The putative L-lactate dehydrogenase from Methanococcus jannaschii is an NADPH-dependent L-malate dehydrogenase

Biochemical, structural and dynamical characterizations of the lactate dehydrogenase from Selenomonas ruminantium provide information about an intermediate evolutionary step prior to complete allosteric regulation acquisition in the super family of lactate and malate dehydrogenases

In this work, we investigated the lactate dehydrogenase (LDH) from Selenomonas ruminantium (S. rum), an enzyme that differs at key amino acid positions from canonical allosteric LDHs. The wild type (Wt) of this enzyme recognises pyuvate as all LDHs. However, introducing a single point mutation in the active site loop (I85R) allows S. Rum LDH to recognize the oxaloacetate substrate as a typical malate dehydrogenase (MalDH), whilst maintaining homotropic activation as an LDH. We report the tertiary structure of the Wt and I85RLDH mutant. The Wt S. rum enzyme structure binds NADH and malonate, whilst also resembling the typical compact R-active state of canonical LDHs. The structure of the mutant with I85R was solved in the Apo State (without ligand), and shows no large conformational reorganization such as that observed with canonical allosteric LDHs in Apo state. This is due to a local structural feature typical of S. rum LDH that prevents large-scale conformational reorganization. The S. rum LDH was also studied using Molecular Dynamics simulations, probing specific local deformations of the active site that allow the S. rum LDH to sample the T-inactive state. We propose that, with respect to the LDH/MalDH superfamily, the S. rum enzyme possesses a specificstructural and dynamical way to ensure homotropic activation.

Lactate Dehydrogenase Superfamily in Rice and Arabidopsis: Understanding the Molecular Evolution and Structural Diversity

Lactate/malate dehydrogenases (Ldh/Maldh) are ubiquitous enzymes involved in the central metabolic pathway of plants and animals. The role of malate dehydrogenases in the plant system is very well documented. However, the role of its homolog L-lactate dehydrogenases still remains elusive. Though its occurrence is experimentally proven in a few plant species, not much is known about its role in rice. Therefore, a comprehensive genome-wide in silico investigation was carried out to identify all Ldh genes in model plants, rice and Arabidopsis, which revealed Ldh to be a multigene family encoding multiple proteins. Publicly available data suggest its role in a wide range of abiotic stresses such as anoxia, salinity, heat, submergence, cold and heavy metal stress, as also confirmed by our qRT-PCR analysis, especially in salinity and heavy metal mediated stresses. A detailed protein modelling and docking analysis using Schrodinger Suite reveals the presence of three putatively functional L-lactate dehydrogenases in rice, namely OsLdh3, OsLdh7 and OsLdh9. The analysis also highlights the important role of Ser-219, Gly-220 and His-251 in the active site geometry of OsLdh3, OsLdh7 and OsLdh9, respectively. In fact, these three genes have also been found to be highly upregulated under salinity, hypoxia and heavy metal mediated stresses in rice.

Phylogenetics and biochemistry elucidate the evolutionary link between l-malate and l-lactate dehydrogenases and disclose an intermediate group of sequences with mix functional properties

The NAD(P)-dependent malate dehydrogenases (MDH) (EC 1.1.1.37) and NAD-dependent lactate dehydrogenases (LDH) (EC. 1.1.1.27) form a large superfamily that has been characterized in organisms belonging to the three Domains of Life. MDH catalyzes the reversible conversion of the oxaloacetate into malate, while LDH operates at the late stage of glycolysis by converting pyruvate into lactate. Phylogenetic studies proposed that the LDH/MDH superfamily encompasses five main groups of enzymes. Here, starting from 16,052 reference proteomes, we reinvestigated the relationships between MDH and LDH. We showed that the LDH/MDH superfamily encompasses three main families: MDH1, MDH2, and a large family encompassing MDH3, LDH, and L-2-hydroxyisocaproate dehydrogenases (HicDH) sequences. An in-depth analysis of the phylogeny of the MDH3/LDH/HicDH family and of the nature of three important amino acids, located within the catalytic site and involved in binding and substrate discrimination, revealed a large group of sequences displaying unexpected combinations of amino acids at these three critical positions. This group branched in-between canonical MDH3 and LDH sequences. The functional characterization of several enzymes from this intermediate group disclosed a mix of functional properties, indicating that the MDH3/LDH/HicDH family is much more diverse than previously thought, and blurred the frontier between MDH3 and LDH enzymes. Present-days enzymes of the intermediate group are a valuable material to study the evolutionary steps that led to functional diversity and emergence of allosteric regulation within the LDH/MDH superfamily.

