Induced fit and the catalytic mechanism of isocitrate dehydrogenase

Tartrate Dehydrogenase in Bacillus Species: Deciphering Unique Catalytic Diversity Through Kinetic, Structural and Molecular Docking Analysis

Divergently evolved Tartrate dehydrogenase (TDH) exhibits multiple catalytic activities at a single active site; the enzyme from P. putida (pTDH) being structurally and biochemically well-characterized. Occurrence of TDH-associated ability to aerobically metabolize L-tartrate in Bacillus isolates and limited resemblance of ycsA-encoded protein sequences with pTDH rendered Bacillus TDH as an intriguing enzyme with possible catalytic diversity as well as evolutionary significance. The present study explores substrate interactions of TDHs from B. subtilis 168 (168bTDH) and B. licheniformis DSM-13 (429bTDH) through kinetic, structural and molecular docking-based analysis. Heterologously expressed bTDHs, purified from insoluble fractions of E. coli BL21(DE3) cells, could significantly catalyze L-tartrate and meso-tartrate as substrates in forward reaction. Unlike pTDH, bTDHs distinctly and more efficiently catalyzed the reverse reaction using dihydroxyfumarate substrate following sigmoidal kinetics; the ability being ~ 4 fold higher in 168bTDH. Their binding energies predicted from molecular docking, further substantiated the relative substrate specificities, while revealing major residues involved in protein-ligand interactions at active site. The kinetic analysis and homology modelling validated using Ramachandran Plot analysis predicted a dimeric nature for bTDH. Collectively, the results highlight unique catalytic potential of phylogenetically recent bTDHs, offering an important protein engineering target to mediate efficient enantioselective enzymatic biotransformations.

Drug Design in the Exascale Era: A Perspective from Massively Parallel QM/MM Simulations

The initial phases of drug discovery - in silico drug design - could benefit from first principle Quantum Mechanics/Molecular Mechanics (QM/MM) molecular dynamics (MD) simulations in explicit solvent, yet many applications are currently limited by the short time scales that this approach can cover. Developing scalable first principle QM/MM MD interfaces fully exploiting current exascale machines - so far an unmet and crucial goal - will help overcome this problem, opening the way to the study of the thermodynamics and kinetics of ligand binding to protein with first principle accuracy. Here, taking two relevant case studies involving the interactions of ligands with rather large enzymes, we showcase the use of our recently developed massively scalable Multiscale Modeling in Computational Chemistry (MiMiC) QM/MM framework (currently using DFT to describe the QM region) to investigate reactions and ligand binding in enzymes of pharmacological relevance. We also demonstrate for the first time strong scaling of MiMiC-QM/MM MD simulations with parallel efficiency of ∼70% up to >80,000 cores. Thus, among many others, the MiMiC interface represents a promising candidate toward exascale applications by combining machine learning with statistical mechanics based algorithms tailored for exascale supercomputers.

MiMiCPy: An Efficient Toolkit for MiMiC-Based QM/MM Simulations

MiMiC is a highly flexible, extremely scalable multiscale modeling framework. It couples the CPMD (quantum mechanics, QM) and GROMACS (molecular mechanics, MM) codes. The code requires preparing separate input files for the two programs with a selection of the QM region. This can be a tedious procedure prone to human error, especially when dealing with large QM regions. Here, we present MiMiCPy, a user-friendly tool that automatizes the preparation of MiMiC input files. It is written in Python 3 with an object-oriented approach. The main subcommand PrepQM can be used to generate MiMiC inputs directly from the command line or through a PyMOL/VMD plugin for visually selecting the QM region. Many other subcommands are also provided for debugging and fixing MiMiC input files. MiMiCPy is designed with a modular structure that allows seamless extensions to new program formats depending on the requirements of MiMiC.

