Preliminary H NMR investigation of sialic acid transfer by the trans-sialidase from Trypanosoma cruzi

¹H NMR spectroscopy has been used to investigate the transfer of sialic acid from sialic acid donor molecules to acceptor molecules using the trans-sialidase from Typanosoma cruzi. It is clearly demonstrated that NMR spectroscopy is an efficient and powerful means of monitoring the trans-sialidase promoted transfer of sialic acid from donor to acceptor.     

NMR spectroscopic and molecular modeling investigations of the trans-sialidase from Trypanosoma cruzi

Nuclear magnetic resonance (NMR) spectroscopy was used to investigate the transfer of sialic acid from a range of sialic acid donor compounds to acceptor molecules, catalyzed by Trypanosoma cruzi trans-sialidase (TcTS). We demonstrate here that NMR spectroscopy is a powerful tool to monitor the trans-sialidase enzyme reaction for a variety of donor and acceptor molecules. The hydrolysis or transfer reactions that are catalyzed by TcTS were also investigated using a range of N-acetylneuraminosyl-based donor substrates and asialo acceptor molecules. These studies showed that the synthetic N-acetylneuraminosyl donor 4-methylumbelliferyl alpha-d-N-acetylneuraminide (MUN) is hydrolyzed by the enzyme approximately 3-5 times faster than either the disaccharide Neu5Acalpha(2,3)Galbeta1Me or the trisaccharide Neu5Acalpha(2,3)Lacbeta1Me. In the transfer reaction, we show that Neu5Acalpha(2,3)Lacbeta1Me is the most favorable substrate for TcTS and is a better substrate than the naturally-occurring N-acetylneuraminosyl donor alpha1-acid glycoprotein. In the case of MUN as the donor molecule, the transfer of Neu5Ac to different acceptors is significantly slower than when other N-acetylneuraminosyl donors are used. We hypothesize that when MUN is bound by the enzyme, the orientation and steric bulk of the umbelliferyl aglycon moiety may restrict the access for the correct positioning of an acceptor molecule. AutoDock studies support our hypothesis and show that the umbelliferyl aglycon moiety undergoes a strong pi-stacking interaction with Trp-312. The binding properties of TcTS towards acceptor (lactose) and donor substrate (Neu5Ac) molecules have also been investigated using saturation transfer difference (STD) NMR experiments. These experiments, taken together with other published data, have clearly demonstrated that lactose in the absence of other coligands does not bind to the TcTS active site or other binding domains. However, in the presence of the sialic acid donor, lactose (an asialo acceptor) was observed by NMR spectroscopy to interact with the enzyme's active site. The association of the asialo acceptor with the active site is an absolute requirement for the transfer reaction to proceed.     

Specificity of sialytransferase: structure of alpha1-acid glycoprotein sialylated in vitro. A new methodology for the determination of the structure of the linkage formed by glycosyltransferase action on galactosyl-oligosaccharide-protein acceptors.

The terminal galactosyl units of desialylated alpha1-acid glycoprotein were selectively labeled with tritium by a galactose oxidase/NaB3H4 procedure. The 3H-labeled glycoprotein was effective as an acceptor in sialytransferase reactions catalyzed by rat liver microsomes in vitro with unlabeled CMP-N-acetyl-neuramininic acid as sialic acid donor. Permethylation/hydrolysis of glycopeptides derived from the resialylated 3H-labeled glycoprotein yielded radioactive 2,3,4-trimethylgalactose indicating that rat liver microsomes are capable of transferring sialic acid to position C-6 of the terminal galactosyl units of desialylated alpha1-acid glycoprotein. No indication was obtained for transfer of sialic acid to other positions. This result is discussed in view of the multiplicity of positions of attachment of sialic acid to galactosyl residues in native alpha1-acid glycoprotein.     

On the biosynthesis of alternating alpha-2,9/alpha-2,8 heteropolymer of sialic acid catalyzed by the sialyltransferase of Escherichia coli Bos-12.