The archaeal LDH-like malate dehydrogenase from Ignicoccus islandicus displays dual substrate recognition, hidden allostery and a non-canonical tetrameric oligomeric organization

The NAD(P)-dependent malate dehydrogenases (MalDHs) and NAD-dependent lactate dehydrogenases (LDHs) are homologous enzymes involved in central metabolism. They display a common protein fold and the same catalytic mechanism, yet have a stringent capacity to discriminate between their respective substrates. The MalDH/LDH superfamily is divided into several phylogenetically related groups. It has been shown that the canonical LDHs and LDH-like group of MalDHs are primarily tetrameric enzymes that diverged from a common ancestor. In order to gain understanding of the evolutionary history of the LDHs and MalDHs, the biochemical properties and crystallographic structure of the LDH-like MalDH from the hyperthermophilic archaeon Ignicoccus islandicus (I. isl) were determined. I. isl MalDH recognizes oxaloacetate as main substrate, but it is also able to use pyruvate. Surprisingly, with pyruvate, the enzymatic activity profile looks like that of allosteric LDHs, suggesting a hidden allosteric capacity in a MalDH. The I. isl MalDH tetrameric structure in the apo state is considerably different from those of canonical LDH-like MalDHs and LDHs, representing an alternative oligomeric organization. A comparison with MalDH and LDH counterparts provides strong evidence that the divergence between allosteric and non-allosteric members of the superfamily involves homologs with intermediate, atypical properties.

Analysis of the Mycoplasma bovis lactate dehydrogenase reveals typical enzymatic activity despite the presence of an atypical catalytic site motif

The lactate dehydrogenase (LDH) of Mycoplasma genitalium has been predicted to also act as a malate dehydrogenase (MDH), but there has been no experimental validation of this hypothesized dual function for any mollicute. Our analysis of the metabolite profile of Mycoplasma bovis using gas chromatography/mass spectrometry (GC/MS) and liquid chromatography/mass spectrometry (LC/MS) detected malate, suggesting that there may be MDH activity in M. bovis. To investigate whether the putative l-LDH enzyme of M. bovis has a dual function (MDH and LDH), we performed bioinformatic and functional biochemical analyses. Although the amino acid sequence and predicted structural analysis of M. bovisl-LDH revealed unusual residues within the catalytic site, suggesting that it may have the flexibility to possess a dual function, our biochemical studies using recombinant M. bovis -LDH did not detect any MDH activity. However, we did show that the enzyme has typical LDH activity that could be inhibited by both MDH substrates oxaloacetate (OAA) and malate, suggesting that these substrates may be able to bind to M. bovis LDH. Inhibition of the conversion of pyruvate to lactate by OAA may be one method the mycoplasma cell uses to reduce the potential for accumulation of intracellular lactate.

N5 ,N10 -methylenetetrahydromethanopterin reductase from Methanocaldococcus jannaschii also serves as a methylglyoxal reductase

In Methanocaldococcus jannaschii, methylglyoxal (MG) is required for aromatic amino acid biosynthesis. Previously, the reduction of MG to lactaldehyde in Methanocaldococcus jannaschii cell extracts using either NADPH or F₄₂₀ H₂ was demonstrated; however, the enzyme responsible was not identified. Using NADPH as the reductant, the unknown enzyme was purified from cell extracts of Methanocaldococcus jannaschii and determined to be the F₄₂₀ -dependent N⁵ ,N¹⁰ -methylenetetrahydromethanopterin reductase (Mer). Here, we report that the recombinantly overexpressed Mer is able to use NADPH and MG (K_M of 1.6 and 1.0 mm, respectively) to produce lactaldehyde. Additionally, Mer does not catalyze the reduction of MG to lactaldehyde in the presence of reduced Fo, the precursor of F₄₂₀ .

