Inositol-1-phosphate synthase from Archaeoglobus fulgidus is a class II aldolase

In Vitro and In Silico Explorations of the Protein Conformational Changes of Corynebacterium glutamicum MshA, a Model Retaining GT-B Glycosyltransferase

MshA is a GT-B glycosyltransferase catalyzing the first step in the biosynthesis of mycothiol. While many GT-B enzymes undergo an open-to-closed transition, MshA is unique because its 97° rotation is beyond the usual range of 10-25°. Molecular dynamics (MD) simulations were carried out for MshA in both ligand bound and unbound states to investigate the effect of ligand binding on localized protein dynamics and its conformational free energy landscape. Simulations showed that both the unliganded "opened" and liganded "closed" forms of the enzyme sample a wide degree of dihedral angles and interdomain distances with relatively low overlapping populations. Calculation of the free energy surface using replica exchange MD for the apo "opened" and an artificial generated apo "closed" structure revealed overlaps in the geometries sampled, allowing calculation of a barrier of 2 kcal/mol for the open-to-closed transition in the absence of ligands. MD simulations of fully liganded MshA revealed a smaller sampling of the dihedral angles. The localized protein fluctuation changes suggest that UDP-GlcNAc binding activates the motions of loops in the 1-l-myo-inositol-1-phosphate (I1P)-binding site despite little change in the interactions with UDP-GlcNAc. Circular dichroism, intrinsic fluorescence spectroscopy, and mutagenesis studies were used to confirm the ligand-induced structural changes in MshA. The results support a proposed mechanism where UDP-GlcNAc binds with rigid interactions to the C-terminal domain of MshA and activates flexible loops in the N-terminal domain for binding and positioning of I1P. This model can be used for future structure-based drug development of inhibitors of the mycothiol biosynthetic pathway.

Carrier-Free Immobilization of Multi-Enzyme Complex Facilitates In Vitro Synthetic Enzymatic Biosystem for Biomanufacturing

A method is developed for carrier-free immobilization of multi-enzyme complexes with more than four enzymes by utilization of polypeptide interactions (SpyCatcher-SpyTag and dockerin-cohesin) and enzyme component self-oligomerization. Two pairs of scaffoldins with different arrangements of SpyCatcher-SpyTag and cohesins are prepared to recruit the four dockerin-containing cascade enzymes (i. e., alpha-glucan phosphorylase, phosphoglucomutase, inositol 1-phosphate synthase, and inositol 1-phosphatase) that can convert starch into inositol, forming multi-enzyme complexes. These self-assembled enzyme complexes show higher initial reaction rates than the four-enzyme cocktail. Moreover, water-insoluble self-assembled multi-enzyme complexes are observed, being the carrier-free immobilized multi-enzyme complex aggregates. These immobilized enzyme complexes can be recycled easily by simple centrifuging followed by resuspension for another round of reaction. Not only can these immobilized enzyme complexes be obtained by mixing the purified enzyme components, but also by the mixing of crude cell extracts. Therefore, the strategy for the carrier-free immobilization of enzyme complex sheds light on improving the catalytic capability of in vitro synthetic enzymatic biosystems.

Open Issues for Protein Function Assignment in Haloferax volcanii and Other Halophilic Archaea

Background: Annotation ambiguities and annotation errors are a general challenge in genomics. While a reliable protein function assignment can be obtained by experimental characterization, this is expensive and time-consuming, and the number of such Gold Standard Proteins (GSP) with experimental support remains very low compared to proteins annotated by sequence homology, usually through automated pipelines. Even a GSP may give a misleading assignment when used as a reference: the homolog may be close enough to support isofunctionality, but the substrate of the GSP is absent from the species being annotated. In such cases, the enzymes cannot be isofunctional. Here, we examined a variety of such issues in halophilic archaea (class Halobacteria), with a strong focus on the model haloarchaeon Haloferax volcanii. Results: Annotated proteins of Hfx. volcanii were identified for which public databases tend to assign a function that is probably incorrect. In some cases, an alternative, probably correct, function can be predicted or inferred from the available evidence, but this has not been adopted by public databases because experimental validation is lacking. In other cases, a probably invalid specific function is predicted by homology, and while there is evidence that this assigned function is unlikely, the true function remains elusive. We listed 50 of those cases, each with detailed background information, so that a conclusion about the most likely biological function can be drawn. For reasons of brevity and comprehension, only the key aspects are listed in the main text, with detailed information being provided in a corresponding section of the Supplementary Materials. Conclusions: Compiling, describing and summarizing these open annotation issues and functional predictions will benefit the scientific community in the general effort to improve the evaluation of protein function assignments and more thoroughly detail them. By highlighting the gaps and likely annotation errors currently in the databases, we hope this study will provide a framework for experimentalists to systematically confirm (or disprove) our function predictions or to uncover yet more unexpected functions.

A Cotton (Gossypium hirsutum) Myo-Inositol-1-Phosphate Synthase (GhMIPS1D) Gene Promotes Root Cell Elongation in Arabidopsis

Myo-inositol-1-phosphate synthase (MIPS, EC 5.5.1.4) plays important roles in plant growth and development, stress responses, and cellular signal transduction. MIPS genes were found preferably expressed during fiber cell initiation and early fast elongation in upland cotton (Gossypium hirsutum), however, current understanding of the function and regulatory mechanism of MIPS genes to involve in cotton fiber cell growth is limited. Here, by genome-wide analysis, we identified four GhMIPS genes anchoring onto four chromosomes in G. hirsutum and analyzed their phylogenetic relationship, evolutionary dynamics, gene structure and motif distribution, which indicates that MIPS genes are highly conserved from prokaryotes to green plants, with further exon-intron structure analysis showing more diverse in Brassicales plants. Of the four GhMIPS members, based on the significant accumulated expression of GhMIPS1D at the early stage of fiber fast elongating development, thereby, the GhMIPS1D was selected to investigate the function of participating in plant development and cell growth, with ectopic expression in the loss-of-function Arabidopsis mips1 mutants. The results showed that GhMIPS1D is a functional gene to fully compensate the abnormal phenotypes of the deformed cotyledon, dwarfed plants, increased inflorescence branches, and reduced primary root lengths in Arabidopsis mips1 mutants. Furthermore, shortened root cells were recovered and normal root cells were significantly promoted by ectopic expression of GhMIPS1D in Arabidopsis mips1 mutant and wild-type plants respectively. These results serve as a foundation for understanding the MIPS family genes in cotton, and suggest that GhMIPS1D may function as a positive regulator for plant cell elongation.