Escherichia coli Bos-12 synthesizes a heteropolymer of sialic acids with alternating alpha-2,9/alpha-2,8 glycosidic linkages (1). In this study, we have shown that the polysialyltransferase of the E. coli Bos-12 recognizes an alpha-2,8 glycosidic linkage of sialic acid at the nonreducing end of an exogenous acceptor of either the alpha-2,8 homopolymer of sialic acid or the alternating alpha-2,9/alpha-2,8 heteropolymer of sialic acid and catalyzes the transfer of Neu5Ac from CMP-Neu5Ac to this residue. When the exogenous acceptor is an alpha-2,8-linked oligomer of sialic acid, the main product synthesized is derived from the addition of a single residue of [14C]Neu5Ac to form either an alpha-2,8 glycosidic linkage or an alpha-2,9 glycosidic linkage at the nonreducing end, at an alpha-2, 8/alpha-2,9 ratio of approximately 2:1. When the acceptor is the alternating alpha-2,9/alpha-2,8 heteropolymer of sialic acid, chain elongation takes place four to five times more efficiently than the alpha-2,8-linked homopolymer of sialic acid as an acceptor. It was found that the alpha-2,9-linked homopolymer of sialic acid and the alpha-2,8/alpha-2,9-linked hetero-oligomer of sialic acid with alpha-2,9 at the nonreducing end not only failed to serve as an acceptor for the E. coli Bos-12 polysialyltransferase for the transfer of [14C]Neu5Ac, but they inhibited the de novo synthesis of polysialic acid catalyzed by this enzyme. The results obtained in this study favor the proposal that the biosynthesis of the alpha-2, 9/alpha-2,8 heteropolymer of sialic acid catalyzed by the E. coli Bos-12 polysialyltransferase involves a successive transfer of a preformed alpha-2,8-linked dimer of sialic acid at the nonreducing terminus of the acceptor to form an alpha-2,9 glycosidic linkage between the incoming dimer and the acceptor. The glycosidic linkage at the nonreducing end of the alternating alpha-2,9/alpha-2,8 heteropolymer of sialic acid produced by E. coli Bos-12 should be an alpha-2,8 glycosidic bond and not an alpha-2,9 glycosidic linkage.     

Structural analysis of sialyltransferase PM0188 from Pasteurella multocida complexed with donor analogue and acceptor sugar

PM0188 is a newly identified sialyltransferase from P. multocida which transfers sialic acid from cytidine 5'-monophosphonuraminic acid (CMP-NeuAc) to an acceptor sugar. Although sialyltransferases are involved in important biological functions like cell-cell recognition, cell differentiation and receptor-ligand interactions, little is known about their catalytic mechanism. Here, we report the X-ray crystal structures of PM0188 in the presence of an acceptor sugar and a donor sugar analogue, revealing the precise mechanism of sialic acid transfer. Site-directed mutagenesis, kinetic assays, and structural analysis show that Asp141, His311, Glu338, Ser355 and Ser356 are important catalytic residues; Asp141 is especially crucial as it acts as a general base. These complex structures provide insights into the mechanism of sialyltransferases and the structure-based design of specific inhibitors.     

Comparative rates of sialylation by recombinant trans-sialidase and inhibitor properties of synthetic oligosaccharides from Trypanosoma cruzi mucins-containing galactofuranose and galactopyranose

The mucin-like glycoproteins of Trypanosoma cruzi have novel O-linked oligosaccharides that are acceptors of sialic acid in the trans-sialidase (TcTS) reaction. The transference of sialic acid from host glycoconjugates to the mucins is involved in infection and pathogenesis. The O-linked chains may contain galactofuranose in addition to the acceptor galactopyranose units. Thus far, the galactofuranose form was found in the mucins of strains belonging to the less infective lineage. The acceptor properties of the chemically synthesized oligosaccharides were now studied in order to correlate their structure with the ability to act as substrates. Recombinant TcTS and sialyllactose as donor were used. The reactions were followed by HPAEC-PAD. The K(m) values were calculated for the free sugars, the sugar alditols and the benzyl glycosides. All the compounds showed to be good acceptors of sialic acid. Thus, the introduction of galactofuranose in the mucins of the strains of lineage 1 would not be responsible for the diminished virulence of the strains. The oligosaccharides and derivatives inhibited the transfer of sialic acid to the substrate N-acetyllactosamine with IC(50) values between 0.6 and 4 mM.     

Resialylation of sialidase-treated sheep and human erythrocytes by Trypanosoma cruzi trans-sialidase: restoration of complement resistance of desialylated sheep erythrocytes.

Trypanosoma cruzi trans-sialidase (TS) is a recently described enzyme which transfers alpha(2-3)-linked sialic acid from host-derived sialylated glycoconjugates to parasite surface molecules [Schenkman et al. (1991) Cell, 65, 1117]. We report here on the ability of TS to transfer sialic acid from donor sialyl-alpha(2-3)lactose to sialidase-treated sheep and human erythrocytes. Up to approximately 50% resialylation of both desialylated red cells could be attained. Resialylation of desialylated sheep erythrocytes restores their resistance to lysis by human complement. This ascribes a possible biological role for T. cruzi TS and demonstrates directly that sialic acid is solely responsible for preventing alternative pathway activation of human complement by sheep erythrocytes.     

