Structural basis of substrate specificity in malate dehydrogenases: crystal structure of a ternary complex of porcine cytoplasmic malate dehydrogenase, alpha-ketomalonate and tetrahydoNAD

Allostery and Evolution: A Molecular Journey Through the Structural and Dynamical Landscape of an Enzyme Super Family

Allosteric regulation is a powerful mechanism for controlling the efficiency of enzymes. Deciphering the evolutionary mechanisms by which allosteric properties have been acquired in enzymes is of fundamental importance. We used the malate (MalDH) and lactate deydrogenases (LDHs) superfamily as model to elucidate this phenomenon. By introducing a few of mutations associated to the emergence of allosteric LDHs into the non-allosteric MalDH from Methanopyrus kandleri, we have gradually shifted its enzymatic profile toward that typical of allosteric LDHs. We first investigated the process triggering homotropic activation. The structures of the resulting mutants show the typical compact organization of the R-active state of LDHs, but a distorted (T-like) catalytic site demonstrating that they corresponds to hybrid states. Molecular dynamics simulations and free energy calculations confirmed the capability of these mutants to sample the T-inactive state. By adding a final single mutation to fine-tune the flexibility of the catalytic site, we obtained an enzyme with both sigmoid (homotropic) and hyperbolic (heterotropic) substrate activation profiles. Its structure shows a typical extended T-state as in LDHs, whereas its catalytic state has as a restored configuration favorable for catalysis. Free energy calculations indicate that the T and R catalytic site configurations are in an equilibrium that depends on solvent conditions. We observed long-range communication between monomers as required for allosteric activation. Our work links the evolution of allosteric regulation in the LDH/MDH superfamily to the ensemble model of allostery at molecular level, and highlights the important role of the underlying protein dynamics.

Phosphorylation of mammalian cytosolic and mitochondrial malate dehydrogenase: insights into regulation

Malate dehydrogenase (MDH) is a key enzyme in mammalian metabolic pathways in cytosolic and mitochondrial compartments. Regulation of MDH through phosphorylation remains an underexplored area. In this review we consolidate evidence supporting the potential role of phosphorylation in modulating the function of mammalian MDH. Parallels are drawn with the phosphorylation of lactate dehydrogenase, a homologous enzyme, to reveal its regulatory significance and to suggest a similar regulatory strategy for MDH. Comprehensive mining of phosphorylation databases, provides substantial experimental (primarily mass spectrometry) evidence of MDH phosphorylation in mammalian cells. Experimentally identified phosphorylation sites are overlaid with MDH's functional domains, offering perspective on how these modifications could influence enzyme activity. Preliminary results are presented from phosphomimetic mutations (serine/threonine residues changed to aspartate) generated in recombinant MDH proteins serving as a proof of concept for the regulatory impact of phosphorylation. We also examine and highlight several approaches to probe the structural and cellular impact of phosphorylation. This review highlights the need to explore the dynamic nature of MDH phosphorylation and calls for identifying the responsible kinases and the physiological conditions underpinning this modification. The synthesis of current evidence and experimental data aims to provide insights for future research on understanding MDH regulation, offering new avenues for therapeutic interventions in metabolic disorders and cancer.

Exploring the uncharted territory of the potential protein-protein interactions of cytosolic malate dehydrogenase

In this review, we examine the protein-protein interactions of cytosolic malate dehydrogenase (MDH), an under-studied area in cellular metabolism. We provide a comprehensive overview of MDH involvement in metabolism, especially its interactions with metabolic partners and dynamics of changing metabolism. We present an analysis of the biophysical nature of these interactions and the current methods used to study them. Our review includes an assessment of computational docking studies, which offer initial hypotheses about potential MDH interaction partners. Furthermore, we provide a summary of the sparse yet insightful experimental evidence available, establishing a foundation for future research. By integrating biophysical analysis and methodological advancements, this paper aims to illuminate the intricate network of interactions involving cytosolic MDH and their metabolic implications. This work not only contributes to our understanding of MDH's role in metabolism but also highlights the potential impact of these interactions in metabolic disorders.

Catalytic mechanism and kinetics of malate dehydrogenase

Malate dehydrogenase (MDH) is a ubiquitous and central enzyme in cellular metabolism, found in all kingdoms of life, where it plays vital roles in the cytoplasm and various organelles. It catalyzes the reversible NAD+-dependent reduction of L-malate to oxaloacetate. This review describes the reaction mechanism for MDH and the effects of mutations in and around the active site on catalytic activity and substrate specificity, with a particular focus on the loop that encloses the active site after the substrates have bound. While MDH exhibits selectivity for its preferred substrates, mutations can alter the specificity of MDH for each cosubstrate. The kinetic characteristics and similarities of a variety of MDH isozymes are summarized, and they illustrate that the KM values are consistent with the relative concentrations of the substrates in cells. As a result of its existence in different cellular environments, MDH properties vary, making it an attractive model enzyme for studying enzyme activity and structure under different conditions.

The structural biology and dynamics of malate dehydrogenases

Malate dehydrogenase (MDH) enzymes catalyze the reversible oxidoreduction of malate to oxaloacetate using NAD(P) as a cofactor. This reaction is vital for metabolism and the exchange of reducing equivalents between cellular compartments. There are more than 100 structures of MDH in the Protein Data Bank, representing species from archaea, bacteria, and eukaryotes. This conserved family of enzymes shares a common nucleotide-binding domain, substrate-binding domain, and subunits associate to form a dimeric or a tetrameric enzyme. Despite the variety of crystallization conditions and ligands in the experimental structures, the conformation and configuration of MDH are similar. The quaternary structure and active site dynamics account for most conformational differences in the experimental MDH structures. Oligomerization appears essential for activity despite each subunit having a structurally independent active site. There are two dynamic regions within the active site that influence substrate binding and possibly catalysis, with one of these regions adjoining the subunit interface. In this review, we introduce the reader to the general structural framework of MDH highlighting the conservation of certain features and pointing out unique differences that regulate MDH enzyme activity.

Molecular Characteristics of the Malate Dehydrogenase (MDH) Gene Family in Spirometra mansoni (Cestoda: Diphyllobothriidea)

The plerocercoid larva of Spirometra mansoni can cause a parasitic zoonosis-sparganosis. Malate dehydrogenase (MDH) plays a very important role in the life activities of parasites. However, little is known about the MDH family in S. mansoni. We identified eight new MDH members in S. mansoni in this study. Clustering analysis divided SmMDHs into two groups and revealed patterns similar to the conserved motif organization. RT-qPCR suggested that five MDHs were highly expressed in the mature proglottid and that three MDHs were highly expressed in the gravid proglottid. Phylogenetic analysis revealed that SmMDHs contain both conserved family members and members in the process of further diversification. rSmMDH has an NAD binding domain, a dimer interface and a substrate binding domain. Natural SmMDH was immunolocalized in the tissues and follicles around the uterus in the mature or gravid proglottid and eggshells. The maximum forward and reverse reaction activities of rSmMDH were observed at pH 8.5 and 9.0, respectively. The optimum temperature for enzyme activity was 37 °C in the forward reaction and 40 °C in the reverse reaction. These results lay the foundation for studying the molecular functions and mechanisms of MDHs in S. mansoni and related taxa.