Pcal_1699, an extremely thermostable malate dehydrogenase from hyperthermophilic archaeon Pyrobaculum calidifontis

Two malate dehydrogenase homologs, Pcal_0564 and Pcal_1699, have been found in the genome of Pyrobaculum calidifontis. The gene encoding Pcal_1699 consisted of 927 nucleotides corresponding to a polypeptide of 309 amino acids. To examine the properties of Pcal_1699, the structural gene was cloned, expressed in Escherichia coli and the purified gene product was characterized. Pcal_1699 was NADH specific enzyme exhibiting a high malate dehydrogenase activity (886 U/mg) at optimal pH (10) and temperature (90 °C). Unfolding studies suggested that urea could not induce complete unfolding and inactivation of Pcal_1699 even at a final concentration of 8 M; however, in the presence of 4 M guanidine hydrochloride enzyme structure was unfolded with complete loss of enzyme activity. Thermostability experiments revealed that Pcal_1699 is the most thermostable malate dehydrogenase, reported to date, retaining more than 90 % residual activity even after heating for 6 h in boiling water.

Refolding, characterization and crystal structure of (S)-malate dehydrogenase from the hyperthermophilic archaeon Aeropyrum pernix

Tartrate oxidation activity was found in the crude extract of an aerobic hyperthermophilic archaeon Aeropyrum pernix, and the enzyme was identified as (S)-malate dehydrogenase (MDH), which, when produced in Escherichia coli, was mainly obtained as an inactive inclusion body. The inclusion body was dissolved in 6 M guanidine-HCl and gradually refolded to the active enzyme through dilution of the denaturant. The purified recombinant enzyme consisted of four identical subunits with a molecular mass of about 110 kDa. NADP was preferred as a coenzyme over NAD for (S)-malate oxidation and, unlike MDHs from other sources, this enzyme readily catalyzed the oxidation of (2S,3S)-tartrate and (2S,3R)-tartrate. The tartrate oxidation activity was also observed in MDHs from the hyperthermophilic archaea Methanocaldococcus jannaschii and Archaeoglobus fulgidus, suggesting these hyperthermophilic MDHs loosely bind their substrates. The refolded A. pernix MDH was also crystallized, and the structure was determined at a resolution of 2.9 A. Its overall structure was similar to those of the M. jannaschii, Chloroflexus aurantiacus, Chlorobium vibrioforme and Cryptosporidium parvum [lactate dehydrogenase-like] MDHs with root-mean-square-deviation values between 1.4 and 2.1 A. Consistent with earlier reports, Ala at position 53 was responsible for coenzyme specificity, and the next residue, Arg, was important for NADP binding. Structural comparison revealed that the hyperthermostability of the A. pernix MDH is likely attributable to its smaller cavity volume and larger numbers of ion pairs and ion-pair networks, but the molecular strategy for thermostability may be specific for each enzyme.

Fundamental and biotechnological applications of neutron scattering measurements for macromolecular dynamics

To explore macromolecular dynamics on the picosecond timescale, we used neutron spectroscopy. First, molecular dynamics were analyzed for the hyperthermophile malate dehydrogenase from Methanococcus jannaschii and a mesophilic homologue, the lactate dehydrogenase from Oryctolagus cunniculus muscle. Hyperthermophiles have elaborate molecular mechanisms of adaptation to extremely high temperature. Using a novel elastic neutron scattering approach that provides independent measurements of the global flexibility and of the structural resilience (rigidity), we have demonstrated that macromolecular dynamics represents one of these molecular mechanisms of thermoadaptation. The flexibilities were found to be similar for both enzymes at their optimal activity temperature and the resilience is higher for the hyperthermophilic protein. Secondly, macromolecular motions were examined in a native and immobilized dihydrofolate reductase (DHFR) from Escherichia coli. The immobilized mesophilic enzyme has increased stability and decreased activity, so that its properties are changed to resemble those of the thermophilic enzyme. Are these changes reflected in dynamical behavior? For this study, we performed quasielastic neutron scattering measurements to probe the protein motions. The residence time is 7.95 ps for the native DHFR and 20.36 ps for the immobilized DHFR. The average height of the potential barrier to local motions is therefore increased in the immobilized DHFR, with a difference in activation energy equal to 0.54 kcal/mol, which is, using the theoretical rate equation, of the same order than expected from calculation.