Phosphatidylcholine exchange protein catalyzes the net transfer of phosphatidylcholine to model membranes

2-Stearoyl spin-labeled phosphatidylcholine (PC*) has been introduced into the phosphatidylcholine exchange protein from bovine liver and its electron spin resonance (ESR) spectrum determined. The spin-labeled group in the PC*- exchange protein complex was strongly immobilized. Addition of sodium deoxycholate micelles released PC* from its binding site, producing a mobile signal. This was also observed when micelles of lysophosphatidylcholine and vesicles of phosphatidic acid were added, indicating that the exchange protein can insert its endogenous PC* into interfaces devoid of phosphatidylcholine. ESR spectroscopy was used to measure transfer of PC* from spin-labeled "donor" vesicles to unlabeled "acceptor" vesicles as described by Machida & Ohnishi [Machida, K., & Ohnishi, S. (1978) Biochim. Biophys. Acta 507, 156-164]. The donor vesicles consisted of PC* and phosphatidic acid (75:25 mol%) and the acceptor vesicles of phosphatidylethanolamine and phosphatidic acid (81:19 mol%). Addition of exchange protein catalyzed a net transfer of PC* from donor to acceptor vesicles. This transfer proceeded until the acceptor vesicles contained approximately 2 mol% of PC*. A spontaneous transfer of PC* was not observed. As for the mode of action, it appears that the exchange protein, after insertion of its endogenous PC* into the acceptor, leaves the interface without a bound phospholipid molecule yet continues to shuttle PC* from donor to acceptor.     

Kinetics and mechanism of exchange of apolipoprotein C-III molecules from very low density lipoprotein particles

Transfer of apolipoprotein (apo) molecules between lipoprotein particles is an important factor in modulating the metabolism of the particles. Although the phenomenon is well established, the kinetics and molecular mechanism of passive apo exchange/transfer have not been defined in detail. In this study, the kinetic parameters governing the movement of radiolabeled apoC molecules from human very low density lipoprotein (VLDL) to high density lipoprotein (HDL3) particles were measured using a manganese phosphate precipitation assay to rapidly separate the two types of lipoprotein particles. In the case of VLDL labeled with human [14C]apoCIII1, a large fraction of the apoCIII1 transfers to HDL3 within 1 minute of mixing the two lipoproteins at either 4 degrees or 37 degrees C. As the diameter of the VLDL donor particles is decreased from 42-59 to 23-25 nm, the size of this rapidly transferring apoCIII1 pool increases from about 50% to 85%. There is also a pool of apoCIII1 existing on the donor VLDL particles that transfers more slowly. This slow transfer follows a monoexponential rate equation; for 35-40 nm donor VLDL particles the pool size is approximately 20% and the t1/2 is approximately 3 h. The flux of apoCIII molecules between VLDL and HDL3 is bidirectional and all of the apoCIII seems to be available for exchange so that equilibrium is attained. It is likely that the two kinetic pools of apoCIII are related to conformational variations of individual apo molecules on the surface of VLDL particles. The rate of slow transfer of apoCIII1 from donor VLDL (35-40 nm) to acceptor HDL3 is unaffected by an increase in the acceptor to donor ratio, indicating that the transfer is not dependent on collisions between donor and acceptor particles. Consistent with this, apoCIII1 molecules can transfer from donor VLDL to acceptor HDL3 particles across a 50 kDa molecular mass cutoff semipermeable membrane separating the lipoprotein particles. These results indicate that apoC molecules transfer between VLDL and HDL3 particles by an aqueous diffusion mechanism.     