Characterization of recombinant fructose-1,6-bisphosphatase gene mutations: evidence of inhibition/activation of FBPase protein by gene mutation

Specific residues of the highly regulated fructose-1,6-bisphosphatase (FBPase) enzyme serve as important contributors to the catalytic activity of the enzyme. Previous clinical studies exploring the genetic basis of hypoglycemia revealed two significant mutations in the coding region of the FBPase gene in patients with hypoglycemia, linking the AMP-binding site to the active site of the enzyme. In the present study, a full kinetic analysis of similar mutants was performed. Kinetic results of mutants Y164A and M177A revealed an approximate two to three-fold decrease in inhibitory constants (K _i's) for natural inhibitors AMP and fructose-2,6-bisphosphate (F2,6-BP) compared with the Wild-type enzyme (WT). A separate mutation (M248D) was performed in the active site of the enzyme to investigate whether the enzyme could be activated. This mutant displayed an approximate seven-fold increase in K _i for F2,6-BP. Interfacial mutants L56A and L73A exhibited an increase in K _i for F2,6-BP by approximately five-fold. Mutations in the AMP-binding site (K112A and Y113A) demonstrated an eight to nine-fold decrease in AMP inhibition. Additionally, mutant M248D displayed a four-fold decrease in its apparent Michelis constant (K _m), and a six-fold increase in catalytic efficiency (CE). The importance-and medical relevance-of specific residues for FBPase structural/functional relationships in both the catalytic site and AMP-binding site is discussed.

Myo-inositol-1-phosphate synthase (Ino-1) functions as a protection mechanism in Corynebacterium glutamicum under oxidative stress

Reactive oxygen species (ROS) generated in aerobic metabolism and oxidative stress lead to macromolecules damage, such as to proteins, lipids, and DNA, which can be eliminated by the redox buffer mycothiol (AcCys-GlcN-Ins, MSH). Myo-inositol-phosphate synthase (Ino-1) catalyzes the first committed step in the synthesis of MSH, thus playing a critical role in the growth of the organism. Although Ino-1s have been systematically studied in eukaryotes, their physiological and biochemical functions remain largely unknown in bacteria. In this study, we report that Ino-1 plays an important role in oxidative stress resistance in the gram-positive Actinobacteria Corynebacterium glutamicum. Deletion of the ino-1 gene resulted in a decrease in cell viability, an increase in ROS production, and the aggravation of protein carbonylation levels under various stress conditions. The physiological roles of Ino-1 in the resistance to oxidative stresses were corroborated by the absence of MSH in the Δino-1 mutant. In addition, we found that the homologous expression of Ino-1 in C. glutamicum yielded a functionally active protein, while when expressed in Escherichia coliBL21(DE3), it lacked measurable activity. An examination of the molecular mass (Mr) suggested that Ino-1 expressed in E. coliBL21(DE3) was not folded in a catalytically competent conformation. Together, the results unequivocally showed that Ino-1 was important for the mediation of oxidative resistance by C. glutamicum.

An evolutionary analysis identifies a conserved pentapeptide stretch containing the two essential lysine residues for rice L-myo-inositol 1-phosphate synthase catalytic activity

A molecular evolutionary analysis of a well conserved protein helps to determine the essential amino acids in the core catalytic region. Based on the chemical properties of amino acid residues, phylogenetic analysis of a total of 172 homologous sequences of a highly conserved enzyme, L-myo-inositol 1-phosphate synthase or MIPS from evolutionarily diverse organisms was performed. This study revealed the presence of six phylogenetically conserved blocks, out of which four embrace the catalytic core of the functional protein. Further, specific amino acid modifications targeting the lysine residues, known to be important for MIPS catalysis, were performed at the catalytic site of a MIPS from monocotyledonous model plant, Oryza sativa (OsMIPS1). Following this study, OsMIPS mutants with deletion or replacement of lysine residues in the conserved blocks were made. Based on the enzyme kinetics performed on the deletion/replacement mutants, phylogenetic and structural comparison with the already established crystal structures from non-plant sources, an evolutionarily conserved peptide stretch was identified at the active pocket which contains the two most important lysine residues essential for catalytic activity.

An In Vitro Enzyme System for the Production of myo-Inositol from Starch

We developed an in vitro enzyme system to produce myo-inositol from starch. Four enzymes were used, maltodextrin phosphorylase (MalP), phosphoglucomutase (PGM), myo-inositol-3-phosphate synthase (MIPS), and inositol monophosphatase (IMPase). The enzymes were thermostable: MalP and PGM from the hyperthermophilic archaeon Thermococcus kodakarensis, MIPS from the hyperthermophilic archaeon Archaeoglobus fulgidus, and IMPase from the hyperthermophilic bacterium Thermotoga maritima The enzymes were individually produced in Escherichia coli and partially purified by subjecting cell extracts to heat treatment and removing denatured proteins. The four enzyme samples were incubated at 90°C with amylose, phosphate, and NAD⁺, resulting in the production of myo-inositol with a yield of over 90% at 2 h. The effects of varying the concentrations of reaction components were examined. When the system volume was increased and NAD⁺ was added every 2 h, we observed the production of 2.9 g myo-inositol from 2.9 g amylose after 7 h, achieving gram-scale production with a molar conversion of approximately 96%. We further integrated the pullulanase from T. maritima into the system and observed myo-inositol production from soluble starch and raw potato with yields of 73% and 57 to 61%, respectively.IMPORTANCEmyo-Inositol is an important nutrient for human health and provides a wide variety of benefits as a dietary supplement. This study demonstrates an alternative method to produce myo-inositol from starch with an in vitro enzyme system using thermostable maltodextrin phosphorylase (MalP), phosphoglucomutase (PGM), myo-inositol-3-phosphate synthase, and myo-inositol monophosphatase. By utilizing MalP and PGM to generate glucose 6-phosphate, we can avoid the addition of phosphate donors such as ATP, the use of which would not be practical for scaled-up production of myo-inositol. myo-Inositol was produced from amylose on the gram scale with yields exceeding 90%. Conversion rates were also high, producing over 2 g of myo-inositol within 4 h in a 200-ml reaction mixture. By adding a thermostable pullulanase, we produced myo-inositol from raw potato with yields of 57 to 61% (wt/wt). The system developed here should provide an attractive alternative to conventional methods that rely on extraction or microbial production of myo-inositol.

Has Inositol Played Any Role in the Origin of Life?

Phosphorus, as phosphate, plays a paramount role in biology. Since phosphate transfer reactions are an integral part of contemporary life, phosphate may have been incorporated into the initial molecules at the very beginning. To facilitate the studies into early phosphate utilization, we should look retrospectively to phosphate-rich molecules present in today’s cells. Overlooked by origin of life studies until now, inositol and the inositol phosphates, of which some species possess more phosphate groups that carbon atoms, represent ideal molecules to consider in this context. The current sophisticated association of inositol with phosphate, and the roles that some inositol phosphates play in regulating cellular phosphate homeostasis, intriguingly suggest that inositol might have played some role in the prebiotic process of phosphate exploitation. Inositol can be synthesized abiotically and, unlike glucose or ribose, is chemically stable. This stability makes inositol the ideal candidate for the earliest organophosphate molecules, as primitive inositol phosphates. I also present arguments suggesting roles for some inositol phosphates in early chemical evolution events. Finally, the possible prebiotic synthesis of inositol pyrophosphates could have generated high-energy molecules to be utilized in primitive trans-phosphorylating processes.

