metabolism are a heterogeneous group of diseases with a wide variety of clinical symptoms, particular metabolic profiles and variable enzymatic defects. Mutations in NFU1 were recently identified in patients with fatal encephalopathy displaying a biochemical phenotype consistent with defects in lipoic acid-dependent enzymatic activities and respiratory chain complexes. This discovery highlighted the molecular function of NFU1 as an iron-sulfur(Fe-S) cluster protein necessary for lipoic acid biosynthesis and respiratory chain complexes activities. To understand the pathophysiological mechanisms underlying this disease we have characterized the protein expression profiles of patients carrying NFU1 mutations. Fibroblasts from patients with the p.Gly208Cys mutation showed complete absence of protein-bound lipoic acid and decreased SDHA and SDHB subunits of complex II. In contrast, subunits of other respiratory chain complexes were normal. Protein lipoylation was also decreased in muscle and liver but not in other tissues available (brain, kidney, lung) from NFU1 patients. Although levels of the respiratory chain subunits were unaltered in tissues, BN-PAGE showed an assembly defect for complex II in muscle, consistent with the low enzymatic activity of this complex. This study provides new insights into the molecular bases of NFU1 disease as well as into the regulation of NFU1 protein in human tissues. We demonstrate a ubiquitous expression of NFU1 protein and further suggest that defects in lipoic acid biosynthesis and complex II are the main molecular signature of this disease, particularly in patients carrying the p.Gly208Cys mutation.

Site-specific protein modification using lipoic acid ligase and bis-aryl hydrazone formation.

A screen of Trp37 mutants of Escherichia coli lipoic acid ligase (LplA) revealed enzymes capable of ligating an aryl-aldehyde or aryl-hydrazine substrate to LplA's 13-residue acceptor peptide. Once site-specifically attached to recombinant proteins fused to this peptide, aryl-aldehydes could be chemoselectively derivatized with hydrazine-probe conjugates, and aryl-hydrazines could be derivatized in an analogous manner with aldehyde-probe conjugates. Such two-step labeling was demonstrated for AlexaFluor568 targeting to monovalent streptavidin in vitro, and to neurexin-1ß on the surface of living mammalian cells. To further highlight this technique, we labeled the low-density lipoprotein receptor on the surface of live cells with fluorescent phycoerythrin protein to allow single-molecule imaging and tracking over time.

Protein-protein interactions in assembly of lipoic acid on the 2-oxoacid dehydrogenases of aerobic metabolism.

Lipoic acid is a covalently attached cofactor essential for the activity of 2-oxoacid dehydrogenases and the glycine cleavage system. In the absence of lipoic acid modification, the dehydrogenases are inactive, and aerobic metabolism is blocked. In Escherichia coli, two pathways for the attachment of lipoic acid exist, a de novo biosynthetic pathway dependent on the activities of the LipB and LipA proteins and a lipoic acid scavenging pathway catalyzed by the LplA protein. LipB is responsible for octanoylation of the E2 components of 2-oxoacid dehydrogenases to provide the substrates of LipA, an S-adenosyl-L-methionine radical enzyme that inserts two sulfur atoms into the octanoyl moiety to give the active lipoylated dehydrogenase complexes. We report that the intact pyruvate and 2-oxoglutarate dehydrogenase complexes specifically copurify with both LipB and LipA. Proteomic, genetic, and dehydrogenase activity data indicate that all of the 2-oxoacid dehydrogenase components are present. In contrast, LplA, the lipoate protein ligase enzyme of lipoate salvage, shows no interaction with the 2-oxoacid dehydrogenases. The interaction is specific to the dehydrogenases in that the third lipoic acid-requiring enzyme of Escherichia coli, the glycine cleavage system H protein, does not copurify with either LipA or LipB. Studies of LipB interaction with engineered variants of the E2 subunit of 2-oxoglutarate dehydrogenase indicate that binding sites for LipB reside both in the lipoyl domain and catalytic core sequences. We also report that LipB forms a very tight, albeit noncovalent, complex with acyl carrier protein. These results indicate that lipoic acid is not only assembled on the dehydrogenase lipoyl domains but that the enzymes that catalyze the assembly are also present "on site."

Dihydrolipoamide dehydrogenase (DLD) deficiency in a Spanish patient with myopathic presentation due to a new mutation in the interface domain.