A CRISPRi mediated self-inducible system for dynamic regulation of TCA cycle and improvement of itaconic acid production in Escherichia coli

Itaconic acid (ITA), an effective alternative fossil fuel, derives from the bypass pathway of the tricarboxylic acid (TCA) cycle. Therefore, the imbalance of metabolic flux between TCA cycle and ITA biosynthetic pathway seriously limits the production of ITA. The optimization of flux distribution between biomass and production has the potential to the productivity of ITA. Based on the previously constructed strain Escherichia coli MG1655 Δ1-SAS-3 (ITA titer: 1.87 g/L), a CRISPRi-mediated self-inducible system (CiMS), which contained a responsive module based on the ITA biosensor YpItcR/P ccl and a regulative CRISPRi-mediated interferential module, was developed to regulate the flux of the TCA cycle and to enhance the capacity of the strain to produce ITA. First, a higher ITA-yielding strain, Δ4-P rmd -SAS-3 (ITA titer: 3.20 g/L), derived from Δ1-SAS-3, was constructed by replacing the promoter P J23100 , for the expression of ITA synthesis genes, with P rmd and knocking out the three bypass genes poxB, pflB, and ldhA. Subsequently, the CiMS was used to inhibit the expression of key genes icd, pykA, and sucCD to dynamically balance the metabolic flux between TCA cycle and ITA biosynthetic pathway during the ITA production stage. The constructed strain Δ4-P rmd -SAS-3 under the dynamic regulation of the CiMS, showed a 23% increase in the ITA titer, which reached 3.93 g/L. This study indicated that CiMS was a practical strategy to dynamically and precisely regulated the metabolic flux in microbial cell factories.

Photocontrol of Itaconic Acid Synthesis in Escherichia coli

Metabolic engineering aims to control cellular metabolic flow and maximize the production of a product of interest. Photocontrol of the activities of proteins is an effective method for accurately regulating metabolic pathways. In this study, we inserted the photosensor light-oxygen-voltage-sensing domain 2 of Avena sativa (AsLOV2) into selected sites of isocitrate dehydrogenase (IDH), the key enzyme in the competitive pathway of itaconic acid (ITA) synthesis, to construct photoswitchable IDH-AsLOV2 (ILOVs). These engineered light-sensitive proteins were used to regulate the metabolic flux of the tricarboxylic acid (TCA) cycle in Escherichia coli to improve ITA production. The engineered fusion proteins ILOV2, ILOV3, ILOV6, and ILOV7 exhibited effective reversibility under the oscillation of darkness and blue light illumination in vitro. The efficacies of the intracellular photoswitches were evaluated, and an optimal photocontrol strategy was established in vivo. The ITA titer was significantly enhanced to 3.30 g/L for strain ITAΔ43, which displayed superior photoswitchable potency for ITA production compared with the strains that completely deleted the icd gene. The photocontrol strategy developed here can be extended for process optimization and titer improvement of other high-value bioengineering chemicals.

6-Phosphogluconate dehydrogenase and its crystal structures

6-Phosphogluconate dehydrogenase (6PGDH; EC 1.1.1.44) catalyses the oxidative decarboxylation of 6-phosphogluconate to ribulose 5-phosphate in the context of the oxidative part of the pentose phosphate pathway. Depending on the species, it can be a homodimer or a homotetramer. Oligomerization plays a functional role not only because the active site is at the interface between subunits but also due to the interlocking tail-modulating activity, similar to that of isocitrate dehydrogenase and malic enzyme, which catalyse a similar type of reaction. Since the pioneering crystal structure of sheep liver 6PGDH, which allowed motifs common to the β-hydroxyacid dehydrogenase superfamily to be recognized, several other 6PGDH crystal structures have been solved, including those of ternary complexes. These showed that more than one conformation exists, as had been suggested for many years from enzyme studies in solution. It is inferred that an asymmetrical conformation with a rearrangement of one of the two subunits underlies the homotropic cooperativity. There has been particular interest in the presence or absence of sulfate during crystallization. This might be related to the fact that this ion, which is a competitive inhibitor that binds in the active site, can induce the same 6PGDH configuration as in the complexes with physiological ligands. Mutagenesis, inhibitors, kinetic and binding studies, post-translational modifications and research on the enzyme in cancer cells have been complementary to the crystallographic studies. Computational modelling and new structural studies will probably help to refine the understanding of the functioning of this enzyme, which represents a promising therapeutic target in immunity, cancer and infective diseases. 6PGDH also has applied-science potential as a biosensor or a biobattery. To this end, the enzyme has been efficiently immobilized on specific polymers and nanoparticles. This review spans the 6PGDH literature and all of the 6PGDH crystal structure data files held by the Protein Data Bank.