An in vitro synthetic biology platform for the industrial biomanufacturing of myo-inositol from starch

Myo-Inositol (vitamin B8) is widely used in the drug, cosmetic, and food & feed industries. Here, we present an in vitro non-fermentative enzymatic pathway that converts starch to inositol in one vessel. This in vitro pathway is comprised of four enzymes that operate without ATP or NAD⁺ supplementation. All enzyme BioBricks are carefully selected from hyperthermophilic microorganisms, that is, alpha-glucan phosphorylase from Thermotoga maritima, phosphoglucomutase from Thermococcus kodakarensis, inositol 1-phosphate synthase from Archaeoglobus fulgidus, and inositol monophosphatase from T. maritima. They were expressed efficiently in high-density fermentation of Escherichia coli BL21(DE3) and easily purified by heat treatment. The four-enzyme pathway supplemented with two other hyperthermophilic enzymes (i.e., 4-α-glucanotransferase from Thermococcus litoralis and isoamylase from Sulfolobus tokodaii) converts branched or linear starch to inositol, accomplishing a very high product yield of 98.9 ± 1.8% wt./wt. This in vitro (aeration-free) biomanufacturing has been successfully operated on 20,000-L reactors. Less costly inositol would be widely added in heath food, low-end soft drink, and animal feed, and may be converted to other value-added biochemicals (e.g., glucarate). This biochemical is the first product manufactured by the in vitro synthetic biology platform on an industrial scale. Biotechnol. Bioeng. 2017;114: 1855-1864. © 2017 Wiley Periodicals, Inc.

A biocatalytic approach towards the stereoselective synthesis of protected inositols

Immobilized TbINO1 produces >400 mg of chiral inositol 1-phosphate in a biocatalytic flow process.

Recombinant expression of a functional myo-inositol-1-phosphate synthase (MIPS) in Mycobacterium smegmatis

Myo-inositol-1-phosphate synthase (MIPS, E.C. 5.5.1.4) catalyzes the first step in inositol production-the conversion of glucose-6-phosphate (Glc-6P) to myo-inositol-1-phosphate. While the three dimensional structure of MIPS from Mycobacterium tuberculosis has been solved, biochemical studies examining the in vitro activity have not been reported to date. Herein we report the in vitro activity of mycobacterial MIPS expressed in E. coli and Mycobacterium smegmatis. Recombinant expression in E. coli yields a soluble protein capable of binding the NAD(+) cofactor; however, it has no significant activity with the Glc-6P substrate. In contrast, recombinant expression in M. smegmatis mc(2)4517 yields a functionally active protein. Examination of structural data suggests that MtMIPS expressed in E. coli adopts a fold that is missing a key helix containing two critical (conserved) Lys side chains, which likely explains the inability of the E. coli expressed protein to bind and turnover the Glc-6P substrate. Recombinant expression in M. smegmatis may yield a protein that adopts a fold in which this key helix is formed enabling proper positioning of important side chains, thereby allowing for Glc-6P substrate binding and turnover. Detailed mechanistic studies may be feasible following optimization of the recombinant MIPS expression protocol in M. smegmatis.

Coordination, microprotonation equilibria and conformational changes of myo-inositol hexakisphosphate with pertinence to its biological function

Within all the eukaryotic cells there is an important group of biomolecules that has been potentially related to signalling functions: the myo-inositol phosphates (InsPs). In nature, the most abundant member of this family is the so called InsP6 (phytate, L(12-)), for which our group has strived in the past to elucidate its intricate chemical behaviour. In this work we expand on our earlier findings, shedding light on the inframolecular details of its protonation and complexation processes. We evaluate systematically the chemical performance of InsP6 in the presence and absence of alkali and alkaline earth metal ions, through (31)P NMR measurements, in a non-interacting medium and over a wide pH range. The analysis of the titration curves by means of a model based on the cluster expansion method allows us to describe in detail the distribution of the different protonated microspecies of the ligand. With the aid of molecular modelling tools, we assess the energetic and geometrical characteristics of the protonation sequence and the conformational transition suffered by InsP6 as the pH changes. By completely characterizing the protonation pattern, conformation and geometry of the metal complexes, we unveil the chemical and structural basis behind the influence that the physiologically relevant cations, Na(+), K(+), Mg(2+) and Ca(2+) have over the phytate chemical reactivity. This information is essential in the process of gaining reliable structural knowledge about the most important InsP6 species in the in vitro and in vivo experiments, and how these features modulate their probable biological functions.

Interaction of myo-inositol hexakisphosphate with biogenic and synthetic polyamines

myo-Inositol hexakisphosphate (phytate) forms very stable adducts with biogenic and synthetic polyamines in aqueous solution.

Reconstructed ancestral Myo-inositol-3-phosphate synthases indicate that ancestors of the Thermococcales and Thermotoga species were more thermophilic than their descendants

The bacterial genomes of Thermotoga species show evidence of significant interdomain horizontal gene transfer from the Archaea. Members of this genus acquired many genes from the Thermococcales, which grow at higher temperatures than Thermotoga species. In order to study the functional history of an interdomain horizontally acquired gene we used ancestral sequence reconstruction to examine the thermal characteristics of reconstructed ancestral proteins of the Thermotoga lineage and its archaeal donors. Several ancestral sequence reconstruction methods were used to determine the possible sequences of the ancestral Thermotoga and Archaea myo-inositol-3-phosphate synthase (MIPS). These sequences were predicted to be more thermostable than the extant proteins using an established sequence composition method. We verified these computational predictions by measuring the activities and thermostabilities of purified proteins from the Thermotoga and the Thermococcales species, and eight ancestral reconstructed proteins. We found that the ancestral proteins from both the archaeal donor and the Thermotoga most recent common ancestor recipient were more thermostable than their descendants. We show that there is a correlation between the thermostability of MIPS protein and the optimal growth temperature (OGT) of its host, which suggests that the OGT of the ancestors of these species of Archaea and the Thermotoga grew at higher OGTs than their descendants.

Phosphorylation regulates myo-inositol-3-phosphate synthase: a novel regulatory mechanism of inositol biosynthesis

myo-Inositol-3-phosphate synthase (MIPS) plays a crucial role in inositol homeostasis. Transcription of the coding gene INO1 is highly regulated. However, regulation of the enzyme is not well defined. We previously showed that MIPS is indirectly inhibited by valproate, suggesting that the enzyme is post-translationally regulated. Using (32)Pi labeling and phosphoamino acid analysis, we show that yeast MIPS is a phosphoprotein. Mass spectrometry analysis identified five phosphosites, three of which are conserved in the human MIPS. Analysis of phosphorylation-deficient and phosphomimetic site mutants indicated that the three conserved sites in yeast (Ser-184, Ser-296, and Ser-374) and humans (Ser-177, Ser-279, and Ser-357) affect MIPS activity. Both S296A and S296D yeast mutants and S177A and S177D human mutants exhibited decreased enzymatic activity, suggesting that a serine residue is critical at that location. The phosphomimetic mutations S184D (human S279D) and S374D (human S357D) but not the phosphodeficient mutations decreased activity, suggesting that phosphorylation of these two sites is inhibitory. The double mutation S184A/S374A caused an increase in MIPS activity, conferred a growth advantage, and partially rescued sensitivity to valproate. Our findings identify a novel mechanism of regulation of inositol synthesis by phosphorylation of MIPS.

