H, C NMR and UV spectroscopy studies of gold(III)-tetracyanide complex with l-cysteine, glutathione, captopril, l-methionine and dl-seleno-methionine in aqueous solution

Auricyanide [Au(CN)₄]⁻ interaction with biologically important thiols, thioether and selenoether were carried out and monitored using ¹H, ¹³C NMR and UV spectroscopy. These ligands include l-cysteine, glutathione, captopril, l-methionine and dl-seleno-methionine. Thiols show very strong affinity to be oxidized into the disulfide by auricyanide, which gets reduced to aurocyanide [Au(CN)₂]⁻. l-cysteine reaction mechanism with [Au(CN)₄]⁻ was found to be dependent on reactants mole ratio. While l-methionine was completely inert toward auricyanide, dl-Se-methionine showed some reactivity with [Au(CN)₄]⁻ after raising solution pH to 12 that facilitated cyanide exchange.     

Crystal structures of D-tagatose 3-epimerase from Pseudomonas cichorii and its complexes with D-tagatose and D-fructose

Pseudomonas cichoriiid-tagatose 3-epimerase (P. cichoriid-TE) can efficiently catalyze the epimerization of not only d-tagatose to d-sorbose, but also d-fructose to d-psicose, and is used for the production of d-psicose from d-fructose. The crystal structures of P. cichoriid-TE alone and in complexes with d-tagatose and d-fructose were determined at resolutions of 1.79, 2.28, and 2.06 Å, respectively. A subunit of P. cichoriid-TE adopts a (β/α)₈ barrel structure, and a metal ion (Mn²⁺) found in the active site is coordinated by Glu152, Asp185, His211, and Glu246 at the end of the β-barrel. P. cichoriid-TE forms a stable dimer to give a favorable accessible surface for substrate binding on the front side of the dimer. The simulated omit map indicates that O2 and O3 of d-tagatose and/or d-fructose coordinate Mn²⁺, and that C3-O3 is located between carboxyl groups of Glu152 and Glu246, supporting the previously proposed mechanism of deprotonation/protonation at C3 by two Glu residues. Although the electron density is poor at the 4-, 5-, and 6-positions of the substrates, substrate-enzyme interactions can be deduced from the significant electron density at O6. The O6 possibly interacts with Cys66 via hydrogen bonding, whereas O4 and O5 in d-tagatose and O4 in d-fructose do not undergo hydrogen bonding to the enzyme and are in a hydrophobic environment created by Phe7, Trp15, Trp113, and Phe248. Due to the lack of specific interactions between the enzyme and its substrates at the 4- and 5-positions, P. cichoriid-TE loosely recognizes substrates in this region, allowing it to efficiently catalyze the epimerization of d-tagatose and d-fructose (C4 epimer of d-tagatose) as well. Furthermore, a C3-O3 proton-exchange mechanism for P. cichoriid-TE is suggested by X-ray structural analysis, providing a clear explanation for the regulation of the ionization state of Glu152 and Glu246.     

Plant isoprenoid biosynthesis via the MEP pathway: In vivo IPP/DMAPP ratio produced by (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase in tobacco BY-2 cell cultures

Feeding tobacco BY-2 cells with [2-¹³C,4-²H]deoxyxylulose revealed from the ¹³C labeling that the plastid isoprenoids, synthesized via the MEP pathway, are essentially derived from the labeled precursor. The ca. 15% ²H retention observed in all isoprene units corresponds to the isopentenyl diphosphate (IPP)/dimethylallyl diphosphate (DMAPP) ratio (85:15) directly produced by the hydroxymethylbutenyl diphosphate reductase, the last enzyme of the MEP pathway. ²H retention characterizes the isoprene units derived from the DMAPP branch, whereas ²H loss represents the signature of the IPP branch. Taking into account the enantioselectivity of the reactions catalyzed by the (E)-4-hydroxy-3-methylbut-2-enyl diphosphate reductase, the IPP isomerase and the trans-prenyl transferase, a single biogenetic scheme allows to interpret all labeling patterns observed in bacteria or plants upon incubation with ²H labeled deoxyxylulose.     