Biochemical Characterization of Recombinant Isocitrate Dehydrogenase and Its Putative Role in the Physiology of an Acidophilic Micrarchaeon

Despite several discoveries in recent years, the physiology of acidophilic Micrarchaeota, such as "Candidatus Micrarchaeum harzensis A_DKE", remains largely enigmatic, as they highly express numerous genes encoding hypothetical proteins. Due to a lacking genetic system, it is difficult to elucidate the biological function of the corresponding proteins and heterologous expression is required. In order to prove the viability of this approach, A_DKE's isocitrate dehydrogenase (MhIDH) was recombinantly produced in Escherichia coli and purified to electrophoretic homogeneity for biochemical characterization. MhIDH showed optimal activity around pH 8 and appeared to be specific for NADP+ yet promiscuous regarding divalent cations as cofactors. Kinetic studies showed KM-values of 53.03 ± 5.63 µM and 1.94 ± 0.12 mM and kcat-values of 38.48 ± 1.62 and 43.99 ± 1.46 s-1 resulting in kcat/KM-values of 725 ± 107.62 and 22.69 ± 2.15 mM-1 s-1 for DL-isocitrate and NADP+, respectively. MhIDH's exceptionally low affinity for NADP+, potentially limiting its reaction rate, can likely be attributed to the presence of a proline residue in the NADP+ binding pocket, which might cause a decrease in hydrogen bonding of the cofactor and a distortion of local secondary structure.

Crystal structures of NAD+-linked isocitrate dehydrogenase from the green alga Ostreococcus tauri and its evolutionary relationship with eukaryotic NADP+-linked homologs

NAD+-linked isocitrate dehydrogenases (NAD-IDHs) catalyze the oxidative decarboxylation of isocitrate into α-ketoglutarate. Previously, we identified a novel phylogenetic clade including NAD-IDHs from several algae in the type II subfamily, represented by homodimeric NAD-IDH from Ostreococcus tauri (OtIDH). However, due to its lack of a crystalline structure, the molecular mechanisms of the ligand binding and catalysis of OtIDH are little known. Here, we elucidate four high-resolution crystal structures of OtIDH in a ligand-free and various ligand-bound forms that capture at least three states in the catalytic cycle: open, semi-closed, and fully closed. Our results indicate that OtIDH shows several novel interactions with NAD+, unlike type I NAD-IDHs, as well as a strictly conserved substrate binding mode that is similar to other homologs. The central roles of Lys283' in dual coenzyme recognition and Lys234 in catalysis were also revealed. In addition, the crystal structures obtained here also allow us to understand the catalytic mechanism. As expected, structural comparisons reveal that OtIDH has a very high structural similarity to eukaryotic NADP+-linked IDHs (NADP-IDHs) within the type II subfamily rather than with the previously reported NAD-IDHs within the type I subfamily. It has also been demonstrated that OtIDH exhibits substantial conformation changes upon ligand binding, similar to eukaryotic NADP-IDHs. These results unambiguously support our hypothesis that OtIDH and OtIDH-like homologs are possible evolutionary ancestors of eukaryotic NADP-IDHs in type II subfamily.

Isocitrate dehydrogenase 1 from Acinetobacter baummanii (AbIDH1) enzymatic characterization and its regulation by phosphorylation

Acinetobacter baumannii encodes all enzymes required in the tricarboxylic acid (TCA) cycle and glyoxylate bypass except for isocitrate dehydrogenase kinase/phosphatase (IDHKP), which can phosphorylate isocitrate dehydrogenase (IDH) at a substrate-binding Ser site and control the carbon flux in enterobacteria, such as Escherichia coli. The potential kinase was not successfully pulled down from A. baumannii cell lyase; therefore, whether the IDH 1 from A. baumannii (AbIDH1) can be phosphorylated to regulate intracellular carbon flux has not been clarified. Herein, the AbIDH1 gene was cloned, the encoded protein was expressed and purified to homogeneity, and phosphorylation and enzyme kinetics were evaluated in vitro. Gel filtration and SDS-PAGE analyses showed that AbIDH1 is an 83.5 kDa homodimer in solution. The kinetics showed that AbIDH1 is a fully active NADP-dependent enzyme. The Michaelis constant K_m is 46.6 (Mn²⁺) and 18.1 μM (Mg²⁺) for NADP⁺ and 50.5 (Mn²⁺) and 65.4 μM (Mg²⁺) for the substrate isocitrate. Phosphorylation experiments in vitro indicated that AbIDH1 is a substrate for E. coli IDHKP. The activity of AbIDH1 treated with E. coli IDHKP immediately decreased by 80% within 9 min. Mass spectrometry indicated that the conserved Ser113 of AbIDH1 is phosphorylated. Continuous phosphorylation-mimicking mutants (Ser113Glu and Ser113Asp) lack almost all enzymatic activity. Side-chain mutations at Ser113 (Ser113Thr, Ser113Ala, Ser113Gly and Ser113Tyr) remarkably reduce the enzymatic activity. Understanding the potential of AbIDH1 phosphorylation enables further investigations of the AbIDH1 physiological functions in A. baumannii.