We present a 32-year-old patient who, from age 7 months, developed photophobia, left-eye ptosis and progressive muscular weakness. At age 7 years, she showed normal psychomotor development, bilateral ptosis and exercise-induced weakness with severe acidosis. Basal blood and urine lactate were normal, increasing dramatically after effort. PDHc deficiency was demonstrated in muscle and fibroblasts without detectable PDHA1 mutations. Ketogenic diet was ineffective, however thiamine gave good response although bilateral ptosis and weakness with acidosis on exercise persisted. Recently, DLD gene analysis revealed a homozygous missense mutation, c.1440 A>G (p.I480M), in the interface domain. Both parents are heterozygous and DLD activity in the patient's fibroblasts is undetectable. The five patients that have been reported with DLD-interface mutations suffered fatal deteriorations. Our patient's disease is milder, only myopathic, more similar to that due to mutation p.G229C in the NAD(+)-binding domain. Two of the five patients presented mutations (p.D479V and p.R482G) very close to the present case (p.I480M). Despite differing degrees of clinical severity, all three had minimal clues to DLD deficiency, with occasional minor increases in α-ketoglutarate and branched-chain amino acids. In the two other patients, hypertrophic cardiomyopathy was a significant feature that has been attributed to moonlighting proteolytic activity of monomeric DLD, which can degrade other mitochondrial proteins, such as frataxin. Our patient does not have cardiomyopathy, suggesting that p.I480M may not affect the DLD ability to dimerize to the same extent as p.D479V and p.R482G. Our patient, with a novel mutation in the DLD interface and mild clinical symptoms, further broadens the spectrum of this enzyme defect.

Crystal structure of lipoate-protein ligase A from Escherichia coli. Determination of the lipoic acid-binding site.

Lipoate-protein ligase A (LplA) catalyzes the formation of lipoyl-AMP from lipoate and ATP and then transfers the lipoyl moiety to a specific lysine residue on the acyltransferase subunit of alpha-ketoacid dehydrogenase complexes and on H-protein of the glycine cleavage system. The lypoyllysine arm plays a pivotal role in the complexes by shuttling the reaction intermediate and reducing equivalents between the active sites of the components of the complexes. We have determined the X-ray crystal structures of Escherichia coli LplA alone and in a complex with lipoic acid at 2.4 and 2.9 angstroms resolution, respectively. The structure of LplA consists of a large N-terminal domain and a small C-terminal domain. The structure identifies the substrate binding pocket at the interface between the two domains. Lipoic acid is bound in a hydrophobic cavity in the N-terminal domain through hydrophobic interactions and a weak hydrogen bond between carboxyl group of lipoic acid and the Ser-72 or Arg-140 residue of LplA. No large conformational change was observed in the main chain structure upon the binding of lipoic acid.

Phylogenetic and immunological definition of four lipoylated proteins from Novosphingobium aromaticivorans, implications for primary biliary cirrhosis.

Novosphingobium aromaticivorans, a unique ubiquitous bacterium that metabolizes xenobiotics and activates environmental estrogens, has been suggested as a pathogenic factor in the development of primary biliary cirrhosis (PBC). To define the molecular basis of PBC sera reactivity, we investigated the characteristic of the bacterial antigens involved. We cloned and sequenced four genes from N. aromaticivorans coding for immunoreactive proteins, arbitrarily named Novo 1 through Novo 4. We subsequently analyzed these proteins for their homology to known mitochondrial proteins and defined their reactivity using monoclonal antibodies (mAbs), rabbit anti-lipoic acid antibody, and PBC/control sera. Moreover, we studied their phylogenetic relation with the known PBC autoantigens. Novo proteins have an extraordinary degree of amino acid homology with all of the major human mitochondrial autoantigens PDC-E2 (Novo 1 and 2), OGDC-E2 (Novo 3), and BCOADC-E2 (Novo 4). Moreover, Novo 1-4 contain a lipoylated domain, are recognized by AMA-positive sera, and react with specific mAbs to mitochondrial antigens. Interestingly, the phylogenetic relation of the proteins emphasizes the conservation of the lipoylated domain. In conclusion, our data provide a high degree of confidence that N. aromaticivorans may potentiate the breakdown of self tolerance in genetically susceptible individuals.