Ectopic expression of the ABA-inducible dehydration-responsive chickpea L-myo-inositol 1-phosphate synthase 2 (CaMIPS2) in Arabidopsis enhances tolerance to salinity and dehydration stress

Myo-inositol participates in many different aspects of plant physiology and myo-inositol 1-phosphate synthase (MIPS; EC 5.5.1.4) catalyzes the rate limiting step of inositol biosynthetic pathway. Chickpea (Cicer arietinum), a drought-tolerant leguminous crop plant, is known to accumulate increased inositol during dehydration stress. Previously, we reported two differentially expressed divergent genes (CaMIPS1 and CaMIPS2) encoding two MIPS isoforms in chickpea. In this communication, we demonstrated that CaMIPS2 is an early dehydration-responsive gene and is also rapidly induced by exogenous ABA application, while CaMIPS1 expression is not much influenced by dehydration or ABA. The regulation of expression of these two genes has been studied by examining their promoter activity through GUS reporter gene and differential promoter activity has been observed. Moreover, unlike CaMIPS1 promoter, CaMIPS2 promoter contains CRT/DRE cis-regulatory element which seems to play a key role in dehydration-induced expression of CaMIPS2. Furthermore, CaMIPS1 and CaMIPS2 have been successfully complemented and shown to repair the defect of seedling growth and altered seed phenotype of Atmips1 mutant. Moreover, Arabidopsis transgenic plants overexpressing CaMIPS1 or CaMIPS2 exhibit improved tolerance to salinity and dehydration stresses and such tolerance of transgenic plants is correlated with their elevated level of inositol. Remarkably, CaMIPS2 transgenic lines perform better in all attributes than CaMIPS1 transformants under such stress conditions, due to comparatively unabated production of inositol by CaMIPS2 enzyme, as this enzyme retains significant activity under stress conditions.

L-myo-inositol-1-phosphate synthase expressed in developing organ: isolation and characterisation of the enzyme from human fetal liver

L-myo-inositol-1-phosphate synthase (MIPS; EC: 5.5.1.4) activity has been detected and partially purified for the first time from human fetal liver. Crude homogenate from the fetal liver was subjected to streptomycin sulphate precipitation and 0-60 % ammonium sulphate fractionation followed by successive chromatography through DEAE cellulose and BioGel A 0.5-m columns. After the final chromatography, the enzyme was purified 51-fold and 3.46 % of MIPS could be recovered. The human fetal liver MIPS specifically utilised D-glucose-6-phosphte and NAD(+) as its substrate and coenzyme, respectively. It shows pH optima between 7.0 and 7.5 while the temperature maximum was at 40 °C. The enzyme activity was remarkably stimulated by NH (4) (+) , slightly stimulated by K(+) and Ca(2+) and highly inhibited by Zn(2+), Cu(2+) and Hg(2+). The K (m) values of MIPS for D-glucose-6-phosphate and NAD(+) were found to be as 1.15 and 0.12 mM respectively while the V (max) values were 280 nM and 252 nM for D-glucose-6-phosphate and NAD(+) correspondingly. The apparent molecular weight of the native enzyme was determined to be 170 kDa.

Crystal structure of a trapped catalytic intermediate suggests that forced atomic proximity drives the catalysis of mIPS

1-L-myo-inositol-phosphate synthase (mIPS) catalyzes the first step of the unique, de novo pathway of inositol biosynthesis. However, details about the complex mIPS catalytic mechanism, which requires oxidation, enolization, intramolecular aldol cyclization, and reduction, are not fully known. To gain further insight into this mechanism, we determined the crystal structure of the wild-type mIPS from Archaeoglobus fulgidus at 1.7 Å, as well as the crystal structures of three active-site mutants. Additionally, we obtained the structure of mIPS with a trapped 5-keto-glucose-6-phosphate intermediate at 2 Å resolution by a novel (to our knowledge) process of activating the crystal at high temperature. A comparison of all of the crystal structures of mIPS described in this work suggests a novel type of catalytic mechanism that relies on the forced atomic proximity of functional groups. The lysine cluster is contained in a small volume in the active site, where random motions of these side chains are responsible for the progress of the complex multistep reaction as well as for the low rate of catalysis. The mechanism requires that functional groups of Lys-274, Lys-278, Lys-306, and Lys-367 assume differential roles in the protonation/deprotonation steps that must occur during the mIPS reaction. This mechanism is supported by the complete loss of activity of the enzyme caused by the Leu-257 mutation to Ala that releases the lysine containment.

A limitation of the continuous spectrophotometric assay for the measurement of myo-inositol-1-phosphate synthase activity

Myo-inositol-1-phosphate synthase (MIPS) catalyzes the conversion of glucose-6-phosphate to myo-inositol-1-phosphate. The reaction catalyzed by MIPS is the first step in the biosynthesis of inositol and inositol-containing molecules that serve important roles in both eukaryotes and prokaryotes. Consequently, MIPS is a target for the development of therapeutic agents for the treatment of infectious diseases and bipolar disorder. We recently reported a continuous spectrophotometric method for measuring MIPS activity using a coupled assay that allows the rapid characterization of MIPS in a multiwell plate format. Here we validate the continuous assay as a high-throughput alternative for measuring MIPS activity and report on one limitation of this assay-the inability to examine the effect of divalent metal ions (at high concentrations) on MIPS activity. In addition, we demonstrate that the activity of MIPS from Arabidopsis thaliana is moderately enhanced by the addition Mg(2+) and is not enhanced by other divalent metal ions (Zn(2+) and Mn(2+)), consistent with what has been observed for other eukaryotic MIPS enzymes. Our findings suggest that the continuous assay is better suited for characterizing eukaryotic MIPS enzymes that require monovalent cations as cofactors than for characterizing bacterial or archeal MIPS enzymes that require divalent metal ions as cofactors.

Role of myo-inositol phosphate synthase and sucrose synthase genes in plant seed development

The aim of this review is to highlight the role of myo-inositol phosphate synthase (MIPS), which catalyses the first step in inositol biosynthesis and of sucrose synthase (Sus), an enzyme involved in UDP-glucose formation, the principal nucleoside diphosphate in the sucrose cleavage reaction and in trehalose biosynthesis. These two enzymes are involved in various physiological processes including seed growth and resistance to biotic and abiotic stresses. The study of mutated MIPS and Sus genes in some crops, such as soybean and cotton, has shown that these two proteins are directly involved in embryogenesis. They exhibit several isoforms that are essential for normal seed development. The possible role of both genes in seed development is discussed in this review.