Production of L-xylose from L-xylulose using Escherichia coli L-fucose isomerase

l-Xylulose was used as a raw material for the production of l-xylose with a recombinantly produced Escherichia colil-fucose isomerase as the catalyst. The enzyme had a very alkaline pH optimum (over 10.5) and displayed Michaelis-Menten kinetics for l-xylulose with a K_m of 41 mM and a V_max of 0.23 μmol/(mg min). The half-lives determined for the enzyme at 35 °C and at 45 °C were 6 h 50 min and 1 h 31 min, respectively. The reaction equilibrium between l-xylulose and l-xylose was 15:85 at 35 °C and thus favored the formation of l-xylose. Contrary to the l-rhamnose isomerase catalyzed reaction described previously [14]l-lyxose was not detected in the reaction mixture with l-fucose isomerase. Although xylitol acted as an inhibitor of the reaction, even at a high ratio of xylitol to l-xylulose the inhibition did not reach 50%.     

Characterization of Mesorhizobium loti L-Rhamnose Isomerase and Its Application to L-Talose Production

The L-rhamnose isomerase gene (rhi) of Mesorhizobium loti was cloned and expressed in Escherichia coli, and then characterized. The enzyme exhibited activity with respect to various aldoses, including D-allose and L-talose. Application of it in L-talose production from galactitol was achieved by a two-step reaction, indicating that it can be utilized in the large-scale production of L-talose.     

Protein structural change upon ligand binding correlates with enzymatic reaction mechanism

Overall structural changes of enzymes in response to ligand binding were investigated by database analysis of 62 non-redundant enzymes whose ligand-unbound and ligand-bound forms were available in the Protein Data Bank. The results of analysis indicate that transferases often undergo large rigid-body domain motions upon ligand binding, while other enzymes, most typically, hydrolases, change their structures to a small extent. It was also found that the solvent accessibility of the substrate molecule was low in transferases but high in hydrolases. These differences are explained by the enzymatic reaction mechanisms. The transferase reaction requires the catalytic groups to be insulated from the water environment, and thus transferases bury the ligand molecule inside the protein by closing the cleft. On the other hand, the hydrolase reaction involves the surrounding water molecules and occurs at the protein surface, requiring only a small structural change.     

Enzymatic synthesis of D-glucosone 6-phosphate (D-arabino-hexos-2-ulose 6-(dihydrogen phosphate)) and NMR analysis of its isomeric forms

D-Glucosone 6-phosphate (D-arabino-hexos-2-ulose 6-(dihydrogen phosphate)) was prepared from D-glucosone (D-arabino-hexos-2-ulose) by enzymatic conversion with hexokinase. The isomeric composition of D-glucosone 6-phosphate in aqueous solution was quantitatively determined by NMR spectroscopy and compared to D-glucosone. The main isomers are the alpha-anomer (58%) and the beta-anomer (28%) of the hydrated pyranose form, and the beta-D-fructofuranose form (14%).     

D-ribose-5-phosphate isomerase B from Escherichia coli is also a functional D-allose-6-phosphate isomerase, while the Mycobacterium tuberculosis enzyme is not