Neutron Scattering Reveals the Dynamic Basis of Protein Adaptation to Extreme Temperature

To explore protein adaptation to extremely high temperatures, two parameters related to macromolecular dynamics, the mean square atomic fluctuation and structural resilience, expressed as a mean force constant, were measured by neutron scattering for hyperthermophilic malate dehydrogenase from Methanococcus jannaschii and a mesophilic homologue, lactate dehydrogenase from Oryctolagus cunniculus (rabbit) muscle. The root mean square fluctuations, defining flexibility, were found to be similar for both enzymes (1.5 A) at their optimal activity temperature. Resilience values, defining structural rigidity, are higher by an order of magnitude for the high temperature-adapted protein (0.15 Newtons/meter for O. cunniculus lactate dehydrogenase and 1.5 Newtons/meter for M. jannaschii malate dehydrogenase). Thermoadaptation appears to have been achieved by evolution through selection of appropriate structural rigidity in order to preserve specific protein structure while allowing the conformational flexibility required for activity.

Molecular adaptation: the malate dehydrogenase from the extreme halophilic bacterium Salinibacter ruber behaves like a non-halophilic protein

Malate dehydrogenase from the extreme halophilic bacterium, Salinibacter ruber (Sr MalDH) was purified and characterised as a tetramer by sedimentation velocity measurements, showing the enzyme belongs to the LDH-like group of MalDHs. In contrast to most other halophilic enzymes, which unfold when incubated at low salt concentration, Sr MalDH is completely stable in absence of salt. Its amino acid composition does not display the strong acidic character specific of halophilic proteins. The enzyme displays a strong KCl-concentration dependent variation in K(m) for oxaloacetate, but not for the NADH co-factor. Its activity is reduced by high salt concentration, but remains sufficient for the enzyme to sustain catalysis at approximately 30% of its maximal rates in 3 M KCl. The properties of the protein were compared with those from other LDH-like MalDHs of bacterial and archaeal origins, showing that Sr MalDH in fact behaves like a non-halophilic enzyme.

Shotgun Proteomics of Methanococcus jannaschii and Insights into Methanogenesis

Methanococcus jannaschii is an autotrophic hyperthermophilic archaeon isolated from an oceanic hydrothermal vent. Its primary pathway for energy production is methanogenesis from H2 and CO2. High-throughput Multidimensional Protein Identification Technology based on microcapillary LC/LC/ MS/MS was used to investigate the proteome of M. jannaschii and the methanogenesis pathway in cells grown in complex medium with high H2 supply. A total of 963 proteins have been unambiguously identified. The identified proteins represent approximately 54% of the whole genome of M. jannaschii. About 44% of the identified proteins are either conserved hypothetical or hypothetical proteins. We identified 83-95% of the proteins predicted to be involved in amino acid biosynthesis, cellular processes, central intermediary metabolism, energy metabolism, protein synthesis, transcription, and purine, pyridine, nucleoside, and nucleotide synthesis. Over 40% of these proteins have better than 50% sequence coverage. Approximately 90% of the predicted methanogenesis proteins were detected. In contrast, only 27-37% of predicted hypothetical proteins, proteins involved in transport and binding, and proteins with regulatory functions were identified. High peptide number, spectrum count, and sequence coverage have been used as indicators of high expression levels and are in good agreement with codon bias analysis. Predicted intein peptides were detected in MJ1043 (DNA-directed RNA polymerase, subunit A"), MJ0542 (phosphoenolpyruvate synthase), MJ0782 (transcription initiation factor IIB), and MJ1422 (putative replication factor C subunit). New peptides created by protein splicing were detected in MJ0885 (DNA dependent DNA polymerase), MJ0542, and MJ0782. The methanogenesis pathway and the enzymes involved are also discussed.