A naturally isolated symbiotic bacterium suppresses flavivirus transmission by Aedes mosquitoes

The commensal microbiota of the mosquito gut plays a complex role in determining the vector competence for arboviruses. In this study, we identified a bacterium from the gut of field Aedes albopictus mosquitoes named Rosenbergiella sp. YN46 (Rosenbergiella_YN46) that rendered mosquitoes refractory to infection with dengue and Zika viruses. Inoculation of 1.6 × 103 colony forming units (CFUs) of Rosenbergiella_YN46 into A. albopictus mosquitoes effectively prevents viral infection. Mechanistically, this bacterium secretes glucose dehydrogenase (RyGDH), which acidifies the gut lumen of fed mosquitoes, causing irreversible conformational changes in the flavivirus envelope protein that prevent viral entry into cells. In semifield conditions, Rosenbergiella_YN46 exhibits effective transstadial transmission in field mosquitoes, which blocks transmission of dengue virus by newly emerged adult mosquitoes. The prevalence of Rosenbergiella_YN46 is greater in mosquitoes from low-dengue areas (52.9 to ~91.7%) than in those from dengue-endemic regions (0 to ~6.7%). Rosenbergiella_YN46 may offer an effective and safe lead for flavivirus biocontrol.

Three amino acid substitutions contributing to thermostability of phosphoglucose isomerase in the Glanville fritillary butterfly

Temperature is one of the most important environmental factors that affect organisms, especially ectotherms, due to its effects on protein stability. Understanding the general rules that govern thermostability changes in proteins to adapt high-temperature environments is crucial. Here, we report the amino acid substitutions of phosphoglucose isomerase (PGI) related to thermostability in the Glanville fritillary butterfly (Melitaea cinxia, Lepidoptera: Nymphalidae). The PGI encoded by the most common allele in M. cinxia in the Chinese population (G3-PGI), which is more thermal tolerant, is more stable under heat stress than that in the Finnish population (D1-PGI). There are 5 amino acid substitutions between G3-PGI and D1-PGI. Site-directed mutagenesis revealed that the combination of amino acid substitutions of H35Q, M49T, and I64V may increase PGI thermostability. These substitutions alter the 3D structure to increase the interaction between 2 monomers of PGI. Through molecular dynamics simulations, it was found that the amino acid at site 421 is more stable in G3-PGI, confining the motion of the α-helix 420-441 and stabilizing the interaction between 2 PGI monomers. The strategy for high-temperature adaptation through these 3 amino acid substitutions is also adopted by other butterfly species (Boloria eunomia, Aglais urticae, Colias erate, and Polycaena lua) concurrent with M. cinxia in the Tianshan Mountains of China, i.e., convergent evolution in butterflies.

Design, Synthesis and Biological Evaluation of Novel MDH Inhibitors Targeting Tumor Microenvironment

MDH1 and MDH2 enzymes play an important role in the survival of lung cancer. In this study, a novel series of dual MDH1/2 inhibitors for lung cancer was rationally designed and synthesized, and their SAR was carefully investigated. Among the tested compounds, compound 50 containing a piperidine ring displayed an improved growth inhibition of A549 and H460 lung cancer cell lines compared with LW1497. Compound 50 reduced the total ATP content in A549 cells in a dose-dependent manner; it also significantly suppressed the accumulation of hypoxia-inducible factor 1-alpha (HIF-1α) and the expression of HIF-1α target genes such as GLUT1 and pyruvate dehydrogenase kinase 1 (PDK1) in a dose-dependent manner. Furthermore, compound 50 inhibited HIF-1α-regulated CD73 expression under hypoxia in A549 lung cancer cells. Collectively, these results indicate that compound 50 may pave the way for the development of next-generation dual MDH1/2 inhibitors to target lung cancer.

Warburg Effect as a Novel Mechanism for Homocysteine-Induced Features of Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a major cause of blindness. Recent studies have reported impaired glycolysis in AMD patients with a high lactate/pyruvate ratio. Elevated homocysteine (Hcy) (Hyperhomocysteinemia, HHcy) was observed in several clinical studies, reporting an association between HHcy and AMD. We established the effect of HHcy on barrier function, retinal pigment epithelium (RPE) structure, and induced choroidal neovascularization (CNV) in mice. We hypothesize that HHcy contributes to AMD by inducing a metabolic switch in the mitochondria, in which cells predominantly produce energy by the high rate of glycolysis, or "Warburg", effect. Increased glycolysis results in an increased production of lactate, cellular acidity, activation of angiogenesis, RPE barrier dysfunction, and CNV. Evaluation of cellular energy production under HHcy was assessed by seahorse analysis, immunofluorescence, and western blot experiments. The seahorse analysis evaluated the extracellular acidification rate (ECAR) as indicative of glycolysis. HHcy showed a significant increase in ECAR both in vivo using (Cystathionine β-synthase) cbs^+/- and cbs^-/- mice retinas and in vitro (Hcy-treated ARPE-19) compared to wild-type mice and RPE cells. Moreover, HHcy up-regulated glycolytic enzyme (Glucose transporter-1 (GlUT-1), lactate dehydrogenase (LDH), and hexokinase 1 (HK1)) in Hcy-treated ARPE-19 and primary RPE cells isolated from cbs^+/+, cbs^+/-, and cbs^-/- mice retinas. Inhibition of GLUT-1 or blocking of N-methyl-D-aspartate receptors (NMDAR) reduced glycolysis in Hcy-treated RPE and improved albumin leakage and CNV induction in Hcy-injected mice eyes. The current study suggests that HHcy causes a metabolic switch in the RPE cells from mitochondrial respiration to glycolysis during AMD and confirms the involvement of NMDAR in this process. Therefore, targeting Glycolysis or NMDAR could be a novel therapeutic target for AMD.

