Isopentenyl diphosphate isomerase catalyzed reactions in D2O: product release limits the rate of this sluggish enzyme-catalyzed reaction

A guanidinium group is an effective mimic of the tertiary carbocation formed by isopentenyl diphosphate isomerase

Type I isopentenyl diphosphate isomerase is a metal-dependent enzyme that generates a tertiary carbocation intermediate during catalysis. This study describes an inhibitor (2-guanidinoethyl(dihydroxyphosphorylmethyl)phosphinate) of the isomerase that bears a guanidinium as a carbocation mimic and a phosphinylphosphonate as a non-hydrolyzable metal binding functionality. Inhibition kinetics show that the compound acts in a competitive manner with a K_i value of 129 nM (K_M,IPP/K_i = 27). An analogous inhibitor bearing a tertiary ammonium as the carbocation mimic was 50-fold less potent, suggesting that the planar guanidinium is a more effective carbocation mimic. Inhibitors bearing an acylated methanesulfonamide or a hydroxamate group in place of the pyrophosphate inhibited the enzyme at millimolar concentrations indicating that the isomerase is highly specific for binding to the diphosphate portion of the intermediate.

Enabling Role of Ligand-Driven Conformational Changes in Enzyme Evolution

Many enzymes that show a large specificity in binding the enzymatic transition state with a higher affinity than the substrate utilize substrate binding energy to drive protein conformational changes to form caged substrate complexes. These protein cages provide strong stabilization of enzymatic transition states. Using part of the substrate binding energy to drive the protein conformational change avoids a similar strong stabilization of the Michaelis complex and irreversible ligand binding. A seminal step in the development of modern enzyme catalysts was the evolution of enzymes that couple substrate binding to a conformational change. These include enzymes that function in glycolysis (triosephosphate isomerase), the biosynthesis of lipids (glycerol phosphate dehydrogenase), the hexose monophosphate shunt (6-phosphogluconate dehydrogenase), and the mevalonate pathway (isopentenyl diphosphate isomerase), catalyze the final step in the biosynthesis of pyrimidine nucleotides (orotidine monophosphate decarboxylase), and regulate the cellular levels of adenine nucleotides (adenylate kinase). The evolution of enzymes that undergo ligand-driven conformational changes to form active protein-substrate cages is proposed to proceed by selection of variants, in which the selected side chain substitutions destabilize a second protein conformer that shows compensating enhanced binding interactions with the substrate. The advantages inherent to enzymes that incorporate a conformational change into the catalytic cycle provide a strong driving force for the evolution of flexible protein folds such as the TIM barrel. The appearance of these folds represented a watershed event in enzyme evolution that enabled the rapid propagation of enzyme activities within enzyme superfamilies.

Adaptation of hydroxymethylbutenyl diphosphate reductase enables volatile isoprenoid production

Volatile isoprenoids produced by plants are emitted in vast quantities into the atmosphere, with substantial effects on global carbon cycling. Yet, the molecular mechanisms regulating the balance between volatile and non-volatile isoprenoid production remain unknown. Isoprenoids are synthesised via sequential condensation of isopentenyl pyrophosphate (IPP) to dimethylallyl pyrophosphate (DMAPP), with volatile isoprenoids containing fewer isopentenyl subunits. The DMAPP:IPP ratio could affect the balance between volatile and non-volatile isoprenoids, but the plastidic DMAPP:IPP ratio is generally believed to be similar across different species. Here we demonstrate that the ratio of DMAPP:IPP produced by hydroxymethylbutenyl diphosphate reductase (HDR/IspH), the final step of the plastidic isoprenoid production pathway, is not fixed. Instead, this ratio varies greatly across HDRs from phylogenetically distinct plants, correlating with isoprenoid production patterns. Our findings suggest that adaptation of HDR plays a previously unrecognised role in determining in vivo carbon availability for isoprenoid emissions, directly shaping global biosphere-atmosphere interactions.

Investigation of the methylerythritol 4-phosphate pathway for microbial terpenoid production through metabolic control analysis

BACKGROUND

Terpenoids are of high interest as chemical building blocks and pharmaceuticals. In microbes, terpenoids can be synthesized via the methylerythritol phosphate (MEP) or mevalonate (MVA) pathways. Although the MEP pathway has a higher theoretical yield, metabolic engineering has met with little success because the regulation of the pathway is poorly understood.