Biosynthesis and functions of mycothiol, the unique protective thiol of Actinobacteria

Mycothiol (MSH; AcCys-GlcN-Ins) is the major thiol found in Actinobacteria and has many of the functions of glutathione, which is the dominant thiol in other bacteria and eukaryotes but is absent in Actinobacteria. MSH functions as a protected reserve of cysteine and in the detoxification of alkylating agents, reactive oxygen and nitrogen species, and antibiotics. MSH also acts as a thiol buffer which is important in maintaining the highly reducing environment within the cell and protecting against disulfide stress. The pathway of MSH biosynthesis involves production of GlcNAc-Ins-P by MSH glycosyltransferase (MshA), dephosphorylation by the MSH phosphatase MshA2 (not yet identified), deacetylation by MshB to produce GlcN-Ins, linkage to Cys by the MSH ligase MshC, and acetylation by MSH synthase (MshD), yielding MSH. Studies of MSH mutants have shown that the MSH glycosyltransferase MshA and the MSH ligase MshC are required for MSH production, whereas mutants in the MSH deacetylase MshB and the acetyltransferase (MSH synthase) MshD produce some MSH and/or a closely related thiol. Current evidence indicates that MSH biosynthesis is controlled by transcriptional regulation mediated by sigma(B) and sigma(R) in Streptomyces coelicolor. Identified enzymes of MSH metabolism include mycothione reductase (disulfide reductase; Mtr), the S-nitrosomycothiol reductase MscR, the MSH S-conjugate amidase Mca, and an MSH-dependent maleylpyruvate isomerase. Mca cleaves MSH S-conjugates to generate mercapturic acids (AcCySR), excreted from the cell, and GlcN-Ins, used for resynthesis of MSH. The phenotypes of MSH-deficient mutants indicate the occurrence of one or more MSH-dependent S-transferases, peroxidases, and mycoredoxins, which are important targets for future studies. Current evidence suggests that several MSH biosynthetic and metabolic enzymes are potential targets for drugs against tuberculosis. The functions of MSH in antibiotic-producing streptomycetes and in bioremediation are areas for future study.

Structural and enzymatic analysis of MshA from Corynebacterium glutamicum: substrate-assisted catalysis

The glycosyltransferase termed MshA catalyzes the transfer of N-acetylglucosamine from UDP-N-acetylglucosamine to 1-L-myo-inositol-1-phosphate in the first committed step of mycothiol biosynthesis. The structure of MshA from Corynebacterium glutamicum was determined both in the absence of substrates and in a complex with UDP and 1-L-myo-inositol-1-phosphate. MshA belongs to the GT-B structural family whose members have a two-domain structure with both domains exhibiting a Rossman-type fold. Binding of the donor sugar to the C-terminal domain produces a 97 degrees rotational reorientation of the N-terminal domain relative to the C-terminal domain, clamping down on UDP and generating the binding site for 1-L-myo-inositol-1-phosphate. The structure highlights the residues important in binding of UDP-N-acetylglucosamine and 1-L-myo-inositol-1-phosphate. Molecular models of the ternary complex suggest a mechanism in which the beta-phosphate of the substrate, UDP-N-acetylglucosamine, promotes the nucleophilic attack of the 3-hydroxyl group of 1-L-myo-inositol-1-phosphate while at the same time promoting the cleavage of the sugar nucleotide bond.

Bifunctional CTP:inositol-1-phosphate cytidylyltransferase/CDP-inositol:inositol-1-phosphate transferase, the key enzyme for di-myo-inositol-phosphate synthesis in several (hyper)thermophiles

The pathway for the synthesis of di-myo-inositol-phosphate (DIP) was recently elucidated on the basis of the detection of the relevant activities in cell extracts of Archaeoglobus fulgidus and structural characterization of products by nuclear magnetic resonance (NMR) (N. Borges, L. G. Gonçalves, M. V. Rodrigues, F. Siopa, R. Ventura, C. Maycock, P. Lamosa, and H. Santos, J. Bacteriol. 188:8128-8135, 2006). Here, a genomic approach was used to identify the genes involved in the synthesis of DIP. Cloning and expression in Escherichia coli of the putative genes for CTP:l-myo-inositol-1-phosphate cytidylyltransferase and DIPP (di-myo-inositol-1,3'-phosphate-1'-phosphate, a phosphorylated form of DIP) synthase from several (hyper)thermophiles (A. fulgidus, Pyrococcus furiosus, Thermococcus kodakaraensis, Aquifex aeolicus, and Rubrobacter xylanophilus) confirmed the presence of those activities in the gene products. The DIPP synthase activity was part of a bifunctional enzyme that catalyzed the condensation of CTP and l-myo-inositol-1-phosphate into CDP-l-myo-inositol, as well as the synthesis of DIPP from CDP-l-myo-inositol and l-myo-inositol-1-phosphate. The cytidylyltransferase was absolutely specific for CTP and l-myo-inositol-1-P; the DIPP synthase domain used only l-myo-inositol-1-phosphate as an alcohol acceptor, but CDP-glycerol, as well as CDP-l-myo-inositol and CDP-d-myo-inositol, were recognized as alcohol donors. Genome analysis showed homologous genes in all organisms known to accumulate DIP and for which genome sequences were available. In most cases, the two activities (l-myo-inositol-1-P cytidylyltransferase and DIPP synthase) were fused in a single gene product, but separate genes were predicted in Aeropyrum pernix, Thermotoga maritima, and Hyperthermus butylicus. Additionally, using l-myo-inositol-1-phosphate labeled on C-1 with carbon 13, the stereochemical configuration of all the metabolites involved in DIP synthesis was established by NMR analysis. The two inositol moieties in DIP had different stereochemical configurations, in contradiction of previous reports. The use of the designation di-myo-inositol-1,3'-phosphate is recommended to facilitate tracing individual carbon atoms through metabolic pathways.

Functional identification of sll1383 from Synechocystis sp PCC 6803 as L-myo-inositol 1-phosphate phosphatase (EC 3.1.3.25): molecular cloning, expression and characterization

The genome sequence of the cyanobacterium Synechocystis sp. PCC6803 revealed four Open reading frame (ORF) encoding putative inositol monophosphatase or inositol monophosphatase-like proteins. One of the ORFs, sll1383, is approximately 870 base pair long and has been assigned as a probable myo-inositol 1 (or 4) monophosphatase (IMPase; EC 3.1.3.25). IMPase is the second enzyme in the inositol biosynthesis pathway and catalyses the conversion of L-myo-inositol 1-phosphate to free myo-inositol. The present work describes the functional assignment of ORF sll1383 as myo-inositol 1-phosphate phosphatase (IMPase) through molecular cloning, bacterial overexpression, purification and biochemical characterization of the gene product. Affinity (K (m)) of the recombinant protein for the substrate DL-myo-inositol 1-phosphate was found to be much higher (0.0034 +/- 0.0003 mM) compared to IMPase(s) from other sources but in comparison V (max) ( approximately 0.033 mumol Pi/min/mg protein) was low. Li(+) was found to be an inhibitor (IC(50) 6.0 mM) of this enzyme, other monovalent metal ions (e.g. Na(+), K(+) NH (4) (+) ) having no significant effect on the enzyme activity. Like other IMPase(s), the activity of this enzyme was found to be totally Mg(2+) dependent, which can be substituted partially by Mn(2+). However, unlike other IMPase(s), the enzyme is optimally active at approximately 42 degrees C. To the best of our knowledge, sll1383 encoded IMPase has the highest substrate affinity and specificity amongst the known examples from other prokaryotic sources. A possible application of this recombinant protein in the enzymatic coupled assay of L-myo-inositol 1-phosphate synthase (MIPS) is discussed.