Interconversion of d-ribose-5-phosphate (R5P) and d-ribulose-5-phosphate is an important step in the pentose phosphate pathway. Two unrelated enzymes with R5P isomerase activity were first identified in Escherichia coli, RpiA and RpiB. In this organism, the essential 5-carbon sugars were thought to be processed by RpiA, while the primary role of RpiB was suggested to instead be interconversion of the rare 6-carbon sugars d-allose-6-phosphate (All6P) and d-allulose-6-phosphate. In Mycobacterium tuberculosis, where only an RpiB is found, the 5-carbon sugars are believed to be the enzyme's primary substrates. Here, we present kinetic studies examining the All6P isomerase activity of the RpiBs from these two organisms and show that only the E. coli enzyme can catalyze the reaction efficiently. All6P instead acts as an inhibitor of the M. tuberculosis enzyme in its action on R5P. X-ray studies of the M. tuberculosis enzyme co-crystallized with All6P and 5-deoxy-5-phospho-d-ribonohydroxamate (an inhibitor designed to mimic the 6-carbon sugar) and comparison with the E. coli enzyme's structure allowed us to identify differences in the active sites that explain the kinetic results. Two other structures, that of a mutant E. coli RpiB in which histidine 99 was changed to asparagine and that of wild-type M. tuberculosis enzyme, both co-crystallized with the substrate ribose-5-phosphate, shed additional light on the reaction mechanism of RpiBs generally.     

Two novel oligosaccharides isolated from a beverage produced by fermentation of a plant extract

An extract from 50 kinds of fruits and vegetables was fermented to produce a new beverage. Natural fermentation of the extract was carried out mainly by lactic acid bacteria (Leuconostoc spp.) and yeast (Zygosaccharomyces spp. and Pichia spp.). Two new saccharides were found in this fermented beverage. The saccharides were isolated using carbon-Celite column chromatography and preparative high performance liquid chromatography. Gas liquid chromatography analysis of methylated derivatives as well as MALDI-TOF MS and NMR measurements were used for structural confirmation. The (1)H and (13)C NMR signals of each saccharide were assigned using 2D-NMR including COSY, HSQC, HSQC-TOCSY, CH(2)-HSQC-TOCSY, and CT-HMBC experiments. The saccharides were identified as beta-D-fructopyranosyl-(2-->6)-beta-D-glucopyranosyl-(1-->3)-D-glucopyranose and beta-D-fructopyranosyl-(2-->6)-[beta-D-glucopyranosyl-(1-->3)]-D-glucopyranose.     

Dynamic requirements for a functional protein hinge

The enzyme triosephosphate isomerase (TIM) is a model of catalytic efficiency. The 11 residue loop 6 at the TIM active site plays a major role in this enzymatic prowess. The loop moves between open and closed states, which facilitate substrate access and catalysis, respectively. The N and C-terminal hinges of loop 6 control this motion. Here, we detail flexibility requirements for hinges in a comparative solution NMR study of wild-type (WT) TIM and a quintuple mutant (PGG/GGG). The latter contained glycine substitutions in the N-terminal hinge at Val167 and Trp168, which follow the essential Pro166, and in the C-terminal hinge at Lys174, Thr175, and Ala176. Previous work demonstrated that PGG/GGG has a tenfold higher Km value and 10(3)-fold reduced k(cat) relative to WT with either d-glyceraldehyde 3-phosphate or dihyrdroxyacetone phosphate as substrate. Our NMR results explain this in terms of altered loop-6 dynamics in PGG/GGG. In the mutant, loop 6 exhibits conformational heterogeneity with corresponding motional rates <750 s(-1) that are an order of magnitude slower than the natural WT loop 6 motion. At the same time, nanosecond timescale motions of loop 6 are greatly enhanced in the mutant relative to WT. These differences from WT behavior occur in both apo PGG/GGG and in the form bound to the reaction-intermediate analog, 2-phosphoglycolate (2-PGA). In addition, as indicated by 1H, 15N and 13CO chemical-shifts, the glycine substitutions diminished the enzyme's response to ligand, and induced structural perturbations in apo and 2-PGA-bound forms of TIM that are atypical of WT. These data show that PGG/GGG exists in multiple conformations that are not fully competent for ligand binding or catalysis. These experiments elucidate an important principle of catalytic hinge design in proteins: structural rigidity is essential for focused motional freedom of active-site loops.     