RESULTS

We applied metabolic control analysis to the MEP pathway in Escherichia coli expressing a heterologous isoprene synthase gene (ispS). The expression of ispS led to the accumulation of isopentenyl pyrophosphate (IPP)/dimethylallyl pyrophosphate (DMAPP) and severely impaired bacterial growth, but the coexpression of ispS and isopentenyl diphosphate isomerase (idi) restored normal growth and wild-type IPP/DMAPP levels. Targeted proteomics and metabolomics analysis provided a quantitative description of the pathway, which was perturbed by randomizing the ribosome binding site in the gene encoding 1-deoxyxylulose 5-phosphate synthase (Dxs). Dxs has a flux control coefficient of 0.35 (i.e., a 1% increase in Dxs activity resulted in a 0.35% increase in pathway flux) in the isoprene-producing strain and therefore exerted significant control over the flux though the MEP pathway. At higher dxs expression levels, the intracellular concentration of 2-C-methyl-D-erythritol-2,4-cyclopyrophosphate (MEcPP) increased substantially in contrast to the other MEP pathway intermediates, which were linearly dependent on the abundance of Dxs. This indicates that 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase (IspG), which consumes MEcPP, became saturated and therefore limited the flux towards isoprene. The higher intracellular concentrations of MEcPP led to the efflux of this intermediate into the growth medium.

DISCUSSION

These findings show the importance of Dxs, Idi and IspG and metabolite export for metabolic engineering of the MEP pathway and will facilitate further approaches for the microbial production of valuable isoprenoids.

Uncovering the Role of Key Active-Site Side Chains in Catalysis: An Extended Brønsted Relationship for Substrate Deprotonation Catalyzed by Wild-Type and Variants of Triosephosphate Isomerase

We report results of detailed empirical valence bond simulations that model the effect of several amino acid substitutions on the thermodynamic (ΔG°) and kinetic activation (ΔG^⧧) barriers to deprotonation of dihydroxyacetone phosphate (DHAP) and d-glyceraldehyde 3-phosphate (GAP) bound to wild-type triosephosphate isomerase (TIM), as well as to the K12G, E97A, E97D, E97Q, K12G/E97A, I170A, L230A, I170A/L230A, and P166A variants of this enzyme. The EVB simulations model the observed effect of the P166A mutation on protein structure. The E97A, E97Q, and E97D mutations of the conserved E97 side chain result in ≤1.0 kcal mol^-1 decreases in the activation barrier for substrate deprotonation. The agreement between experimental and computed activation barriers is within ±1 kcal mol^-1, with a strong linear correlation between ΔG^⧧ and ΔG° for all 11 variants, with slopes β = 0.73 (R² = 0.994) and β = 0.74 (R² = 0.995) for the deprotonation of DHAP and GAP, respectively. These Brønsted-type correlations show that the amino acid side chains examined in this study function to reduce the standard-state Gibbs free energy of reaction for deprotonation of the weak α-carbonyl carbon acid substrate to form the enediolate phosphate reaction intermediate. TIM utilizes the cationic side chain of K12 to provide direct electrostatic stabilization of the enolate oxyanion, and the nonpolar side chains of P166, I170, and L230 are utilized for the construction of an active-site cavity that provides optimal stabilization of the enediolate phosphate intermediate relative to the carbon acid substrate.

Mechanistic Studies of the Protonation-Deprotonation Reactions for Type 1 and Type 2 Isopentenyl Diphosphate:Dimethylallyl Diphosphate Isomerase

Type 1 and type 2 isopentenyl diphosphate:dimethylallyl diphosphate isomerase (IDI-1 and IDI-2) catalyze the interconversion of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP), the fundamental building blocks for biosynthesis of isoprenoid compounds. Previous studies indicate that both isoforms of IDI catalyze isomerization by a protonation-deprotonation mechanism. IDI-1 and IDI-2 are "sluggish" enzymes with turnover times of ∼10 s^-1 and ∼1 s^-1, respectively. We measured incorporation of deuterium into IPP and DMAPP in D₂O buffer for IDI-1 and IDI-2 under conditions where newly synthesized DMAPP is immediately and irreversibly removed by coupling its release to condensation with l-tryptophan catalyzed by dimethylallyltrytophan synthase. During the course of the reactions, we detected formation of d₁, d₂, and d₃ isotopologues of IPP and DMAPP, which were formed during up to five isomerizations between IPP and DMAPP during each turnover. The patterns for deuterium incorporation into IPP show that d₂-IPP is formed in preference to d₁-IPP for both enzymes. Analysis of the patterns of deuterium incorporation are consistent with a mechanism involving addition and removal of protons by a concerted asynchronous process, where addition substantially precedes removal, or a stepwise process through a short-lived (<3 ps) tertiary carbocationic intermediate. Previous work with mechanism-based inhibitors and related model studies supports a concerted asynchronous mechanism for the enzyme-catalyzed isomerizations.