Methanoarchaeal sulfolactate dehydrogenase: prototype of a new family of NADH-dependent enzymes

The crystal structure of the sulfolactate dehydrogenase from the hyperthermophilic and methanogenic archaeon Methanocaldococcus jannaschii was solved at 2.5 A resolution (PDB id. 1RFM). The asymmetric unit contains a tetramer of tight dimers. This structure, complexed with NADH, does not contain a cofactor-binding domain with 'Rossmann-fold' topology. Instead, the tertiary and quaternary structures indicate a novel fold. The NADH is bound in an extended conformation in each active site, in a manner that explains the pro-S specificity. Cofactor binding involves residues belonging to both subunits within the tight dimers, which are therefore the smallest enzymatically active units. The protein was found to be a homodimer in solution by size-exclusion chromatography, analytical ultracentrifugation and small-angle neutron scattering. Various compounds were tested as putative substrates. The results indicate the existence of a substrate discrimination mechanism, which involves electrostatic interactions. Based on sequence homology and phylogenetic analyses, several other enzymes were classified as belonging to this novel family of homologous (S)-2-hydroxyacid dehydrogenases.

Alpha-proteobacterial relationship of apicomplexan lactate and malate dehydrogenases

We have cloned and sequenced a lactate dehydrogenase (LDH) gene from Cryptosporidium parvum (CpLDH1). With this addition, and that of four recently deposited alpha-proteobacterial malate dehydrogenase (MDH) genes, the phylogenetic relationships among apicomplexan LDH and bacterial MDH were re-examined. Consistent with previous studies, our maximum likelihood (ML) analysis using the quartet-puzzling method divided 105 LDH/MDH enzymes into five clades, and confirmed that mitochondrial MDH is a sister clade to those of y-proteobacteria, rather than to alpha-proteobacteria. In addition, a Cryptosporidium parvum MDH (CpMDH1) was identified from the ongoing Cryptosporidium genome project that appears to belong to a distinct clade (III) comprised of 22 sequences from one archaebacterium, numerous eubacteria, and several apicomplexans. Using the ML puzzling test and bootstrapping analysis with protein distance and parsimony methods, the resulting trees not only robustly confirmed the alpha-proteobacterial relationship of apicomplexan LDH/MDH, but also supported a monophyletic relationship of CpLDH1 with CpMDHI. These data suggest that, unlike most other eukaryotes, the Apicomplexa may be one of the few lineages retaining an alpha-proteobacterial-type MDH that could have been acquired from an ancestral alpha-proteobacterium through primary endosymbiosis giving rise to the mitochondria, or through an unknown lateral gene transfer (LGT) event.

Desulfovibrio sp. genes involved in the respiration of sulfate during metabolism of hydrogen and lactate

To develop a better understanding of respiration by sulfate-reducing bacteria, we examined transcriptional control of respiratory genes during growth with lactate or hydrogen as an electron donor. RNA extracts of Desulfovibrio desulfuricans subsp. aestuarii were analyzed by using random arbitrarily primed PCR. RNA was reverse transcribed under low-stringency conditions with a set of random primers, and candidate cDNAs were cloned, sequenced, and characterized by BLAST analysis. Putative differentially expressed transcripts were confirmed by Northern blot analysis. Interestingly, dissimilatory bisulfite reductase was upregulated in the presence of hydrogen. To link these transcriptional changes to the physiology of sulfate-reducing bacteria, sulfide was measured during growth of several strains of Desulfovibrio on hydrogen or lactate, and this revealed that hydrogen-grown cells produced more sulfide per unit of cell mass than lactate-grown cells. Transcription of other redox proteins was characterized by Northern blotting to determine whether or not they were also transcribed to higher levels in hydrogen-grown cells. Growth on lactate produced greater transcription of [NiFe] hydrogenase. H(2)-grown cells transcribed the adenylylsulfate reductase b subunit and HmcA to higher levels. The results we describe here provide new insight into the continuing debate over how Desulfovibrio species utilize redox components to generate membrane potential and to channel electrons to sulfate, the final electron acceptor.