Krebs cycle enzymes for targeted therapeutics and immunotherapy for anti-leishmanial drug development using: Pathways, potential targets, and future perspectives

Leishmaniasis is a parasitic and neglected tropical disease which majorly impacts poor and developing nations. One of the significant factors that impacts the severity of the pathological condition includes the socioeconomic background of the affected region. The rise of drug-resistant Leishmania is a serious concern for the effectiveness of the present treatment. As a result, the drug options need to be relooked immediately. Leishmania employs Krebs cycle intermediates for its needs after infection for establishing various defense mechanisms to escape the host immune responses. Nevertheless, a variety of immunological reactions are also seen during infection, which clear the parasites. One of the more promising strategies in this regard would involve combining targeted therapy and immunotherapy. The targeted treatments work by obstructing vital pathways that are required for Leishmania to grow and survive. The mechanism of action of immunotherapy is the control of the host immune response, which entails the blockage of molecular pathways essential for the growth and maintenance of the parasite. The Krebs cycle intermediates have important biochemical roles. Additionally, in macrophages and dendritic cells, they play roles as signalling molecules for controlling inflammatory responses. The review brings together the available literature about the importance of Krebs cycle metabolites as potential treatment targets for leishmaniasis.

O-GlcNAcylation promotes pancreatic tumor growth by regulating malate dehydrogenase 1

Oncogenic Kras-activated pancreatic ductal adenocarcinoma (PDAC) cells highly rely on an unconventional glutamine catabolic pathway to sustain cell growth. However, little is known about how this pathway is regulated. Here we demonstrate that Kras mutation induces cellular O-linked β-N-acetylglucosamine (O-GlcNAc), a prevalent form of protein glycosylation. Malate dehydrogenase 1 (MDH1), a key enzyme in the glutamine catabolic pathway, is positively regulated by O-GlcNAcylation on serine 189 (S189). Molecular dynamics simulations suggest that S189 glycosylation on monomeric MDH1 enhances the stability of the substrate-binding pocket and strengthens the substrate interactions by serving as a molecular glue. Depletion of O-GlcNAcylation reduces MDH1 activity, impairs glutamine metabolism, sensitizes PDAC cells to oxidative stress, decreases cell proliferation and inhibits tumor growth in nude mice. Furthermore, O-GlcNAcylation levels of MDH1 are elevated in clinical PDAC samples. Our study reveals that O-GlcNAcylation contributes to pancreatic cancer growth by regulating the metabolic activity of MDH1.

Protein Conformational Space at the Edge of Allostery: Turning a Non-allosteric Malate Dehydrogenase into an "Allosterized" Enzyme using Evolution Guided Punctual Mutations

We unveil the intimate relationship between protein dynamics and allostery by following the trajectories of model proteins in their conformational and sequence spaces. Starting from a nonallosteric hyperthermophilic malate dehydrogenase, we have tracked the role of protein dynamics in the evolution of the allosteric capacity. Based on a large phylogenetic analysis of the malate (MalDH) and lactate dehydrogenase (LDH) superfamily, we identified two amino acid positions that could have had a major role for the emergence of allostery in LDHs, which we targeted for investigation by site-directed mutagenesis. Wild-type MalDH and the single and double mutants were tested with respect to their substrate recognition profiles. The double mutant displayed a sigmoid-shaped profile typical of homotropic activation in LDH. By using molecular dynamics simulations, we showed that the mutations induce a drastic change in the protein sampling of its conformational landscape, making transiently T-like (inactive) conformers, typical of allosteric LDHs, accessible. Our data fit well with the seminal key concept linking protein dynamics and evolvability. We showed that the selection of a new phenotype can be achieved by a few key dynamics-enhancing mutations causing the enrichment of low-populated conformational substates.

Structural Characterization of the Human Cytosolic Malate Dehydrogenase I

The first crystal structure of the human cytosolic malate dehydrogenase I (MDH1) is described. Structure determination at a high resolution (1.65 Å) followed production, isolation, and purification of human MDH1 using a bacterial expression system. The structure is a binary complex of MDH1 with only a bound malonate molecule in the substrate binding site. Comparisons of this structure with malate dehydrogenase enzymes from other species confirm that the human enzyme adopts similar secondary, tertiary, and quaternary structures and that the enzyme retains a similar conformation even when nicotinamide adenine dinucleotide (NAD⁺) is not bound. A comparison to the highly homologous porcine (sus scrofa) MDH1 ternary structures leads to the conclusion that only small conformational differences are needed to accommodate binding by NAD⁺ or other NAD⁺ mimetics. Conformational differences observed in the second subunit show that the NAD⁺ binding elements are nevertheless quite flexible. Comparison of hMDH1 to the human mitochondrial malate dehydrogenase (hMDH2) reveals some key differences in the α7-α8 loop, which lies directly beneath the substrate binding pocket. These differences might be exploited in the structure-assisted design of selective small molecule inhibitors of hMDH1, an emerging target for the development of anticancer therapeutics.

A Specialized Dehydrogenase Provides l-Phenyllactate for FR900359 Biosynthesis

d‐Phenyllactate (PLA) is a component of the selective Gq protein inhibitor and nonribosomal cyclic depsipeptide FR900359 (FR). Here we report a detailed biochemical investigation of pla biosynthesis and its incorporation into the natural product FR. The enzyme FrsC, member of the lactate/malate dehydrogenase superfamily, was shown to catalyze the formation of l‐PLA from phenylpyruvate. FrsC was kinetically characterized and its substrate specificity determined. Incorporation of l‐PLA was probed by assaying the adenylation domain FrsE‐A3 and feeding studies with a Chromobacterium vaccinii ΔfrsC mutant, confirming preferred activation of l‐PLA followed by on‐line epimerization to d‐pla. Finally, detailed bioinformatic analyses of FrsC revealed its close relation to malate dehydrogenases from primary metabolism and suggest extensions in the substrate binding loop to be responsible for its adaptation to accepting larger aromatic substrates with high specificity.

Kinetic characterization and thermostability of C. elegans cytoplasmic and mitochondrial malate dehydrogenases

Malate dehydrogenase (MDH) catalyzes the conversion of NAD⁺ and malate to NADH and oxaloacetate in the citric acid cycle. Eukaryotes have one MDH isozyme that is imported into the mitochondria and one in the cytoplasm. We overexpressed and purified Caenorhabditis elegans cytoplasmic MDH-1 and mitochondrial MDH-2 in E. coli. Our goal was to compare the kinetic and structural properties of these enzymes because C. elegans can survive adverse environmental conditions, such as lack of food and elevated temperatures. In steady-state enzyme kinetics assays, we measured K_M values for oxaloacetate of 54 and 52 μM and K_M values for NADH of 61 and 107 μM for MDH-1 and MDH-2, respectively. We partially purified endogenous MDH-1 and MDH-2 from a mixed population of worms and separated them using anion exchange chromatography. Both endogenous enzymes had a K_M for oxaloacetate similar to that of the corresponding recombinant enzyme. Recombinant MDH-1 and MDH-2 had maximum activity at 40 °C and 35 °C, respectively. In a thermotolerance assay, MDH-1 was much more thermostable than MDH-2. Protein homology modeling predicted that MDH-1 had more intersubunit salt-bridges than mammalian MDH1 enzymes, and these ionic interactions may contribute to its thermostability. In contrast, the MDH-2 homology model predicted fewer intersubunit ionic interactions compared to mammalian MDH2 enzymes. These results suggest that the increased stability of MDH-1 may facilitate its ability to remain active in adverse environmental conditions. In contrast, MDH-2 may use other strategies, such as protein binding partners, to function under similar conditions.