Genomic identification and in vitro reconstitution of a complete biosynthetic pathway for the osmolyte di-myo-inositol-phosphate

Di-myo-inositol 1,1'-phosphate (DIP) is a major osmoprotecting metabolite in a number of hyperthermophilic species of archaea and bacteria. Although the DIP biosynthesis pathway was previously proposed, genes encoding only two of the four required enzymes, inositol-1-phosphate synthase and inositol monophosphatase, were identified. In this study we used a comparative genomic analysis to predict two additional genes of this pathway (termed dipA and dipB) that remained missing. In Thermotoga maritima both candidate genes (in an originally misannotated locus TM1418) form an operon with the inositol-1-phosphate synthase encoding gene (TM1419). A predicted inositol-mono-phosphate cytidylyltransferase activity was directly confirmed for the purified product of T. maritima gene dipA cloned and expressed in Escherichia coli. The entire DIP pathway was reconstituted in E. coli by cloning of the TM1418-TM1419 operon in pBAD expression vector and confirmed to function in the crude lysate. (31)P NMR and MS analysis revealed that DIP synthesis proceeds via a phosphorylated DIP intermediate, P-DIP, which is generated by the dipB-encoded enzyme, now termed P-DIP synthase. This previously unknown intermediate is apparently converted to the final product, DIP, by an inositol monophosphatase-like phosphatase. These findings allowed us to revise the previously proposed DIP pathway. The genomic survey confirmed its presence in the species known to use DIP for osmoprotection. Among several newly identified species with a postulated DIP pathway, Aeropyrum pernix was directly proven to produce this osmolyte.

Biosynthesis of ether-type polar lipids in archaea and evolutionary considerations

This review deals with the in vitro biosynthesis of the characteristics of polar lipids in archaea along with preceding in vivo studies. Isoprenoid chains are synthesized through the classical mevalonate pathway, as in eucarya, with minor modifications in some archaeal species. Most enzymes involved in the pathway have been identified enzymatically and/or genomically. Three of the relevant enzymes are found in enzyme families different from the known enzymes. The order of reactions in the phospholipid synthesis pathway (glycerophosphate backbone formation, linking of glycerophosphate with two radyl chains, activation by CDP, and attachment of common polar head groups) is analogous to that of bacteria. sn-Glycerol-1-phosphate dehydrogenase is responsible for the formation of the sn-glycerol-1-phosphate backbone of phospholipids in all archaea. After the formation of two ether bonds, CDP-archaeol acts as a common precursor of various archaeal phospholipid syntheses. Various phospholipid-synthesizing enzymes from archaea and bacteria belong to the same large CDP-alcohol phosphatidyltransferase family. In short, the first halves of the phospholipid synthesis pathways play a role in synthesis of the characteristic structures of archaeal and bacterial phospholipids, respectively. In the second halves of the pathways, the polar head group-attaching reactions and enzymes are homologous in both domains. These are regarded as revealing the hybrid nature of phospholipid biosynthesis. Precells proposed by Wächtershäuser are differentiated into archaea and bacteria by spontaneous segregation of enantiomeric phospholipid membranes (with sn-glycerol-1-phosphate and sn-glycerol-3-phosphate backbones) and the fusion and fission of precells. Considering the nature of the phospholipid synthesis pathways, we here propose that common phospholipid polar head groups were present in precells before the differentiation into archaea and bacteria.

NAD+ mediated differential thermotolerance between chloroplastic and cytosolic L-myo-inositol-1-phosphate synthase from Diplopterygium glaucum (Thunb.) Nakai

Relative thermotolerance of the enzyme, L-myo-inositol-1-phosphate synthase (MIPS; EC: 5.5.1.4), from the chloroplastic and cytosolic sources of Diplopterygium glaucum was studied. The purification involved streptomycin sulphate precipitation, ammonium sulphate fractionation, ion-exchange chromatography, and molecular sieve chromatography. After the final chromatography, 16.62% of chloroplastic and 13.47% of cytosolic MIPS could be recovered. Between 15 degrees C and 55 degrees C, the two forms of MIPS exhibited differential thermal stability, which is related to the presence of the MIPS co-factor, NAD+. Added NAD+ increased the lower thermotolerance of the chloroplastic MIPS and the removal of 'built-in' NAD+ decreased the higher thermal stability of the cytosolic MIPS.

The structure and mechanism of myo-inositol-1-phosphate synthase

The first and rate-limiting step in the biosynthesis of myo-inositol is the conversion of D-glucose 6-phosphate to 1L-myo-inositol 1-phosphate catalyzed by 1L-myo-inositol 1-phosphate synthase (MIP synthase). MIP synthase has been identified in a wide variety of organisms from bacteria to humans and is relatively well-conserved throughout evolution. It is probably homotetrameric in most if not all cases and always requires NAD+ as a cofactor, with NADH being reconverted to NAD+ in the catalytic cycle. This review focuses on the structure and mechanism of MIP synthase, with a particular emphasis on the mechanistic insights that have come from several recent structures of the enzyme. These include the structure of the enzyme from Saccharomyces cerevisiae, Archeoglobus fulgidus and Mycobacterium tuberculosis.

Biochemistry of the initial steps of mycothiol biosynthesis

Mycothiol is the major thiol produced by mycobacteria and is required for growth of Mycobacterium tuberculosis. The final three steps in the biosynthesis of mycothiol have been fully elucidated but the initial steps have been unclear. A glycosyltransferase, MshA, is required for production of the mycothiol precursor, 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol, but its substrates and immediate products were unknown. In this study, we show that the N-acetylglucosamine donor is UDP-N-acetylglucosamine and that the N-acetylglucosamine acceptor is 1L-myo-inositol 1-phosphate. The reaction generates UDP and 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol 3-phosphate. Using cell-free extracts of M. smegmatis mc(2)155, little activity was obtained with myo-inositol, 1D-myo-inositol 1-phosphate, or myo-inositol 2-phosphate as the N-acetylglucosamine acceptor. A phosphatase, designated MshA2, is required to dephosphorylate 1-O-(2-acetamido-2-deoxy-alpha-glucopyranosyl)-D-myo-inositol 3-phosphate to produce 1-O-(2-acetamido-2-deoxy-alpha-D-glucopyranosyl)-D-myo-inositol. The latter is deacetylated, ligated with cysteine, and the cysteinyl amino group acetylated by acetyl-CoA to complete the mycothiol biosynthesis pathway. Uptake and concentration of myo-[14C]inositol is rapid in Mycobacterium smegmatis and leads to production of radiolabeled inositol 1-phosphate and mycothiol. This demonstrates the presence of a myo-inositol transporter and a kinase that generates 1L-myo-inositol 1-phosphate. The biochemical pathway of mycothiol biosynthesis is now fully elucidated.