FATCAT 2.0: towards a better understanding of the structural diversity of proteins

FATCAT 2.0 server (http://fatcat.godziklab.org/), provides access to a flexible protein structure alignment algorithm developed in our group. In such an alignment, rotations and translations between elements in the structure are allowed to minimize the overall root mean square deviation (RMSD) between the compared structures. This allows to effectively compare protein structures even if they underwent structural rearrangements in different functional forms, different crystallization conditions or as a result of mutations. The major update for the server introduces a new graphical interface, much faster database searches and several new options for visualization of the structural differences between proteins

Tricarboxylic acid cycle dehydrogenases inhibition by naringenin: experimental and molecular modelling evidence

The study of polyphenols’ effects on health has been gaining attention lately. In addition to reacting with important enzymes, altering the cell metabolism, these substances can present either positive or negative metabolic alterations depending on their consumption levels. Naringenin, a citrus flavonoid, already presents diverse metabolic effects. The objective of this work was to evaluate the effect of maternal naringenin supplementation during pregnancy on the tricarboxylic acid cycle activity in offspring’s cerebellum. Adult female Wistar rats were divided into two groups: (1) vehicle (1 ml/kg by oral administration (p.o.)) or (2) naringenin (50 mg/kg p.o.). The offspring were euthanised at 7th day of life, and the cerebellum was dissected to analyse citrate synthase, isocitrate dehydrogenase (IDH), α-ketoglutarate dehydrogenase (α-KGDH) and malate dehydrogenase (MDH) activities. Molecular docking used SwissDock web server and FORECASTER Suite, and the proposed binding pose image was created on UCSF Chimera. Data were analysed by Student’s t test. Naringenin supplementation during pregnancy significantly inhibited IDH, α-KGDH and MDH activities in offspring’s cerebellum. A similar reduction was observed in vitro, using purified α-KGDH and MDH, subjected to pre-incubation with naringenin. Docking simulations demonstrated that naringenin possibly interacts with dehydrogenases in the substrate and cofactor binding sites, inhibiting their function. Naringenin administration during pregnancy may affect cerebellar development and must be evaluated with caution by pregnant women and their physicians.

Molecular basis for the function of the αβ heterodimer of human NAD-dependent isocitrate dehydrogenase

Mammalian mitochondrial NAD-dependent isocitrate dehydrogenase (NAD-IDH) catalyzes the decarboxylation of isocitrate into α-ketoglutarate in the tricarboxylic acid cycle. It exists as the α₂βγ heterotetramer composed of the αβ and αγ heterodimers. Different from the αγ heterodimer that can be allosterically activated by CIT and ADP, the αβ heterodimer cannot be allosterically regulated by the activators; however, the molecular mechanism is unclear. We report here the crystal structures of the αβ heterodimer of human NAD-IDH with the α subunit in apo form and in Ca²⁺-bound, NAD-bound, and NADH-bound forms. Structural analyses and comparisons reveal that the αβ heterodimer has a similar yet more compact overall structure compared with the αγ heterodimer and contains a pseudo-allosteric site that is structurally different from the allosteric site. In particular, the β3-α3 and β12-α8 loops of the β subunit at the pseudo-allosteric site adopt significantly different conformations from those of the γ subunit at the allosteric site and hence impede the binding of the activators, explaining why the αβ heterodimer cannot be allosterically regulated by the activators. The structural data also show that NADH can compete with NAD to bind to the active site and inhibits the activity of the αβ heterodimer. These findings together with the biochemical data reveal the molecular basis for the function of the αβ heterodimer of human NAD-IDH.