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.

Structural analysis and reaction mechanism of malate dehydrogenase from Geobacillus stearothermophilus

Malate dehydrogenase (MDH) catalyzes the reversible reduction of oxaloacetate (OAA) to L-malate using nicotinamide adenine dinucleotide hydrogen (NADH). MDH has two characteristic loops, the mobile loop and the catalytic loop, in the active site. On binding to the substrate, the enzyme undergoes a structural change from the open-form, with an open conformation of the mobile loop, to the closed-form, with the loop in a closed conformation. In this study, three crystals of MDH from a moderate thermophile, Geobacillus stearothermophilus (gs-MDH) were used to determine four different enzyme structures (resolutions, 1.95-2.20 Å), each of which was correspondingly assigned to its four catalytic states. Two OAA-unbound structures exhibited the open-form, while the other two OAA-bound structures exhibited both the open- and closed-form. The structural analysis suggested that the binding of OAA to the open-form gs-MDH promotes conformational change in the mobile loop and simultaneously activates the catalytic loop. The mutations on the key amino acid residues involving the proposed catalytic mechanism significantly affected the gs-MDH activity, supporting our hypothesis. These findings contribute to the elucidation of the detailed molecular mechanism underlying the substrate recognition and structural switching during the MDH catalytic cycle.

Thermal adaptation in the honeybee (Apis mellifera) via changes to the structure of malate dehydrogenase

In honeybees there are three alleles of cytosolic malate dehydrogenase gene: F, M and S. Allele frequencies are correlated with environmental temperature, suggesting that the alleles have temperature-dependent fitness benefits. We determined the enzyme activity of each allele across a range of temperatures in vitro The F and S alleles have higher activity and are less sensitive to high temperatures than the M allele, which loses activity after incubation at temperatures found in the thorax of foraging bees in hot climates. Next, we predicted the protein structure of each allele and used molecular dynamics simulations to investigate their molecular flexibility. The M allozyme is more flexible than the S and F allozymes at 50°C, suggesting a plausible explanation for its loss of activity at high temperatures, and has the greatest structural flexibility at 15°C, suggesting that it can retain some enzyme activity at cooler temperatures. MM bees recovered from 2 h of cold narcosis significantly better than all other genotypes. Combined, these results explain clinal variation in malate dehydrogenase allele frequencies in the honeybee at the molecular level.

Identification and characterization of cytosolic malate dehydrogenase from the liver fluke Fasciola gigantica

The liver fluke zoonoses, Fasciola spp. are parasitic helminths infecting humans and animals globally. Recent sequencing of the genome of Fasciola gigantica has provided a basis to understand the biochemistry of this parasite. Here, we identified the cytosolic malate dehydrogenase in F. gigantica (FgMDH) and characterized the enzyme biochemically and structurally. F. gigantica encodes a single cytosolic MDH, a key enzyme of the citric acid cycle. It catalyzes the reversible oxidation of malate to oxaloacetate using NAD⁺. The Fgmdh gene was amplified and cloned for expression of the recombinant protein. The purified protein showed a molecular weight of ~ 36 kDa that existed in a dimeric form in solution. The recombinant enzyme was catalytically active as it catalyzed both forward and reverse reactions efficiently. The kinetic parameters were determined for both directions. The structure of FgMDH and human MDH were modeled and validated. The superimposition of both the model structures showed overall structural similarity in the active site loop region, however, the conformation of the residues was different. Molecular docking elucidated the binding sites and affinities of the substrates and cofactors to the enzyme. Simulation of molecular dynamics and principal component analysis indicated the stability of the systems and collective motions, respectively. Understanding the structural and functional properties of MDH is important to better understand the roles of this enzyme in the biochemistry of the parasite.

Krebs Cycle Reborn in Macrophage Immunometabolism

A striking change has happened in the field of immunology whereby specific metabolic processes have been shown to be a critical determinant of immune cell activation. Multiple immune receptor types rewire metabolic pathways as a key part of how they promote effector functions. Perhaps surprisingly for immunologists, the Krebs cycle has emerged as the central immunometabolic hub of the macrophage. During proinflammatory macrophage activation, there is an accumulation of the Krebs cycle intermediates succinate and citrate, and the Krebs cycle–derived metabolite itaconate. These metabolites have distinct nonmetabolic signaling roles that influence inflammatory gene expression. A key bioenergetic target for the Krebs cycle, the electron transport chain, also becomes altered, generating reactive oxygen species from Complexes I and III. Similarly, alternatively activated macrophages require α-ketoglutarate-dependent epigenetic reprogramming to elicit anti-inflammatory gene expression. In this review, we discuss these advances and speculate on the possibility of targeting these events therapeutically for inflammatory diseases.

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.

The crystal structure of homoserine dehydrogenase complexed with l-homoserine and NADPH in a closed form

Homoserine dehydrogenase from Thermus thermophilus (TtHSD) is a key enzyme in the aspartate pathway that catalyses the reversible conversion of l-aspartate-β-semialdehyde to l-homoserine (l-Hse) with NAD(P)H. We determined the crystal structures of unliganded TtHSD, TtHSD complexed with l-Hse and NADPH, and Lys99Ala and Lys195Ala mutant TtHSDs, which have no enzymatic activity, complexed with l-Hse and NADP+ at 1.83, 2.00, 1.87 and 1.93 Å resolutions, respectively. Binding of l-Hse and NADPH induced the conformational changes of TtHSD from an open to a closed form: the mobile loop containing Glu180 approached to fix l-Hse and NADPH, and both Lys99 and Lys195 could make hydrogen bonds with the hydroxy group of l-Hse. The ternary complex of TtHSDs in the closed form mimicked a Michaelis complex better than the previously reported open form structures from other species. In the crystal structure of Lys99Ala TtHSD, the productive geometry of the ternary complex was almost preserved with one new water molecule taking over the hydrogen bonds associated with Lys99, while the positions of Lys195 and l-Hse were significantly retained with those of the wild-type enzyme. These results propose new possibilities that Lys99 is the acid-base catalytic residue of HSDs.