The effect of dietary linoleic acid on rat platelet sialic acid

The total sialic acid content of blood platelets from rats raised for 8 weeks or 12 months on a diet containing 1% linoleic acid (1LA) was significantly lower (by over 30%) than that from those raised on an isocalorific diet containing 6% linoleic acid (6LA). The transfer of sialic acid to endogenous glycoprotein acceptor was also significantly lower (up to almost 4-fold) in 1LA platelet and megakaryocyte-rich preparations but the transfer to exogenous glycoprotein acceptor was similar in both 1LA and 6LA platelets. The megakaryocyte-rich fraction of 1LA animals showed a reduced phosphodolichol-sensitive N-acetylglucosaminyl (but not mannosyl) transfer to endogenous glycoprotein compared with 6LA animals. No significant difference was found between the megakaryocytes of 1LA and 6LA animals in the incorporation of radioactive mannose and glucosamine into the glycoprotein of the whole cells. It was concluded that the decreased transfer of sialic acid to glycoproteins of platelets and megakaryocyte of animals on the 1LA diet was due to the decreased availability of sialyl acceptor. The formation of N-linked oligosaccharide was the same in both 1LA and 6LA megakaryocytes, and thus any differences in phosphodolichol-mediated N-glycosylation did not account for this decreased availability of sialyl acceptor.     

Structural changes of yellow Cameleon domains observed by quantitative FRET analysis and polarized fluorescence correlation spectroscopy

Förster resonance energy transfer (FRET) is a widely used method for monitoring interactions between or within biological macromolecules conjugated with suitable donor-acceptor pairs. Donor fluorescence lifetimes in absence and presence of acceptor molecules are often measured for the observation of FRET. However, these lifetimes may originate from interacting and noninteracting molecules, which hampers quantitative interpretation of FRET data. We describe a methodology for the detection of FRET that monitors the rise time of acceptor fluorescence on donor excitation thereby detecting only those molecules undergoing FRET. The large advantage of this method, as compared to donor fluorescence quenching method used more commonly, is that the transfer rate of FRET can be determined accurately even in cases where the FRET efficiencies approach 100% yielding highly quenched donor fluorescence. Subsequently, the relative orientation between donor and acceptor chromophores is obtained from time-dependent fluorescence anisotropy measurements carried out under identical conditions of donor excitation and acceptor detection. The FRET based calcium sensor Yellow Cameleon 3.60 (YC3.60) was used because it changes its conformation on calcium binding, thereby increasing the FRET efficiency. After mapping distances and orientation angles between the FRET moieties in YC3.60, cartoon models of this FRET sensor with and without calcium could be created. Independent support for these representations came from experiments where the hydrodynamic properties of YC3.60 under ensemble and single-molecule conditions on selective excitation of the acceptor were determined. From rotational diffusion times as found by fluorescence correlation spectroscopy and consistently by fluorescence anisotropy decay analysis it could be concluded that the open structure (without calcium) is flexible as opposed to the rather rigid closed conformation. The combination of two independent methods gives consistent results and presents a rapid and specific methodology to analyze structural and dynamical changes in a protein on ligand binding.     

Aggregation state of melittin in lipid vesicle membranes

We have performed time-resolved fluorescence energy transfer measurements using melittin as donor and a modified melittin as acceptor. The melittin molecules were bound to fluid vesicle membranes of dimyristoylphosphatidylcholine. Analysis of the temporal decay of the energy transfer and of its variation with the donor and acceptor concentrations led to the conclusion that melittin in fluid membranes is usually monomeric. Only at the high melittin/lipid molar ratio of 1/200 and high ionic strength evidence for aggregation was obtained, the percentage of aggregated melittin molecules being of the order of 10%. The shortcomings of previous steady-state measurements of fluorescence energy transfer between melittin molecules are discussed.     

Förster-type energy transfer. Simultaneous 'forward' and 'reverse' transfer between unlike fluorophores

The general case of F?rster-type energy transfer is that in which energy is exchanged in both directions between two unlike fluorophores. In such cases, energy is transferred from the conventionally defined donor to the conventionally defined acceptor (forward transfer) and at the same time from the acceptor to the donor (reverse transfer). Expressions are derived to describe the fluorescence intensities and lifetimes of fluorophores undergoing simultaneous forward and reverse transfer; these are compared with corresponding quantities for the case more usually considered, in which only forward transfer is significant. It is shown that the presence of reverse transfer removes the distinction between donor and acceptor, and allows such anomalous effects as 'acceptor quenching'. A confirmatory example is described. It is shown that the equations generally used in distance determination by steady-state fluorescence spectroscopy can also be applied in the presence of reverse transfer, if a correction term is included; however, for lifetime spectroscopy the correction is more complex.     