Proteomic profiling of Sporotrichum thermophile under the effect of ionic liquids: manifestation of an oxidative stress response

Sporotrichum thermophile, a known producer of industrial enzymes exhibited stability in the presence of ionic liquids (ILs).The study reports, for the first time, the stress response of S. thermophile upon exposure to ILs. In vitro assay showed increased anti-oxidative enzyme levels indicating ROS-mediated oxidative stress by ILs. The proteomic profile and identification of differential proteins confirmed the fungal adaptations by (i) increased expression of glycolytic enzymes and ATP synthases (ii) downregulation of TCA cycle and protein synthesis machinery components (iii) expression of HSP70 and catalase/peroxidase. These changes are indicative of metabolic regulation of many important pathways and how ILs can be used to manipulate protein behavior.

Adaptive Landscapes in the Age of Synthetic Biology

For nearly a century adaptive landscapes have provided overviews of the evolutionary process and yet they remain metaphors. We redefine adaptive landscapes in terms of biological processes rather than descriptive phenomenology. We focus on the underlying mechanisms that generate emergent properties such as epistasis, dominance, trade-offs and adaptive peaks. We illustrate the utility of landscapes in predicting the course of adaptation and the distribution of fitness effects. We abandon aged arguments concerning landscape ruggedness in favor of empirically determining landscape architecture. In so doing, we transform the landscape metaphor into a scientific framework within which causal hypotheses can be tested.

Crystal Structures of the Putative Isocitrate Dehydrogenase from Sulfolobus tokodaii Strain 7 in the Apo and NADP+-Bound Forms

Isocitrate dehydrogenase is a catabolic enzyme that acts during the third step of the tricarboxylic acid cycle. The hypothetical protein ST2166 from the archaeon Sulfolobus tokodaii was isolated and crystallized. It shares high primary structure homology with prokaryotic NADP⁺-dependent IDHs, suggesting that these enzymes share a common enzymatic mechanism. The crystal structure of ST2166 was determined at 2.0 Å resolution in the apo form, and then the structure of the crystal soaked with NADP⁺ was also determined at 2.4 Å resolution, which contained NADP⁺ bound at the putative active site. Comparisons between the structures of apo and NADP⁺-bound forms and NADP-IDHs from other prokaryotes suggest that prokaryotic NADP-IDHs recognize their cofactors using conserved Lys335, Tyr336, and Arg386 in ST2166 at the opening cleft before the domain closure.

Crystal structure of Arabidopsis thaliana glutamyl-tRNAGlu reductase in complex with NADPH and glutamyl-tRNAGlu reductase binding protein

In higher plants, the tetrapyrrole biosynthesis pathway starts from the reaction catalyzed by the rate-limiting enzyme, glutamyl-tRNA^Glu reductase (GTR). In Arabidopsis thaliana, GTR is controlled by post-transcriptional regulators such as GTR binding protein (GBP), which stimulates AtGTR activity. The NADPH-binding domain of AtGTR undergoes a substantial movement upon GBP binding. Here, we report the crystal structure of AtGTR-NADPH-GBP ternary complex. NADPH binding causes slight structural changes compared with the AtGTR-GBP binary complex, and possibly take a part of the space needed by the substrate glutamyl-tRNA^Glu. The highly reactive sulfhydryl group of the active-site residue Cys144 shows an obvious rotation, which may facilitate the hydride transfer from NADPH to the thioester intermediate to form glutamate-1-semialdehyde. Furthermore, Lys271, Lys274, Ser275, Asn278, and Gln282 of GBP participate in the interaction between AtGTR and GBP, and the stimulating effect of GBP decreased when all of these residues were mutated to Ala. When the Cys144 of AtGTR was mutated to Ser, AtGTR activity could not be detected even in the presence of GBP.

Characterizing Lysine Acetylation of Isocitrate Dehydrogenase in Escherichia coli

The Escherichia coli isocitrate dehydrogenase (ICDH) is one of the tricarboxylic acid cycle enzymes, playing key roles in energy production and carbon flux regulation. E. coli ICDH was the first bacterial enzyme shown to be regulated by reversible phosphorylation. However, the effect of lysine acetylation on E. coli ICDH, which has no sequence similarity with its counterparts in eukaryotes, is still unclear. Based on previous studies of E. coli acetylome and ICDH crystal structures, eight lysine residues were selected for mutational and kinetic analyses. They were replaced with acetyllysine by the genetic code expansion strategy or substituted with glutamine as a classic approach. Although acetylation decreased the overall ICDH activity, its effects were different site by site. Deacetylation tests demonstrated that the CobB deacetylase could deacetylate ICDH both in vivo and in vitro, but CobB was only specific for lysine residues at the protein surface. On the other hand, ICDH could be acetylated by acetyl-phosphate chemically in vitro. And in vivo acetylation tests indicated that the acetylation level of ICDH was correlated with the amounts of intracellular acetyl-phosphate. This study nicely complements previous proteomic studies to provide direct biochemical evidence for ICDH acetylation.