Primary Deuterium Kinetic Isotope Effects: A Probe for the Origin of the Rate Acceleration for Hydride Transfer Catalyzed by Glycerol-3-Phosphate Dehydrogenase

Large primary deuterium kinetic isotope effects (1° DKIEs) on enzyme-catalyzed hydride transfer may be observed when the transferred hydride tunnels through the energy barrier. The following 1° DKIEs on k_cat/K_m and relative reaction driving force are reported for wild-type and mutant glycerol-3-phosphate dehydrogenase (GPDH)-catalyzed reactions of NADL (L = H, D): wild-type GPDH, ΔΔG^⧧ = 0 kcal/mol, 1° DKIE = 1.5; N270A, 5.6 kcal/mol, 3.1; R269A, 9.1 kcal/mol, 2.8; R269A + 1.0 M guanidine, 2.4 kcal/mol, 2.7; R269A/N270A, 11.5 kcal/mol, 2.4. Similar 1° DKIEs were observed on k_cat. The narrow range of 1° DKIEs (2.4–3.1) observed for a 9.1 kcal/mol change in reaction driving force provides strong evidence that these are intrinsic 1° DKIEs on rate-determining hydride transfer. Evidence is presented that the intrinsic DKIE on wild-type GPDH-catalyzed reduction of DHAP lies in this range. A similar range of 1° DKIEs (2.4–2.9) on (k_cat/K_GA, M^–1 s^–1) was reported for dianion-activated hydride transfer from NADL to glycolaldehyde (GA) [Reyes, A. C.; Amyes, T. L.; Richard, J. P. J. Am. Chem. Soc.2016, 138, 14526–14529]. These 1° DKIEs are much smaller than those observed for enzyme-catalyzed hydrogen transfer that occurs mainly by quantum mechanical tunneling. These results support the conclusion that the rate acceleration for GPDH-catalyzed reactions is due to the stabilization of the transition state for hydride transfer by interactions with the protein catalyst. The small 1° DKIEs reported for mutant GPDH-catalyzed and for wild-type dianion-activated reactions are inconsistent with a model where the dianion binding energy is utilized in the stabilization of a tunneling ready state.

Insights into grapevine defense response against drought as revealed by biochemical, physiological and RNA-Seq analysis

Grapevine is an important and extensively grown fruit crop, which is severely hampered by drought worldwide. So, comprehending the impact of drought on grapevine genetic resources is necessary. In the present study, RNA-sequencing was executed using cDNA libraries constructed from both drought-stress and control plants. Results generated 12,451 differentially expressed genes (DEGs), out of which 8,021 genes were up-regulated, and 4,430 were down-regulated. Further physiological and biochemical investigations were also performed to validate the biological processes associated with the development of grapevine in response to drought stress. Results also revealed that decline in the rate of stomatal conductance, in turn, decrease the photosynthetic activity and CO2 assimilation in the grapevine leaves. Reactive oxygen species, including stress enzymes and their related proteins, and secondary metabolites were also activated in the present study. Likewise, various hormones also induced in response to drought stress. Overall, the present study concludes that these DEGs play both positive and negative roles in drought tolerance by regulating various biological pathways of grapevine. Nevertheless, our findings have provided valuable gene information for future studies of abiotic stress in grapevine and various other fruit crops.

Applications of deuterium oxide in human health

The main aim goal of this review was to gather information about recent publications related to deuterium oxide (D₂O), and its use as a scientific tool related to human health. Searches were made in electronic databases Pubmed, Scielo, Lilacs, Medline and Cochrane. Moreover, the following patent databases were consulted: EPO (Espacenet patent search), USPTO (United States Patent and Trademark Office) and Google Patents, which cover researches worldwide related to innovations using D₂O.