Production of l-allose and d-talose from l-psicose and d-tagatose by l-ribose isomerase

l-ribose isomerase (L-RI) from Cellulomonas parahominis MB426 can convert l-psicose and d-tagatose to l-allose and d-talose, respectively. Partially purified recombinant L-RI from Escherichia coli JM109 was immobilized on DIAION HPA25L resin and then utilized to produce l-allose and d-talose. Conversion reaction was performed with the reaction mixture containing 10% l-psicose or d-tagatose and immobilized L-RI at 40 °C. At equilibrium state, the yield of l-allose and d-talose was 35.0% and 13.0%, respectively. Immobilized enzyme could convert l-psicose to l-allose without remarkable decrease in the enzyme activity over 7 times use and d-tagatose to d-talose over 37 times use. After separation and concentration, the mixture solution of l-allose and d-talose was concentrated up to 70% and crystallized by keeping at 4 °C. l-Allose and d-talose crystals were collected from the syrup by filtration. The final yield was 23.0% l-allose and 7.30% d-talose that were obtained from l-psicose and d-tagatose, respectively.     

Site-directed alkylation of LacY: effect of the proton electrochemical gradient

Previous N-ethylmaleimide-labeling studies show that ligand binding increases the reactivity of single-Cys mutants located predominantly on the periplasmic side of LacY and decreases reactivity of mutants located for the most part of the cytoplasmic side. Thus, sugar binding appears to induce opening of a periplasmic pathway with closing of the cytoplasmic cavity resulting in alternative access of the sugar-binding site to either side of the membrane. Here we describe the use of a fluorescent alkylating reagent that reproduces the previous observations with respect to sugar binding. We then show that generation of an H⁺ electrochemical gradient (Δ_μ¯H+, interior negative) increases the reactivity of single-Cys mutants on the periplasmic side of the sugar-binding site and in the putative hydrophilic pathway. The results suggest that Δ_μ¯H+, like sugar, acts to increase the probability of opening on the periplasmic side of LacY.     

Helix Dynamics in LacY: Helices II and IV

Biochemical and biophysical studies based upon crystal structures of both a mutant and wild-type lactose permease from Escherichia coli (LacY) in an inward-facing conformation have led to a model for the symport mechanism in which both sugar and H⁺ binding sites are alternatively accessible to both sides of the membrane. Previous findings indicate that the face of helix II with Asp68 is important for the conformational changes that occur during turnover. As shown here, replacement of Asp68 at the cytoplasmic end of helix II, particularly with Glu, abolishes active transport but the mutants retain the ability to bind galactopyranoside. In the x-ray structure, Asp68 and Lys131 (helix IV) lie within ∼ 4.2 Å of each other. Although a double mutant with Cys replacements at both position 68 and position 131 cross-links efficiently, single replacements for Lys131 exhibit very significant transport activity. Site-directed alkylation studies show that sugar binding by the Asp68 mutants causes closure of the cytoplasmic cavity, similar to wild-type LacY; however, strikingly, the probability of opening the periplasmic pathway upon sugar binding is markedly reduced. Taken together with results from previous mutagenesis and cross-linking studies, these findings lead to a model in which replacement of Asp68 blocks a conformational transition involving helices II and IV that is important for opening the periplasmic cavity. Evidence suggesting that movements of helices II and IV are coupled functionally with movements in the pseudo-symmetrically paired helices VIII and X is also presented.     

Crystal structure of YihS in complex with D-mannose: structural annotation of Escherichia coli and Salmonella enterica yihS-encoded proteins to an aldose-ketose isomerase