Comparing mutagenesis and simulations as tools for identifying functionally important sequence changes for protein thermal adaptation

Comparative studies of orthologous proteins of species evolved at different temperatures have revealed consistent patterns of temperature-related variation in thermal stabilities of structure and function. However, the precise mechanisms by which interspecific variations in sequence foster these adaptive changes remain largely unknown. Here, we compare orthologs of cytosolic malate dehydrogenase (cMDH) from marine molluscs adapted to temperatures ranging from -1.9 °C (Antarctica) to ∼55 °C (South China coast) and show how amino acid usage in different regions of the enzyme (surface, intermediate depth, and protein core) varies with adaptation temperature. This eukaryotic enzyme follows some but not all of the rules established in comparisons of archaeal and bacterial proteins. To link the effects of specific amino acid substitutions with adaptive variations in enzyme thermal stability, we combined site-directed mutagenesis (SDM) and in vitro protein experimentation with in silico mutagenesis using molecular dynamics simulation (MDS) techniques. SDM and MDS methods generally but not invariably yielded common effects on protein stability. MDS analysis is shown to provide insights into how specific amino acid substitutions affect the conformational flexibilities of mobile regions (MRs) of the enzyme that are essential for binding and catalysis. Whereas these substitutions invariably lie outside of the MRs, they effectively transmit their flexibility-modulating effects to the MRs through linked interactions among surface residues. This discovery illustrates that regions of the protein surface lying outside of the site of catalysis can help establish an enzyme's thermal responses and foster evolutionary adaptation of function.

A Lysine Acetyltransferase Contributes to the Metabolic Adaptation to Hypoxia in Mycobacterium tuberculosis

Upon inhibition of respiration, which occurs in hypoxic or nitric oxide-containing host microenvironments, Mycobacterium tuberculosis (Mtb) adopts a non-replicating "quiescent" state and becomes relatively unresponsive to antibiotic treatment. We used comprehensive mutant fitness analysis to identify regulatory and metabolic pathways that are essential for the survival of quiescent Mtb. This genetic study identified a protein acetyltransferase (Mt-Pat/Rv0998) that promoted survival and altered the flux of carbon from oxidative to reductive tricarboxylic acid (TCA) reactions. Reductive TCA requires malate dehydrogenase (MDH) and maintains the redox state of the NAD+/NADH pool. Genetic or chemical inhibition of MDH resulted in rapid cell death in both hypoxic cultures and in murine lung. These phenotypic data, in conjunction with significant structural differences between human and mycobacterial MDH enzymes that could be exploited for drug development, suggest a new strategy for eradicating quiescent bacteria.

Rational engineering of a malate dehydrogenase for microbial production of 2,4-dihydroxybutyric acid via homoserine pathway

A synthetic pathway for the production of 2,4-dihydroxybutyric acid from homoserine (HMS), composed of two consecutive enzymatic reaction steps has been recently reported. An important step in this pathway consists in the reduction in 2-keto-4-hydroxybutyrate (OHB) into (l)-dihydroxybutyrate (DHB), by an enzyme with OHB reductase activity. In the present study, we used a rational approach to engineer an OHB reductase by using the cytosolic (l)-malate dehydrogenase from Escherichia coli (Ec-Mdh) as the template enzyme. Structural analysis of (l)-malate dehydrogenase and (l)-lactate dehydrogenase enzymes acting on sterically cognate substrates revealed key residues in the substrate and co-substrate-binding sites responsible for substrate discrimination. Accordingly, amino acid changes were introduced in a stepwise manner into these regions of the protein. This rational engineering led to the production of an Ec-Mdh-5E variant (I12V/R81A/M85E/G179D/D86S) with a turnover number (kcat) on OHB that was increased by more than 2000-fold (from 0.03 up to 65.0 s−1), which turned out to be 7-fold higher than that on its natural substrate oxaloacetate. Further kinetic analysis revealed the engineered enzyme to possess comparable catalytic efficiencies (kcat/Km) between natural and synthetic OHB substrates (84 and 31 s−1 mM−1, respectively). Shake-flask cultivation of a HMS-overproducing E. coli strain expressing this improved OHB reductase together with a transaminase encoded by aspC able to convert HMS to OHB resulted in 89% increased DHB production as compared with our previous report using a E. coli host strain expressing an OHB reductase derived from the lactate dehydrogenase A of Lactococcus lactis.

Conformational changes on substrate binding revealed by structures of Methylobacterium extorquens malate dehydrogenase

Three high-resolution X-ray crystal structures of malate dehydrogenase (MDH; EC 1.1.1.37) from the methylotroph Methylobacterium extorquens AM1 are presented. By comparing the structures of apo MDH, a binary complex of MDH and NAD+, and a ternary complex of MDH and oxaloacetate with ADP-ribose occupying the pyridine nucleotide-binding site, conformational changes associated with the formation of the catalytic complex were characterized. While the substrate-binding site is accessible in the enzyme resting state or NAD+-bound forms, the substrate-bound form exhibits a closed conformation. This conformational change involves the transition of an α-helix to a 310-helix, which causes the adjacent loop to close the active site following coenzyme and substrate binding. In the ternary complex, His284 forms a hydrogen bond to the C2 carbonyl of oxaloacetate, placing it in a position to donate a proton in the formation of (2S)-malate.

Heat-resistant cytosolic malate dehydrogenases (cMDHs) of thermophilic intertidal snails (genus Echinolittorina): protein underpinnings of tolerance to body temperatures reaching 55°C

Snails of the genus Echinolittorina are among the most heat-tolerant animals; they experience average body temperatures near 41-44°C in summer and withstand temperatures up to at least 55°C. Here, we demonstrate that heat stability of function (indexed by the Michaelis-Menten constant of the cofactor NADH, K_M^NADH) and structure (indexed by rate of denaturation) of cytosolic malate dehydrogenases (cMDHs) of two congeners (E. malaccana and E. radiata) exceeds values previously found for orthologs of this protein from less thermophilic species. The ortholog of E. malaccana is more heat stable than that of E. radiata, in keeping with the congeners' thermal environments. Only two inter-congener differences in amino acid sequence in these 332 residue proteins were identified. In both cases (positions 48 and 114), a glycine in the E. malaccana ortholog is replaced by a serine in the E. radiata protein. To explore the relationship between structure and function and to characterize how amino acid substitutions alter stability of different regions of the enzyme, we used molecular dynamics simulation methods. These computational methods allow determination of thermal effects on fine-scale movements of protein components, for example, by estimating the root mean square deviation in atom position over time and the root mean square fluctuation for individual residues. The minor changes in amino acid sequence favor temperature-adaptive change in flexibility of regions in and around the active sites. Interspecific differences in effects of temperature on fine-scale protein movements are consistent with the differences in thermal effects on binding and rates of heat denaturation.