Temperature differences for trans-glycosylation and hydrolysis reaction reveal an acceptor binding site in the catalytic mechanism of Trypanosoma cruzi trans-sialidase

Trypanosoma cruzi, the agent of Chagas disease, expresses onits surface a trans-sialidase that catalyzes preferentiallythe transference of -2,3-linked sialic acid to acceptors containingterminal β-galactosyl residues, instead of the typicalhydrolysis reaction, found in most sialidases. The trans-sialidaseis responsible for the acquisition of the host sialic acid bythis protozoan parasite, which does not synthesize sialic acids.Here, we have studied some kinetic properties of a recombinanttrans-sialidase expressed in Escherichia coli We found thatit has sequential-type kinetics for the transferase reaction,as shown for the parasite-derived enzyme. The rates of sialicacid transfer to water (hydrolysis), and to β-galactosylresidues have a unique behavior with respect to the reactiontemperature. While the hydrolysis rate of sialyUactose increasescontinuously up to 35°C, the temperature for the maximalrate of trans-glycosylation depends on the acceptor concentration.At low acceptor concentrations the rate of trans-glycosylationis maximal at 13°C and independent of the amount of sialicacid donors. With increasing acceptor concentrations, maximalrates of trans-glycosylation are shifted to higher temperatures.This finding is explained by an 8-fold increase in the K_m forthe acceptor from 13°C to 33°C. Differences in hydrolysisand transfer rates were also obtained by using 4-methyl-umbelliferyl-N-acetyl-neuraminicacid. However, its hydrolysis rate is much higher than the rateof transference to lactose, suggesting that a long-lived enzyme-sialosylintermediate is not formed. In addition, lactose does not increasethe rate of methyl-umbelliferone release at any temperature,indicating that the rate limiting step is the aglycon release.Based on these results we propose that trans-glycosylation inT.cruzi sialidase is favored by the existence of a binding sitefor β-galactosyl residues, which accepts the new glycosidicbond as sialic acid is released from the donor. With increasingtemperature the affinity for the acceptor decreases, resultingin a concomitant increase in the rate of transfer to water,which, in turn, can be suppressed by increasing the acceptorconcentration. Trypanosoma cruzi sialidase kinetics reaction mechanism temperature     

Structural basis for acceptor substrate recognition of a human glucuronyltransferase, GlcAT-P, an enzyme critical in the biosynthesis of the carbohydrate epitope HNK-1

The HNK-1 carbohydrate epitope is found on many neural cell adhesion molecules. Its structure is characterized by a terminal sulfated glucuronyl acid. The glucuronyltransferases, GlcAT-P and GlcAT-S, are involved in the biosynthesis of the HNK-1 epitope, GlcAT-P as the major enzyme. We overexpressed and purified the recombinant human GlcAT-P from Escherichia coli. Analysis of its enzymatic activity showed that it catalyzed the transfer reaction for N-acetyllactosamine (Galbeta1-4GlcNAc) but not lacto-N-biose (Galbeta1-3GlcNAc) as an acceptor substrate. Subsequently, we determined the first x-ray crystal structures of human GlcAT-P, in the absence and presence of a donor substrate product UDP, catalytic Mn(2+), and an acceptor substrate analogue N-acetyllactosamine (Galbeta1-4GlcNAc) or an asparagine-linked biantennary nonasaccharide. The asymmetric unit contains two independent molecules. Each molecule is an alpha/beta protein with two regions that constitute the donor and acceptor substrate binding sites. The UDP moiety of donor nucleotide sugar is recognized by conserved amino acid residues including a DXD motif (Asp(195)-Asp(196)-Asp(197)). Other conserved amino acid residues interact with the terminal galactose moiety of the acceptor substrate. In addition, Val(320) and Asn(321), which are located on the C-terminal long loop from a neighboring molecule, and Phe(245) contribute to the interaction with GlcNAc moiety. These three residues play a key role in establishing the acceptor substrate specificity.     

A rigorous experimental framework for detecting protein oligomerization using bioluminescence resonance energy transfer

Bioluminescence resonance energy transfer (BRET), which relies on nonradiative energy transfer between luciferase-coupled donors and GFP-coupled acceptors, is emerging as a useful tool for analyzing the quaternary structures of cell-surface molecules. Conventional BRET analyses are generally done at maximal expression levels and single acceptor/donor ratios. We show that under these conditions substantial energy transfer arises from random interactions within the membrane. The dependence of BRET efficiency on acceptor/donor ratio at fixed surface density, or expression level at a defined acceptor/donor ratio, can nevertheless be used to correctly distinguish between well-characterized monomeric and oligomeric proteins, including a very weak dimer. The pitfalls associated with the nonrigorous treatment of BRET data are illustrated for the case of G protein-coupled receptors (GPCRs) proposed to form homophilic and/or mixed oligomers on the basis of previous, conventional BRET experiments.     