Structure and function of an ancestral-type β-decarboxylating dehydrogenase from Thermococcus kodakarensis

β-Decarboxylating dehydrogenases, which are involved in central metabolism, are considered to have diverged from a common ancestor with broad substrate specificity. In a molecular phylogenetic analysis of 183 β-decarboxylating dehydrogenase homologs from 84 species, TK0280 from Thermococcus kodakarensis was selected as a candidate for an ancestral-type β-decarboxylating dehydrogenase. The biochemical characterization of recombinant TK0280 revealed that the enzyme exhibited dehydrogenase activities toward homoisocitrate, isocitrate, and 3-isopropylmalate, which correspond to key reactions involved in the lysine biosynthetic pathway, tricarboxylic acid cycle, and leucine biosynthetic pathway, respectively. In T. kodakarensis, the growth characteristics of the KUW1 host strain and a TK0280 deletion strain suggested that TK0280 is involved in lysine biosynthesis in this archaeon. On the other hand, gene complementation analyses using Thermus thermophilus as a host revealed that TK0280 functions as both an isocitrate dehydrogenase and homoisocitrate dehydrogenase in this organism, but not as a 3-isopropylmalate dehydrogenase, most probably reflecting its low catalytic efficiency toward 3-isopropylmalate. A crystallographic study on TK0280 binding each substrate indicated that Thr71 and Ser80 played important roles in the recognition of homoisocitrate and isocitrate while the hydrophobic region consisting of Ile82 and Leu83 was responsible for the recognition of 3-isopropylmalate. These analyses also suggested the importance of a water-mediated hydrogen bond network for the stabilization of the β3-α4 loop, including the Thr71 residue, with respect to the promiscuity of the substrate specificity of TK0280.

Dual Role of the Active Site Residues of Thermus thermophilus 3-Isopropylmalate Dehydrogenase: Chemical Catalysis and Domain Closure

The key active site residues K185, Y139, D217, D241, D245, and N102 of Thermus thermophilus 3-isopropylmalate dehydrogenase (Tt-IPMDH) have been replaced, one by one, with Ala. A drastic decrease in the kcat value (0.06% compared to that of the wild-type enzyme) has been observed for the K185A and D241A mutants. Similarly, the catalytic interactions (Km values) of these two mutants with the substrate IPM are weakened by more than 1 order of magnitude. The other mutants retained some (1-13%) of the catalytic activity of the wild-type enzyme and do not exhibit appreciable changes in the substrate Km values. The pH dependence of the wild-type enzyme activity (pK = 7.4) is shifted toward higher values for mutants K185A and D241A (pK values of 8.4 and 8.5, respectively). For the other mutants, smaller changes have been observed. Consequently, K185 and D241 may constitute a proton relay system that can assist in the abstraction of a proton from the OH group of IPM during catalysis. Molecular dynamics simulations provide strong support for the neutral character of K185 in the resting state of the enzyme, which implies that K185 abstracts the proton from the substrate and D241 assists the process via electrostatic interactions with K185. Quantum mechanics/molecular mechanics calculations revealed a significant increase in the activation energy of the hydride transfer of the redox step for both D217A and D241A mutants. Crystal structure analysis of the molecular contacts of the investigated residues in the enzyme-substrate complex revealed their additional importance (in particular that of K185, D217, and D241) in stabilizing the domain-closed active conformation. In accordance with this, small-angle X-ray scattering measurements indicated the complete absence of domain closure in the cases of D217A and D241A mutants, while only partial domain closure could be detected for the other mutants. This suggests that the same residues that are important for catalysis are also essential for inducing domain closure.