Molecular characterization and functional analysis of Eimeria tenella malate dehydrogenase

Eimeria tenella is a serious intracellular parasite that actively invades cecal epithelial cells of chickens. The widespread use of drugs causes severe resistance to Eimeria tenella. We detected that malate dehydrogenase (MDH), one of the differentially expressed genes, was upregulated in diclazuril-resistant and maduramicin-resistant strains through transcriptome sequencing. In this study, we cloned and expressed MDH of E. tenella (EtMDH). Quantitative real-time polymerase chain reactions (qPCR) and Western blots were used to analyze the expression of EtMDH in resistant and sensitive strains, indicating EtMDH was upregulated in two resistant strains at the messenger RNA and protein levels. Enzyme activity was tested through absorbance measurement and the EtMDH activity increased in two resistant strains. Expression levels of EtMDH in four developmental stages of E. tenella were tested through qPCR and Western blot. Invasion inhibition assays explored if EtMDH was involved in invasion of DF-1 cells by E. tenella sporozoites. Indirect immunofluorescence assays investigated EtMDH distribution during parasite development in DF-1 cells invaded by E. tenella sporozoites. Experimental results showed that EtMDH may be related to drug resistance of E. tenella during its development and invasion. EtMDH may be an effective molecular marker for detection of E. tenella drug resistance.

Structural flexibility and protein adaptation to temperature: Molecular dynamics analysis of malate dehydrogenases of marine molluscs

Orthologous proteins of species adapted to different temperatures exhibit differences in stability and function that are interpreted to reflect adaptive variation in structural "flexibility." However, quantifying flexibility and comparing flexibility across proteins has remained a challenge. To address this issue, we examined temperature effects on cytosolic malate dehydrogenase (cMDH) orthologs from differently thermally adapted congeners of five genera of marine molluscs whose field body temperatures span a range of ∼60 °C. We describe consistent patterns of convergent evolution in adaptation of function [temperature effects on K_M of cofactor (NADH)] and structural stability (rate of heat denaturation of activity). To determine how these differences depend on flexibilities of overall structure and of regions known to be important in binding and catalysis, we performed molecular dynamics simulation (MDS) analyses. MDS analyses revealed a significant negative correlation between adaptation temperature and heat-induced increase of backbone atom movements [root mean square deviation (rmsd) of main-chain atoms]. Root mean square fluctuations (RMSFs) of movement by individual amino acid residues varied across the sequence in a qualitatively similar pattern among orthologs. Regions of sequence involved in ligand binding and catalysis-termed mobile regions 1 and 2 (MR1 and MR2), respectively-showed the largest values for RMSF. Heat-induced changes in RMSF values across the sequence and, importantly, in MR1 and MR2 were greatest in cold-adapted species. MDS methods are shown to provide powerful tools for examining adaptation of enzymes by providing a quantitative index of protein flexibility and identifying sequence regions where adaptive change in flexibility occurs.

Arginine Methylation of MDH1 by CARM1 Inhibits Glutamine Metabolism and Suppresses Pancreatic Cancer

Distinctive from their normal counterparts, cancer cells exhibit unique metabolic dependencies on glutamine to fuel anabolic processes. Specifically, pancreatic ductal adenocarcinoma (PDAC) cells rely on an unconventional metabolic pathway catalyzed by aspartate aminotransferase, malate dehydrogenase 1 (MDH1), and malic enzyme 1 to rewire glutamine metabolism and support nicotinamide adenine dinucleotide phosphate (NADPH) production. Here, we report that methylation on arginine 248 (R248) negatively regulates MDH1. Protein arginine methyltransferase 4 (PRMT4/CARM1) methylates and inhibits MDH1 by disrupting its dimerization. Knockdown of MDH1 represses mitochondria respiration and inhibits glutamine metabolism, which sensitizes PDAC cells to oxidative stress and suppresses cell proliferation. Meanwhile, re-expression of wild-type MDH1, but not its methylation-mimetic mutant, protects cells from oxidative injury and restores cell growth and clonogenic activity. Importantly, MDH1 is hypomethylated at R248 in clinical PDAC samples. Our study reveals that arginine methylation of MDH1 by CARM1 regulates cellular redox homeostasis and suppresses glutamine metabolism of pancreatic cancer.

Nanocaged enzymes with enhanced catalytic activity and increased stability against protease digestion

Cells routinely compartmentalize enzymes for enhanced efficiency of their metabolic pathways. Here we report a general approach to construct DNA nanocaged enzymes for enhancing catalytic activity and stability. Nanocaged enzymes are realized by self-assembly into DNA nanocages with well-controlled stoichiometry and architecture that enabled a systematic study of the impact of both encapsulation and proximal polyanionic surfaces on a set of common metabolic enzymes. Activity assays at both bulk and single-molecule levels demonstrate increased substrate turnover numbers for DNA nanocage-encapsulated enzymes. Unexpectedly, we observe a significant inverse correlation between the size of a protein and its activity enhancement. This effect is consistent with a model wherein distal polyanionic surfaces of the nanocage enhance the stability of active enzyme conformations through the action of a strongly bound hydration layer. We further show that DNA nanocages protect encapsulated enzymes against proteases, demonstrating their practical utility in functional biomaterials and biotechnology.

Characterization of the kinetics of cardiac cytosolic malate dehydrogenase and comparative analysis of cytosolic and mitochondrial isoforms

Because the mitochondrial inner membrane is impermeable to pyridine nucleotides, transport of reducing equivalents between the mitochondrial matrix and the cytoplasm relies on shuttle mechanisms, including the malate-aspartate shuttle and the glycerol-3-phosphate shuttle. These shuttles are needed for reducing equivalents generated by metabolic reactions in the cytosol to be oxidized via aerobic metabolism. Two isoenzymes of malate dehydrogenase (MDH) operate as components of the malate-aspartate shuttle, in which a reducing equivalent is transported via malate, which when oxidized to oxaloacetate, transfers an electron pair to reduce NAD to NADH. Several competing mechanisms have been proposed for the MDH-catalyzed reaction. This study aims to identify the pH-dependent kinetic mechanism for cytoplasmic MDH (cMDH) catalyzed oxidation/reduction of MAL/OAA. Experiments were conducted assaying the forward and reverse directions with products initially present, varying pH between 6.5 and 9.0. By fitting time-course data to various mechanisms, it is determined that an ordered bi-bi mechanism with coenzyme binding first followed by the binding of substrate is able to explain the kinetic data. The proposed mechanism is similar to, but not identical to, the mechanism recently determined for the mitochondrial isoform, mMDH. cMDH and mMDH mechanisms are also shown to both be reduced versions of a common, more complex mechanism that can explain the kinetic data for both isoforms. Comparing the simulated activity (ratio of initial velocity to the enzyme concentration) under physiological conditions, the mitochondrial MDH (mMDH) activity is predicted to be higher than cMDH activity under mitochondrial matrix conditions while the cMDH activity is higher than mMDH activity under cytoplasmic conditions, suggesting that the functions of the isoforms are kinetically tuned to their individual physiological roles.