Comparative transcriptome analysis of grapevine in response to copper stress

Grapevine is one of the most economically important and widely cultivated fruit crop worldwide. With the industrialization and the popular application of cupric fungicides in grape industry, copper stress and copper pollution are also the factors affecting grape production and berry and wine quality. Here, 3,843 transcripts were significantly differently expressed genes in response to Cu stress by RNA-seq, which included 1,892 up-regulated and 1,951 down-regulated transcripts. During this study we found many known and novel Cu-induced and -repressed genes. Biological analysis of grape samples were indicated that exogenous Cu can influence chlorophylls metabolism and photosynthetic activities of grapevine. Most ROS detoxification systems, including antioxidant enzyme, stress-related proteins and secondary metabolites were strongly induced. Concomitantly, abscisic acid functioned as a negative regulator in Cu stress, in opposite action to ethylene, auxin, jasmonic acid, and brassinolide. This study also identified a set of Cu stress specifically activated genes coding copper transporter, P1B-type ATPase, multidrug transporters. Overall, this work was carried out to gain insights into the copper-regulated and stress-responsive mechanisms in grapevine at transcriptome level. This research can also provide some genetic information that can help us in better vinery management and breeding Cu-resistant grape cultivars.

Enzyme architecture: on the importance of being in a protein cage

Substrate binding occludes water from the active sites of many enzymes. There is a correlation between the burden to enzymatic catalysis of deprotonation of carbon acids and the substrate immobilization at solvent-occluded active sites for ketosteroid isomerase (KSI--small burden, substrate pKa=13), triosephosphate isomerase (TIM, substrate pKa≈18) and diaminopimelate epimerase (DAP epimerase, large burden, substrate pKa≈29) catalyzed reaction. KSI binds substrates at a surface cleft, TIM binds substrate at an exposed 'cage' formed by closure of flexible loops; and, DAP epimerase binds substrate in a tight cage formed by an 'oyster-like' clamping motion of protein domains. Directed evolution of a solvent-occluded active site at a designed protein catalyst of the Kemp elimination reaction is discussed.

Characterization of an isopentenyl diphosphate isomerase involved in the juvenile hormone pathway in Aedes aegypti

Isopentenyl diphosphate isomerase (IPPI) is an enzyme involved in the synthesis of juvenile hormone (JH) in the corpora allata (CA) of insects. IPPI catalyzes the conversion of isopentenyl pyrophosphate (IPP) to dimethylallyl pyrophosphate (DMAPP); afterward IPP and DMAPP condense in a head-to-tail manner to produce geranyl diphosphate (GPP), this head-to-tail condensation can be repeated, by the further reaction of GPP with IPP, yielding the JH precursor farnesyl diphosphate. An IPPI expressed sequence tag (EST) was obtained from an Aedes aegypti corpora-allata + corpora cardiaca library. Its full-length cDNA encodes a 244-aa protein that shows a high degree of similarity with type I IPPIs from other organisms, particularly for those residues that have important roles in catalysis, metal coordination and interaction with the diphosphate moiety of the IPP. Heterologous expression produced a recombinant protein that metabolized IPP into DMAPP; treatment of DMAPP with phosphoric acid produced isoprene, a volatile compound that was measured with an assay based on a solid-phase micro extraction protocol and direct analysis by gas chromatography. A. aegypti IPPI (AaIPPI) required Mg(2+) or Mn(2+) but not Zn(2+) for full activity and it was entirely inhibited by iodoacetamide. Real time PCR experiments showed that AaIPPI is highly expressed in the CA. Changes in AaIPPI mRNA levels in the CA in the pupal and adult female mosquito corresponded well with changes in JH synthesis (Li et al., 2003). This is the first molecular and functional characterization of an isopentenyl diphosphate isomerase involved in the production of juvenile hormone in the CA of an insect.

Naphthyl groups in chiral recognition: structures of salts and esters of 2-methoxy-2-naphthylpropanoic acids

The crystal structures of salt 8, which was prepared from (R)-2-methoxy-2-(2-naphthyl)propanoic acid ((R)-MβNP acid, (R)-2) and (R)-1-phenylethylamine ((R)-PEA, (R)-6), and salt 9, which was prepared from (R)-2-methoxy-2-(1-naphthyl)propanoic acid ((R)-MαNP acid, (R)-1) and (R)-1-(p-tolyl)ethylamine ((R)-TEA, (R)-7), were determined by X-ray crystallography. The MβNP and MαNP anions formed ion-pairs with the PEA and TEA cations, respectively, through a methoxy-group-assisted salt bridge and aromatic CH⋅⋅⋅π interactions. The networks of salt bridges formed 2(1) columns in both salts. Finally, (S)-(2E,6E)-(1-(2) H(1) )farnesol ((S)-13) was prepared from the reaction of (2E,6E)-farnesal (11) with deuterated (R)-BINAL-H (i.e., (R)-BINAL-D). The enantiomeric excess of compound (S)-13 was determined by NMR analysis of (S)-MαNP ester 14. The solution-state structures of MαNP esters that were prepared from primary alcohols were also elucidated.