Structural and energetic basis of isopropylmalate dehydrogenase enzyme catalysis

The three-dimensional structure of the enzyme 3-isopropylmalate dehydrogenase from the bacterium Thermus thermophilus in complex with Mn(2+) , its substrate isopropylmalate and its co-factor product NADH at 2.0 Å resolution features a fully closed conformation of the enzyme. Upon closure of the two domains, the substrate and the co-factor are brought into precise relative orientation and close proximity, with a distance between the C2 atom of the substrate and the C4N atom of the pyridine ring of the co-factor of approximately 3.0 Å. The structure further shows binding of a K(+) ion close to the active site, and provides an explanation for its known activating effect. Hence, this structure is an excellent mimic for the enzymatically competent complex. Using high-level QM/MM calculations, it may be demonstrated that, in the observed arrangement of the reactants, transfer of a hydride from the C2 atom of 3-isopropylmalate to the C4N atom of the pyridine ring of NAD(+) is easily possible, with an activation energy of approximately 15 kcal·mol(-1) . The activation energy increases by approximately 4-6 kcal·mol(-1) when the K(+) ion is omitted from the calculations. In the most plausible scenario, prior to hydride transfer the ε-amino group of Lys185 acts as a general base in the reaction, aiding the deprotonation reaction of 3-isopropylmalate prior to hydride transfer by employing a low-barrier proton shuttle mechanism involving a water molecule.

DATABASE

Structural data have been submitted to the Protein Data Bank under accession number 4F7I.

Escherichia coli D-malate dehydrogenase, a generalist enzyme active in the leucine biosynthesis pathway

The enzymes of the β-decarboxylating dehydrogenase superfamily catalyze the oxidative decarboxylation of D-malate-based substrates with various specificities. Here, we show that, in addition to its natural function affording bacterial growth on D-malate as a carbon source, the D-malate dehydrogenase of Escherichia coli (EcDmlA) naturally expressed from its chromosomal gene is capable of complementing leucine auxotrophy in a leuB(-) strain lacking the paralogous isopropylmalate dehydrogenase enzyme. To our knowledge, this is the first example of an enzyme that contributes with a physiologically relevant level of activity to two distinct pathways of the core metabolism while expressed from its chromosomal locus. EcDmlA features relatively high catalytic activity on at least three different substrates (L(+)-tartrate, D-malate, and 3-isopropylmalate). Because of these properties both in vivo and in vitro, EcDmlA may be defined as a generalist enzyme. Phylogenetic analysis highlights an ancient origin of DmlA, indicating that the enzyme has maintained its generalist character throughout evolution. We discuss the implication of these findings for protein evolution.

Crystal structure studies of NADP+ dependent isocitrate dehydrogenase from Thermus thermophilus exhibiting a novel terminal domain

NADP(+) dependent isocitrate dehydrogenase (IDH) is an enzyme catalyzing oxidative decarboxylation of isocitrate into oxalosuccinate (intermediate) and finally the product α-ketoglutarate. The crystal structure of Thermus thermophilus isocitrate dehydrogenase (TtIDH) ternary complex with citrate and cofactor NADP(+) was determined using X-ray diffraction method to a resolution of 1.80 Å. The overall fold of this protein was resolved into large domain, small domain and a clasp domain. The monomeric structure reveals a novel terminal domain involved in dimerization, very unique and novel domain when compared to other IDH's. And, small domain and clasp domain showing significant differences when compared to other IDH's of the same sub-family. The structure of TtIDH reveals the absence of helix at the clasp domain, which is mainly involved in oligomerization in other IDH's. Also, helices/beta sheets are absent in the small domain, when compared to other IDH's of the same sub family. The overall TtIDH structure exhibits closed conformation with catalytic triad residues, Tyr144-Asp248-Lys191 are conserved. Oligomerization of the protein is quantized using interface area and subunit-subunit interactions between protomers. Overall, the TtIDH structure with novel terminal domain may be categorized as a first structure of subfamily of type IV.

Evolution of a transition state: role of Lys100 in the active site of isocitrate dehydrogenase

An active site lysine essential to catalysis in isocitrate dehydrogenase (IDH) is absent from related enzymes. As all family members catalyze the same oxidative β-decarboxylation at the (2R)-malate core common to their substrates, it seems odd that an amino acid essential to one is not found in all. Ordinarily, hydride transfer to a nicotinamide C4 neutralizes the positive charge at N1 directly. In IDH, the negatively charged C4-carboxylate of isocitrate stabilizes the ground state positive charge on the adjacent nicotinamide N1, opposing hydride transfer. The critical lysine is poised to stabilize-and perhaps even protonate-an oxyanion formed on the nicotinamide 3-carboxamide, thereby enabling the hydride to be transferred while the positive charge at N1 is maintained. IDH might catalyze the same overall reaction as other family members, but dehydrogenation proceeds through a distinct, though related, transition state. Partial activation of lysine mutants by K(+) and NH4 (+) represents a throwback to the primordial state of the first promiscuous substrate family member.