Combined metabolome and proteome analysis of the mantle tissue from Pacific oyster Crassostrea gigas exposed to elevated pCO2

Ocean acidification (OA) has been found to affect an array of normal physiological processes in mollusks, especially posing a significant threat to the fabrication process of mollusk shell. In the current study, the impact of exposure to elevated pCO2 condition was investigated in mantle tissue of Crassostrea gigas by an integrated metabolomic and proteomic approach. Analysis of metabolome and proteome revealed that elevated pCO2 could affect energy metabolism in oyster C. gigas, marked by differentially altered ATP, succinate, MDH, PEPCK and ALDH levels. Moreover, the up-regulated calponin-2, tropomyosins and myosin light chains indicated that elevated pCO2 probably caused disturbances in cytoskeleton structure in mantle tissue of oyster C. gigas. This work demonstrated that a combination of proteomics and metabolomics could provide important insights into the effects of OA at molecular levels.

Interface matters: the stiffness route to stability of a thermophilic tetrameric malate dehydrogenase

In this work we investigate by computational means the behavior of two orthologous bacterial proteins, a mesophilic and a thermophilic tetrameric malate dehydrogenase (MalDH), at different temperatures. Namely, we quantify how protein mechanical rigidity at different length- and time-scales correlates to protein thermophilicity as commonly believed. In particular by using a clustering analysis strategy to explore the conformational space of the folded proteins, we show that at ambient conditions and at the molecular length-scale the thermophilic variant is indeed more rigid that the mesophilic one. This rigidification is the result of more efficient inter-domain interactions, the strength of which is further quantified via ad hoc free energy calculations. When considered isolated, the thermophilic domain is indeed more flexible than the respective mesophilic one. Upon oligomerization, the induced stiffening of the thermophilic protein propagates from the interface to the active site where the loop, controlling the access to the catalytic pocket, anchors down via an extended network of ion-pairs. On the contrary in the mesophilic tetramer the loop is highly mobile. Simulations at high temperature, could not re-activate the mobility of the loop in the thermophile. This finding opens questions on the similarities of the binding processes for these two homologues at their optimal working temperature and suggests for the thermophilic variant a possible cooperative role of cofactor/substrate.

Multi-enzyme complexes on DNA scaffolds capable of substrate channelling with an artificial swinging arm

Swinging arms are a key functional component of multistep catalytic transformations in many naturally occurring multi-enzyme complexes. This arm is typically a prosthetic chemical group that is covalently attached to the enzyme complex via a flexible linker, allowing the direct transfer of substrate molecules between multiple active sites within the complex. Mimicking this method of substrate channelling outside the cellular environment requires precise control over the spatial parameters of the individual components within the assembled complex. DNA nanostructures can be used to organize functional molecules with nanoscale precision and can also provide nanomechanical control. Until now, protein-DNA assemblies have been used to organize cascades of enzymatic reactions by controlling the relative distance and orientation of enzymatic components or by facilitating the interface between enzymes/cofactors and electrode surfaces. Here, we show that a DNA nanostructure can be used to create a multi-enzyme complex in which an artificial swinging arm facilitates hydride transfer between two coupled dehydrogenases. By exploiting the programmability of DNA nanostructures, key parameters including position, stoichiometry and inter-enzyme distance can be manipulated for optimal activity.

An atomic-resolution view of neofunctionalization in the evolution of apicomplexan lactate dehydrogenases

Malate and lactate dehydrogenases (MDH and LDH) are homologous, core metabolic enzymes that share a fold and catalytic mechanism yet possess strict specificity for their substrates. In the Apicomplexa, convergent evolution of an unusual LDH from MDH produced a difference in specificity exceeding 12 orders of magnitude. The mechanisms responsible for this extraordinary functional shift are currently unknown. Using ancestral protein resurrection, we find that specificity evolved in apicomplexan LDHs by classic neofunctionalization characterized by long-range epistasis, a promiscuous intermediate, and few gain-of-function mutations of large effect. In canonical MDHs and LDHs, a single residue in the active-site loop governs substrate specificity: Arg102 in MDHs and Gln102 in LDHs. During the evolution of the apicomplexan LDH, however, specificity switched via an insertion that shifted the position and identity of this 'specificity residue' to Trp107f. Residues far from the active site also determine specificity, as shown by the crystal structures of three ancestral proteins bracketing the key duplication event. This work provides an unprecedented atomic-resolution view of evolutionary trajectories creating a nascent enzymatic function.

Modulation of genes related to specific metabolic pathways in response to cytosolic ascorbate peroxidase knockdown in rice plants

As a central component of the hydrogen peroxide detoxifying system in plant cells, ascorbate peroxidases (APX) play an essential role in the control of intracellular reactive oxygen species (ROS) levels. To characterise the function of cytosolic APX isoforms (OsAPX1 and OsAPX2) in the mechanisms of plant defence, OsAPX1/2 knockdown rice plants were previously obtained. OsAPX1/2 knockdown plants (APx1/2s) exhibited a normal phenotype and development, even though they showed a global reduction of APX activity and increased hydrogen peroxide accumulation. To understand how rice plants compensate for the deficiency of cytosolic APX, expression and proteomic analyses were performed to characterise the global expression pattern of the APx1/2s mutant line compared with non-transformed plants. Our results strongly suggest that deficiencies in cytosolic APX isoforms markedly alter expression of genes associated with several key metabolic pathways, especially of genes involved in photosynthesis and antioxidant defence. These metabolic changes are compensatory because central physiological processes such as photosynthesis and growth were similar to non-transformed rice plants. Our analyses showed modulation of groups of genes and proteins related to specific metabolic pathways. Among the differentially expressed genes, the largest number corresponded to those with catalytic activity. Genes related to oxidative stress, carbohydrate metabolism, photosynthesis and transcription factor-encoding genes were also modulated. These results represent an important step toward understanding of the role played by cytosolic APX isoforms and hydrogen peroxide in the regulation of metabolism by redox modulation in monocots.