Myo-inositol content in pteridophytes and the isolation and characterization of L-myo-inositol-1-phosphate synthase from Diplopterygium glaucum

Myo-inositol is involved in normal growth and development of all living organisms and L-myo-inositol-1-phosphate synthase (MIPS; EC: 5.5.1.4) is responsible for its de novo synthesis. This enzyme has been reported for a number of life forms including plants, animals and bacteria. In the present study free myo-inositol has been detected in the common pteridophytes found in the Darjeeling Himalayas and the enzyme, L-myo-inositol-1-phosphate synthase has been partially purified from Diplopterygium glaucum (Thunb.) Nakai. A crude homogenate from the reproductive pinnules of D. glaucum was subjected to streptomycin sulphate precipitation and 0-70% ammonium sulphate fractionation followed by successive chromatography through DEAE-cellulose, Hexylagarose and BioGel A-0.5m columns. This resulted in a partial purification of the enzyme of about 81-fold with 13.5% recovery. The pteridophytic MIPS specifically utilized D-glucose-6-phosphte and NAD+ as its substrate and co-factor, respectively. It shows a pH optimum between 7.0 and 7.5 while the temperature maximum was 30 °C. The enzyme activity was stimulated by NH4+, slightly inhibited by Na+, Ba2+ and Cd2+, and strongly inhibited by Li+, Zn2+ and Hg2+. EDTA, pCMB and some substrate isomers like glucose-1-phosphate, fructose-6-phosphte and galactose-6-phosphate were inhibitory to the enzyme. The apparent molecular weight of the native D. glaucum MIPS was determined to be approximately 171 kDa.

An insight into the molecular basis of salt tolerance of L-myo-inositol 1-P synthase (PcINO1) from Porteresia coarctata (Roxb.) Tateoka, a halophytic wild rice

The molecular basis of salt tolerance of L-myo-inositol 1-P synthase (MIPS; EC 5.5.1.4) from Porteresia coarctata (Roxb.) Tateoka (PcINO1, AF412340) earlier reported from this laboratory, has been analyzed by in vitro mutant and hybrid generation and subsequent biochemical and biophysical studies of the recombinant proteins. A 37-amino acid stretch between Trp-174 and Ser-210 has been confirmed as the salt-tolerance determinant domain in PcINO1 both by loss or gain of salt tolerance by either deletion or by addition to salt-sensitive MIPS(s) of Oryza (OsINO1) and Brassica juncea (BjINO1). This was further verified by growth analysis under salt environment of Schizosaccharomyces pombe transformed with the various gene constructs and studies on the differential behavior of mutant and wild proteins by Trp fluorescence, aggregation, and circular dichroism spectra in the presence of salt. 4,4'-Dianilino-1,1'-binaphthyl-5,5-disulfonic acid binding experiments revealed a lower hydrophobic surface on PcINO1 than OsINO1, contributed by this 37-amino acid stretch explaining the differential behavior of OsINO1 and PcINO1 both with respect to their enzymatic functions and thermodynamic stability in high salt environment. Detailed amino acid sequence comparison and modeling studies revealed the interposition of polar and charged residues and a well-connected hydrogen-bonding network formed by Ser and Thr in this stretch of PcINO1. On the contrary, hydrophobic residues clustered in two continuous stretches in the corresponding region of OsINO1 form a strong hydrophobic patch on the surface. It is conceivable that salt-tolerant MIPS proteins may be designed out of the salt-sensitive plant MIPS proteins by replacement of the corresponding amino acid stretch by the designated 37-amino acid stretch of PcINO1.

Inositol phosphates and phosphoinositides in health and disease

In the past two decades, considerable progress has been made toward understanding inositol phosphates and PI metabolism. However, there is still much to learn. The present challenge is to understand how inositol phosphates and PIs are compartmentalized, identify new targets of inositol phosphates and PIs, and elucidate the mechanisms underlying spatial and temporal regulation of the enzymes that metabolize inositol phosphates and PIs. Answers to these questions will help clarify the mechanisms of the diseases associated with these molecules and identify new possibilities for drug design.

Probing the mechanism of the Archaeoglobus fulgidus inositol-1-phosphate synthase

myo-Inositol-1-phosphate synthase (mIPS) catalyzes the conversion of glucose-6-phosphate (G-6-P) to inositol-1-phosphate. In the sulfate-reducing archaeon Archaeoglobus fulgidus it is a metal-dependent thermozyme that catalyzes the first step in the biosynthetic pathway of the unusual osmolyte di-myo-inositol-1,1'-phosphate. Several site-specific mutants of the archaeal mIPS were prepared and characterized to probe the details of the catalytic mechanism that was suggested by the recently solved crystal structure and by the comparison to the yeast mIPS. Six charged residues in the active site (Asp225, Lys274, Lys278, Lys306, Asp332, and Lys367) and two noncharged residues (Asn255 and Leu257) have been changed to alanine. The charged residues are located at the active site and were proposed to play binding and/or direct catalytic roles, whereas noncharged residues are likely to be involved in proper binding of the substrate. Kinetic studies showed that only N255A retains any measurable activity, whereas two other mutants, K306A and D332A, can carry out the initial oxidation of G-6-P and reduction of NAD+ to NADH. The rest of the mutant enzymes show major changes in binding of G-6-P (monitored by the 31P line width of inorganic phosphate when G-6-P is added in the presence of EDTA) or NAD+ (detected via changes in the protein intrinsic fluorescence). Characterization of these mutants provides new twists on the catalytic mechanism previously proposed for this enzyme.

A novel salt-tolerant L-myo-inositol-1-phosphate synthase from Porteresia coarctata (Roxb.) Tateoka, a halophytic wild rice: molecular cloning, bacterial overexpression, characterization, and functional introgression into tobacco-conferring salt tolerance phenotype

l-myo-Inositol-1-phosphate synthase (EC 5.5.1.4, MIPS), an evolutionarily conserved enzyme protein, catalyzes the synthesis of inositol, which is implicated in a number of metabolic reactions in the biological kingdom. Here we report on the isolation of the gene (PINO1) for a novel salt-tolerant MIPS from the wild halophytic rice, Porteresia coarctata (Roxb.) Tateoka. Identity of the PINO1 gene was confirmed by functional complementation in a yeast inositol auxotrophic strain. Comparison of the nucleotide and deduced amino acid sequences of PINO1 with that of the homologous gene from Oryza sativa L. (RINO1) revealed distinct differences in a stretch of 37 amino acids, between amino acids 174 and 210. Purified bacterially expressed PINO1 protein demonstrated a salt-tolerant character in vitro compared with the salt-sensitive RINO1 protein as with those purified from the native source or an expressed salt-sensitive mutant PINO1 protein wherein amino acids 174-210 have been deleted. Analysis of the salt effect on oligomerization and tryptophan fluorescence of the RINO1 and PINO1 proteins revealed that the structure of PINO1 protein is stable toward salt environment. Furthermore, introgression of PINO1 rendered transgenic tobacco plants capable of growth in 200-300 mm NaCl with retention of approximately 40-80% of the photosynthetic competence with concomitant increased inositol production compared with unstressed control. MIPS protein isolated from PINO1 transgenics showed salt-tolerant property in vitro confirming functional expression in planta of the PINO1 gene. To our knowledge, this is the first report of a salt-tolerant MIPS from any source.