The three-dimensional structure of a Salmonella enterica hypothetical protein YihS is significantly similar to that of N-acyl-d-glucosamine 2-epimerase (AGE) with respect to a common scaffold, an α₆/α₆-barrel, although the function of YihS remains to be clarified. To identify the function of YihS, Escherichia coli and S. enterica YihS proteins were overexpressed in E. coli, purified, and characterized. Both proteins were found to show no AGE activity but showed cofactor-independent aldose-ketose isomerase activity involved in the interconversion of monosaccharides, mannose, fructose, and glucose, or lyxose and xylulose. In order to clarify the structure/function relationship of YihS, we determined the crystal structure of S. enterica YihS mutant (H248A) in complex with a substrate (d-mannose) at 1.6 Å resolution. This enzyme-substrate complex structure is the first demonstration in the AGE structural family, and it enables us to identify active-site residues and postulate a reaction mechanism for YihS. The substrate, β-d-mannose, fits well in the active site and is specifically recognized by the enzyme. The substrate-binding site of YihS for the mannose C1 and O5 atoms is architecturally similar to those of mutarotases, suggesting that YihS adopts the pyranose ring-opening process by His383 and acidifies the C2 position, forming an aldehyde at the C1 position. In the isomerization step, His248 functions as a base catalyst responsible for transferring the proton from the C2 to C1 positions through a cis-enediol intermediate. On the other hand, in AGE, His248 is thought to abstract and re-adduct the proton at the C2 position of the substrate. These findings provide not only molecular insights into the YihS reaction mechanism but also useful information for the molecular design of novel carbohydrate-active enzymes with the common scaffold, α₆/α₆-barrel.     

Mechanism of the dehydration of D-fructose to 5-hydroxymethylfurfural in dimethyl sulfoxide at 150 degrees C: an NMR study

The anomeric composition of d-fructose in dimethyl sulfoxide changes when the solution is heated from room temperature to 150 °C, with a small increase in the α-furanose form at the expense of the β-pyranose tautomer. Additionally, a small amount of α-pyranose form was also observed at 150 °C. A mechanism is proposed for the dehydration of d-fructose to 5-hydroxymethylfurfural in DMSO at 150 °C, where the solvent acts as the catalyst. A key intermediate in the reaction was identified as (4R,5R)-4-hydroxy-5-hydroxymethyl-4,5-dihydrofuran-2-carbaldehyde by using ¹H and ¹³C NMR spectra of the sample during the reaction.     

Phytosphingosine 1-phosphate: a high affinity ligand for the S1P(4)/Edg-6 receptor

It has been reported recently that the phosphorylated form of the immunomodulator FTY720 activates sphingosine 1-phosphate G protein-coupled receptors [1] and [2]. Therefore, understanding the biology of this new class of receptors will be important in clarifying the immunological function of bioactive lysosphingolipid ligands. The S1P₄ receptor has generated interest due to its lymphoid tissue distribution. While the S1P₄ receptor binds the prototypical ligand, S1P, a survey of other lysosphingolipids demonstrated that 4d-hydroxysphinganine 1-phosphate, more commonly known as phytosphingosine 1-phosphate (PhS1P), binds to S1P₄ with higher affinity. Using radiolabeled S1P (S1³³P), the affinity of PhS1P for the S1P₄ receptor is 1.6 nM, while that of S1P is nearly 50-fold lower (119±20 nM). Radiolabeled PhS1P proved to be superior to S1³³P in routine binding assays due to improved signal-to-noise ratio. The present study demonstrates the utility of a novel radiolabeled probe, PhS1³³P, for in vitro studies of the S1P₄ receptor pharmacology.     

Oxidation of methyl α-d-galactopyranoside by galactose oxidase: products formed and optimization of reaction conditions for production of aldehyde