Crystallization and preliminary X-ray diffraction of malate dehydrogenase from Plasmodium falciparum

The expression, purification, crystallization and preliminary X-ray diffraction characterization of malate dehydrogenase (MDH) from the malarial parasite Plasmodium falciparum (PfMDH) are reported. In order to gain a deeper understanding of the function and role of PfMDH, the protein was purified to homogeneity. The purified protein crystallized in space group P1, with unit-cell parameters a = 72, b = 157, c = 159 Å, α = 105, β = 101, γ = 95°. The resulting crystals diffracted to a maximal resolution of 2.24 Å and the structure has been solved by molecular replacement, with 16 monomers in the asymmetric unit. The 16 monomers are arranged into four independent tetramers, in agreement with previous reports demonstrating the tetrameric solution state of PfMDH. The X-ray structure of PfMDH is expected to clarify the differences in catalysis by PfMDH compared with other MDH family members and to provide a basis for the structure-based design of specific PfMDH inhibitors as well as general MDH inhibitors.

Design and synthesis of novel lactate dehydrogenase A inhibitors by fragment-based lead generation

Lactate dehydrogenase A (LDHA) catalyzes the conversion of pyruvate to lactate, utilizing NADH as a cofactor. It has been identified as a potential therapeutic target in the area of cancer metabolism. In this manuscript we report our progress using fragment-based lead generation (FBLG), assisted by X-ray crystallography to develop small molecule LDHA inhibitors. Fragment hits were identified through NMR and SPR screening and optimized into lead compounds with nanomolar binding affinities via fragment linking. Also reported is their modification into cellular active compounds suitable for target validation work.

Characterization of covalent Ene adduct intermediates in "hydride equivalent" transfers in a dihydropyridine model for NADH reduction reactions

A study of the reactions of an NADH model, 1,4-di(trimethylsilyl)-1,4-dihydropyridine, 7, with a series of α,β-unsaturated cyano and carbonyl compounds has produced the first direct evidence for an obligatory covalent adduct between a dihydropyridine and substrate in a reduction reaction. The reactions were monitored by NMR spectroscopy. In all reactions studied, the covalent adduct was the first new species detected and its decomposition to form products could be observed. Concentrations of adducts were sufficiently high at steady-state that their structures could be determined directly from NMR spectra of the reaction mixtures; adduct structures are those expected from an Ene reaction between 7 and the substrate. This first reaction step results in transfer of the C(4) hydrogen nucleus of 7 to the substrate and formation of a covalent bond between C(2) of the dihydropyridine ring and the substrate α-atom. Discovery of these Ene-adduct intermediates completes the spectrum of mechanisms observed in NADH model reactions to span those with free radical intermediates, no detectable intermediates and now covalent intermediates. The geometry of the transition state for formation of the Ene adduct is compared with those of theoretical transition state models and crystal structures of enzyme-substrate/inhibitor complexes to suggest a relative orientation for the dihydropyridine ring and the substrate in an initial cyclic transition state that is flexible enough to accommodate all observed mechanistic outcomes.

Cloning, sequencing and functional expression of cytosolic malate dehydrogenase from Taenia solium: Purification and characterization of the recombinant enzyme

We report herein the complete coding sequence of a Taenia solium cytosolic malate dehydrogenase (TscMDH). The cDNA fragment, identified from the T. solium genome project database, encodes a protein of 332 amino acid residues with an estimated molecular weight of 36517Da. For recombinant expression, the full length coding sequence was cloned into pET23a. After successful expression and enzyme purification, isoelectrofocusing gel electrophoresis allowed to confirm the calculated pI value at 8.1, as deduced from the amino acid sequence. The recombinant protein (r-TscMDH) showed MDH activity of 409U/mg in the reduction of oxaloacetate, with neither lactate dehydrogenase activity nor NADPH selectivity. Optimum pH for enzyme activity was 7.6 for oxaloacetate reduction and 9.6 for malate oxidation. K(cat) values for oxaloacetate, malate, NAD, and NADH were 665, 47, 385, and 962s(-1), respectively. Additionally, a partial characterization of TsMDH gene structure after analysis of a 1.56Kb genomic contig assembly is also reported.

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.

Temperature adaptation of cytosolic malate dehydrogenases of limpets (genus Lottia): differences in stability and function due to minor changes in sequence correlate with biogeographic and vertical distributions

We characterized functional and structural properties of cytoplasmic malate dehydrogenases (cMDHs) from six limpets of the genus Lottia that have different vertical and latitudinal distributions. Particular attention was given to the cryptic species pair Lottia digitalis (northern occurring) and L. austrodigitalis (southern occurring) because of recent contraction in the southern range of L. digitalis and a northward range extension of L. austrodigitalis. As an index of adaptation of function, we measured the effects of temperature on the apparent Michaelis-Menten constant (K(m)) of the cofactor NADH (K(m)(NADH)). K(m)(NADH) values of cMDHs from the mid- to high-intertidal, low-latitude species L. scabra and L. gigantea were less sensitive to high temperature than those of cMDHs from the low- and mid-intertidal, high-latitude species L. scutum and L. pelta. cMDH of L. digitalis was more sensitive to high temperatures than the cMDH ortholog of L. austrodigitalis. Thermal stability (rate of loss of activity at 42.5 degrees C) showed a similar pattern of interspecific variation. Comparison of the deduced amino acid sequences showed that interspecific differences ranged from one to as many as 17 residues. Differences in K(m)(NADH) and thermal stability between orthologs of L. digitalis and L. austrodigitalis result from a single amino acid substitution. At position 291, the glycine residue in cMDH of L. digitalis is replaced by a serine in cMDH of L. austrodigitalis, a change that favors additional hydrogen bonding and reduced conformational entropy. This difference between closely related congeners demonstrates the role of minor alterations in protein sequence in temperature adaptation and suggests that such variation is important in governing shifts in biogeographic range in response to climate change.

Analysis of quaternary structure of a [LDH-like] malate dehydrogenase of Plasmodium falciparum with oligomeric mutants

L-Malate dehydrogenase (PfMDH) from Plasmodium falciparum, the causative agent for the most severe form of malaria, has shown remarkable similarities to L: -lactate dehydrogenase (PfLDH). PfMDH is more closely related to [LDH-like] MDHs characterized in archae and other prokaryotes. Initial sequence analysis and identification of critical amino acid residues involved in inter-subunit salt-bridge interactions predict tetrameric structure for PfMDH. The catalytically active recombinant PfMDH was characterized as a tetramer. The enzyme is localized primarily in the parasites cytosol. To gain molecular insights into PfMDH/PfLDH relationships and to understand the quaternary structure of PfMDH, dimers were generated by mutation to the potential salt-bridge interacting sites. The R183A and R214G mutations, which snapped the salt bridges between the dimers and resulted in lower dimeric state, did not affect catalytic properties of the enzyme. The mutant dimers of PfMDH were active equally as the wild-type PfMDH. The studies reveal structure of PfMDH as a dimer of dimers. The tetrameric state of PfMDH was not essential for catalytic functions of the enzyme but may be an evolutionary adaptation for cytosolic localization to support its role in NAD/NADH coupling, an important metabolic function for survival of the malaria parasite.