Biochemical characterization of WbpA, a UDP-N-acetyl-D-glucosamine 6-dehydrogenase involved in O-antigen biosynthesis in Pseudomonas aeruginosa PAO1

WbpA (PA3159) is an enzyme involved in the biosynthesis of unusual di-N-acetyl-d-mannosaminuronic acid-derived sugar nucleotides found in the O antigen of Pseudomonas aeruginosa PAO1 (serotype O5). The wbpA gene that encodes this enzyme was cloned into pET-28a, overexpressed as a histidine-tagged fusion protein, and purified by nickel chelation chromatography. Capillary electrophoresis was used to examine substrate conversion by WbpA, and the data revealed that WbpA is a UDP-N-acetyl-D-glucosamine 6-dehydrogenase (EC 1.1.1.136), which uses NAD(+) as a coenzyme. The enzyme reaction product was purified by HPLC and analyzed using NMR spectroscopy. Our results showed unequivocally that the product of the WbpA reaction is UDP-N-acetyl-d-glucosaminuronic acid. WbpA requires either NH(4)(+) or K(+) for activity and the accompanying anions exert secondary effects on activity consistent with their ranking in the Hofmeister series. Kinetic analysis showed positive cooperativity with respect to UDP-N-acetyl-d-glucosamine binding with a K(0.5) of 94 microM, a k(cat) of 86 min(-1), and a Hill coefficient of 1.8. In addition, WbpA has a K(0.5) for NAD(+) of 220 microM, a k(cat) of 86 min(-1), and a Hill coefficient of 1.1. The oligomerization state of WbpA was analyzed by gel filtration, dynamic light scattering, and analytical ultracentrifugation, with all three techniques indicating that WbpA exists as a trimer in solution. However, tertiary structure predictions suggested a tetramer, which was supported by data from transmission electron microscopy. The electron micrograph of negatively stained WbpA samples revealed structures with 4-fold symmetry.

Human 1-D-myo-Inositol-3-phosphate Synthase Is Functional in Yeast

We have cloned, sequenced, and expressed a human cDNA encoding 1-d-myo-inositol-3-phosphate (MIP) synthase (hINO1). The encoded 62-kDa human enzyme converted d-glucose 6-phosphate to 1-d-myo-inositol 3-phosphate, the rate-limiting step for de novo inositol biosynthesis. Activity of the recombinant human MIP synthase purified from Escherichia coli was optimal at pH 8.0 at 37 degrees C and exhibited K(m) values of 0.57 mm and 8 microm for glucose 6-phosphate and NAD(+), respectively. NH(4)(+) and K(+) were better activators than other cations tested (Na(+), Li(+), Mg(2+), Mn(2+)), and Zn(2+) strongly inhibited activity. Expression of the protein in the yeast ino1Delta mutant lacking MIP synthase (ino1Delta/hINO1) complemented the inositol auxotrophy of the mutant and led to inositol excretion. MIP synthase activity and intracellular inositol were decreased about 35 and 25%, respectively, when ino1Delta/hINO1 was grown in the presence of a therapeutically relevant concentration of the anti-bipolar drug valproate (0.6 mm). However, in vitro activity of purified MIP synthase was not inhibited by valproate at this concentration, suggesting that inhibition by the drug is indirect. Because inositol metabolism may play a key role in the etiology and treatment of bipolar illness, functional conservation of the key enzyme in inositol biosynthesis underscores the power of the yeast model in studies of this disorder.

The structure of the 1L-myo-inositol-1-phosphate synthase-NAD+-2-deoxy-D-glucitol 6-(E)-vinylhomophosphonate complex demands a revision of the enzyme mechanism

1l-myo-inositol 1-phosphate (MIP) synthase catalyzes the conversion of d-glucose 6-phosphate to 1l-myo-inositol 1-phosphate, the first and rate-limiting step in the biosynthesis of all inositol-containing compounds. It involves an oxidation, enolization, intramolecular aldol cyclization, and reduction. Here we present the structure of MIP synthase in complex with NAD(+) and a high-affinity inhibitor, 2-deoxy-d-glucitol 6-(E)-vinylhomophosphonate. This structure reveals interactions between the enzyme active site residues and the inhibitor that are significantly different from that proposed for 2-deoxy-d-glucitol 6-phosphate in the previously published structure of MIP synthase-NAD(+)-2-deoxy-d-glucitol 6-phosphate. There are several other conformational changes in NAD(+) and the enzyme active site as well. Based on the new structural data, we propose a new and completely different mechanism for MIP synthase.

Diversification and evolution of L-myo-inositol 1-phosphate synthase

L-myo-Inositol 1-phosphate synthase (MIPS, EC 5.5.1.4), the key enzyme in the inositol and phosphoinositide biosynthetic pathway, is present throughout evolutionarily diverse organisms and is considered an ancient protein/gene. Analysis by multiple sequence alignment, phylogenetic tree generation and comparison of newly determined crystal structures provides new insight into the origin and evolutionary relationships among the various MIPS proteins/genes. The evolution of the MIPS protein/gene among the prokaryotes seems more diverse and complex than amongst the eukaryotes. However, conservation of a 'core catalytic structure' among the MIPS proteins implies an essential function of the enzyme in cellular metabolism throughout the biological kingdom.

Unexpected similarity in regulation between an archaeal inositol monophosphatase/fructose bisphosphatase and chloroplast fructose bisphosphatase

Hyperthermophilic archaea have an unusual phosphatase that exhibits activity toward both inositol-1-phosphate and fructose-1,6-bisphosphate, activities carried out by separate gene products in eukaryotes and bacteria. The structures of phosphatases from Archaeoglobus fulgidus (AF2372) and Methanococcus jannaschii (MJ0109), both anaerobic organisms, resemble the dimeric unit of the tetrameric pig kidney fructose bisphosphatase (FBPase). A striking feature of AF2372, but not of MJ0109, is that the sulfhydryl groups of two cysteines, Cys150 and Cys186, are in close proximity (4 A). A similar arrangement of cysteines has been observed in chloroplast FBPases that are regulated by disulfide formation controlled by redox signaling pathways (ferredoxin/thioredoxin). This mode of regulation has not been detected in any other FBPase enzymes. Biochemical assays show that the AF2372 phosphatase activity can be abolished by incubation with O(2). Full activity is restored by incubation with thiol-containing compounds. Neither the C150S variant of AF2372 nor the equivalent phosphatase from M. jannaschii loses activity with oxidation. Oxidation experiments using Escherichia coli thioredoxin, in analogy with the chloroplast FBPase system, indicate an unexpected mode of regulation for AF2372, a key phosphatase in this anaerobic sulfate reducer.

Crystal structure of a dual activity IMPase/FBPase (AF2372) from Archaeoglobus fulgidus. The story of a mobile loop

Several hyperthermophilic organisms contain an unusual phosphatase that has dual activity toward inositol monophosphates and fructose 1,6-bisphosphate. The structure of the second member of this family, an FBPase/IMPase from Archaeoglobus fulgidus (AF2372), has been solved. This enzyme shares many kinetic and structural similarities with that of a previously solved enzyme from Methanococcus jannaschii (MJ0109). It also shows some kinetic differences in divalent metal ion binding as well as structural variations at the dimer interface that correlate with decreased thermal stability. The availability of different crystal forms allowed us to investigate the effect of the presence of ligands on the conformation of a mobile catalytic loop independently of the crystal packing. This conformational variability in AF2372 is compared with that observed in other members of this structural family that are sensitive or insensitive to submillimolar concentrations of Li(+). This analysis provides support for the previously proposed mechanism of catalysis involving three metal ions. A direct correlation of the loop conformation with strength of Li(+) inhibition provides a useful system of classification for this extended family of enzymes.