The reaction conditions of galactose oxidase-catalyzed, targeted C-6 oxidation of galactose derivatives were optimized for aldehyde production and to minimize the formation of secondary products. Galactose oxidase, produced in transgenic Pichia pastoris carrying the galactose oxidase gene from Fusarium spp., was used as catalyst, methyl α-d-galactopyranoside as substrate, and reaction medium, temperature, concentration, and combinations of galactose oxidase, catalase, and horseradish peroxidase were used as variables. The reactions were followed by ¹H NMR spectroscopy and the main products isolated, characterized, and identified. An optimal combination of all the three enzymes gave aldehyde (methyl α-d-galacto-hexodialdo-1,5-pyranoside) in approximately 90% yield with a substrate concentration of 70 mM in water at 4 °C using air as oxygen source. Oxygen flushing of the reaction mixture was not necessary. The aldehyde existed as a hydrate in water. The main secondary products, a uronic acid (methyl α-d-galactopyranosiduronic acid) and an α,β-unsaturated aldehyde (methyl 4-deoxy-α-d-threo-hex-4-enodialdo-1,5-pyranoside), were observed for the first time to form in parallel. Formation of uronic acid seemed to be the result of impurities in the galactose oxidase preparation. ¹H and ¹³C NMR data of the products are reported for the α,β-unsaturated aldehyde for the first time, and chemical shifts in DMSO-d₆ for all the products for the first time. Oxidation of d-raffinose (α-d-galactopyranosyl-(1-6)-α-d-glucopyranosyl-(1-2)-β-d-fructofuranoside) in the same optimum conditions also proceeded well, resulting in approximately 90% yield of the corresponding aldehyde.     

Thermodynamic determination of the binding constants of angiotensin-converting enzyme inhibitors by a displacement method

Somatic angiotensin I-converting enzyme (s-ACE) plays a central role in blood pressure regulation and has been the target of most antihypertensive drugs. A displacement isothermal titration calorimetry method has been used to accurately determine the binding constant of three strong s-ACE inhibitors. Under the experimental conditions studied in this work, the relative potency of the inhibitors was determined to be enalaprilat>lisinopril>captopril. We analyze the thermodynamic behaviour of the binding process using the new structural information provided by the ACE structures, as well as the conformational changes that occur upon binding.     

Recognition of selected monosaccharides by Pseudomonas aeruginosa Lectin II analyzed by molecular dynamics and free energy calculations

In this study, interactions of selected monosaccharides with the Pseudomonas aeruginosa Lectin II (PA-IIL) are analyzed in detail. An interesting feature of the PA-IIL binding is that the monosaccharide is interacting via two calcium ions and the binding is unusually strong for protein-saccharide interaction. We have used Molecular Mechanics Poisson-Boltzmann Surface Area (MM/PBSA) and normal mode analysis to calculate the free energy of binding. The impact of intramolecular hydrogen bond network for the lectin/monosaccharide interaction is also analyzed.     

Crystal structures of the complexes between vancomycin and cell-wall precursor analogs

The crystal structures of three vancomycin complexes with two vancomycin-sensitive cell-wall precursor analogs (diacetyl-Lys-D-Ala-D-Ala and acetyl-D-Ala-D-Ala) and a vancomycin-resistant cell-wall precursor analog (diacetyl-Lys-D-Ala-D-lactate) were determined at atomic resolutions of 1.80 A, 1.07 A, and 0.93 A, respectively. These structures not only reconfirm the "back-to-back" dimerization of vancomycin monomers and the ligand-binding scheme proposed by previous experiments but also show important structural features of strategies for the generation of new glycopeptide antibiotics. These structural features involve a water-mediated antibiotic-ligand interaction and supramolecular structures such as "side-by-side" arranged dimer-to-dimer structures, in addition to ligand-mediated and "face-to-face" arranged dimer-to-dimer structures. In the diacetyl-Lys-D-Ala-D-lactate complex, the interatomic O...O distance between the carbonyl oxygen of the fourth residue of the antibiotic backbone and the ester oxygen of the D-lactate moiety of the ligand is clearly longer than the corresponding N-H...O hydrogen-bonding distance observed in the two other complexes due to electrostatic repulsion. In addition, two neighboring hydrogen bonds are concomitantly lengthened. These observations provide, at least in part, a molecular basis for the reduced antibacterial activity of vancomycin toward vancomycin-resistant bacteria with cell-wall precursors terminating in -D-Ala-D-lactate.