Complex formation between malate dehydrogenase and isocitrate dehydrogenase from Bacillus subtilis is regulated by tricarboxylic acid cycle metabolites

In Bacillus subtilis, recent in vivo studies revealed that particular enzymes of the tricarboxylic acid cycle form complexes that allow an efficient transfer of metabolites. Remarkably, a complex of the malate dehydrogenase (Mdh) (EC 1.1.1.37) with isocitrate dehydrogenase (Icd) (EC 1.1.1.42) was identified, although both enzymes do not catalyze subsequent reactions. In the present study, the interactions between these enzymes were characterized in vitro by surface plasmon resonance in the absence and presence of their substrates and cofactors. These analyses revealed a weak but specific interaction between Mdh and Icd, which was specifically stimulated by a mixture of substrates and cofactors of Icd: isocitrate, NADP(+) and Mg(2+). Wild-type Icd converted these substrates too fast, preventing any valid quantitative analysis of the interaction with Mdh. Therefore, binding of the IcdS104P mutant to Mdh was quantified because the mutation reduced the enzymatic activity by 174-fold but did not affect the stimulatory effect of substrates and cofactors on Icd-Mdh complex formation. The analysis of the unstimulated Mdh-IcdS104P interaction revealed kinetic constants of k(a) = 2.0 ± 0.2 × 10(2) m(-1) ·s(-1) and k(d) = 1.0 ± 0.1 × 10(-3) ·s(-1) and a K(D) value of 5.0 ± 0.1 μm. Addition of isocitrate, NADP(+) and Mg(2+) stimulated the affinity of IcdS104P to Mdh by 33-fold (K(D) = 0.15 ± 0.01 μm, k(a) = 1.7 ± 0.7 × 10(3) m(-1) ·s(-1), k(d) = 2.6 ± 0.6 × 10(-4) ·s(-1)). Analyses of the enzymatic activities of wild-type Icd and Mdh showed that Icd activity doubles in the presence of Mdh, whereas Mdh activity was slightly reduced by Icd. In summary, these data indicate substrate control of complex formation in the tricarboxylic acid cycle metabolon assembly and maintenance of the α-ketoglutarate supply for amino acid anabolism in vivo.

Structural, kinetic and chemical mechanism of isocitrate dehydrogenase-1 from Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is the leading cause of death due to a bacterial infection. The success of the Mtb pathogen has largely been attributed to the nonreplicating, persistence phase of the life cycle, for which the glyoxylate shunt is required. In Escherichia coli, flux through the shunt is controlled by regulation of isocitrate dehydrogenase (ICDH). In Mtb, the mechanism of regulation is unknown, and currently, there is no mechanistic or structural information about ICDH. We optimized expression and purification to a yield sufficiently high to perform the first detailed kinetic and structural studies of Mtb ICDH-1. A large solvent kinetic isotope effect [(D2O)V = 3.0 ± 0.2, and (D2O)(V/Kisocitrate) = 1.5 ± 0.3] and a smaller primary kinetic isotope effect [(D)V = 1.3 ± 0.1, and (D)(V/K[2R-(2)H]isocitrate) = 1.5 ± 0.2] allowed us to perform the first multiple kinetic isotope effect studies on any ICDH and suggest a chemical mechanism. In this mechanism, protonation of the enolate to form product α-ketoglutarate is the rate-limiting step. We report the first structure of Mtb ICDH-1 to 2.18 Å by X-ray crystallography with NADPH and Mn(2+) bound. It is a homodimer in which each subunit has a Rossmann fold, and a common top domain of interlocking β sheets. Mtb ICDH-1 is most structurally similar to the R132H mutant human ICDH found in glioblastomas. Similar to human R132H ICDH, Mtb ICDH-1 also catalyzes the formation of α-hydroxyglutarate. Our data suggest that regulation of Mtb ICDH-1 is novel.