Crystal structures of Pasteurella multocida sialyltransferase complexes with acceptor and donor analogues reveal substrate binding sites and catalytic mechanism

Sialyltransferases are key enzymes involved in the biosynthesis of biologically and pathologically important sialic acid-containing molecules in nature. Binary X-ray crystal structures of a multifunctional Pasteurella multocida sialyltransferase (Delta24PmST1) with a donor analogue CMP-3F(a)Neu5Ac or CMP-3F(e)Neu5Ac were determined at 2.0 and 1.9 A resolutions, respectively. Ternary X-ray structures of the protein in complex with CMP or a donor analogue CMP-3F(a)Neu5Ac and an acceptor lactose have been determined at 2.0 and 2.27 A resolutions, respectively. This represents the first sialyltransferase structure and the first GT-B-type glycosyltransferase structure that is bound to both a donor analogue and an acceptor simultaneously. The four structures presented here reveal that binding of the nucleotide-activated donor sugar causes a buried tryptophan to flip out of the protein core to interact with the donor sugar and helps define the acceptor sugar binding site. Additionally, key amino acid residues involved in the catalysis have been identified. Structural and kinetic data support a direct displacement mechanism involving an oxocarbenium ion-like transition state assisted with Asp141 serving as a general base to activate the acceptor hydroxyl group.     

Effects of apolipoprotein structure on the kinetics of apolipoprotein transfer between phospholipid vesicles

The kinetics and mechanism of transfer of 14C-labeled human apolipoproteins A-I, A-II and C-III1 between small unilamellar vesicles (SUV) have been investigated. Ion exchange chromatography was used for rapid separation of negatively charged egg phosphatidylcholine (PC)/dicetyl phosphate donor SUV containing bound 14C-labeled apoprotein from neutral egg PC acceptor SUV present in 10-fold molar excess. The transfer kinetics of these apolipoproteins at 37 degrees C are consistent with the existence of fast, slow and apparently 'nontransferrable' pools of SUV-associated lipoprotein: the transfers from these pools occur on timescales of seconds (or less), minutes/hours and days/weeks, respectively. For donor SUV containing about 15 apoprotein molecules per vesicle and at a donor SUV concentration of 0.15 mg phospholipid/ml incubation mixture, the sizes of the fast kinetic pools for apolipoproteins A-I, A-II and C-III1 associated with donor SUV are 2, 10 and 11%, respectively. The sizes of the slow kinetic pools for these apolipoproteins are 16, 71 and 50%, respectively. The transfer of the various apolipoproteins from the slow kinetic pool follows first order kinetics and the half-time (t1/2) values are in the order: apo C-III1 less than apo A-I. Increasing the number of apoprotein molecules per donor SUV enlarges the size of the fast pool and increases the t1/2 of slow transfer. The differences in the kinetics of apolipoprotein transfer between SUV are consequences of the variations in the primary and secondary structures of the apolipoprotein molecules. The slow transfer of apoprotein molecules is mediated by collisions between donor and acceptor SUV; the rate is dependent on the apoprotein molecular weight with larger molecules transferring more slowly from donor SUV containing the same lipid/protein molar ratio. The hydrophobicity of the apoprotein molecule is also significant with less hydrophobic molecules transferring more rapidly. Further understanding of the differences in the kinetics of transfer of these apolipoproteins will require more knowledge of their secondary and tertiary structures.     

Direct detection of N-H[...]N hydrogen bonds in biomolecules by NMR spectroscopy

A nuclear magnetic resonance (NMR) experiment is described for the direct detection of N-H[...]N hydrogen bonds (H-bonds) in 15N isotope-labeled biomolecules. This quantitative HNN-COSY (correlation spectroscopy) experiment detects and quantifies electron-mediated scalar couplings across the H-bond (H-bond scalar couplings), which connect magnetically active (15)N nuclei of the H-bond donor and acceptor. Detectable H-bonds comprise the imino H-bonds in canonical Watson-Crick base pairs, many H-bonds in unusual nucleic acid base pairs and H-bonds between protein backbone or side-chain N-H donor and N acceptor moieties. Unlike other NMR observables, which provide only indirect evidence of the presence of H-bonds, the H-bond scalar couplings identify all partners of the H-bond, the donor, the donor proton and the acceptor in a single experiment. The size of the scalar couplings can be related to H-bond geometries and as a time average to H-bond dynamics. The time required to detect the H-bonds is typically less than 1 d at millimolar concentrations for samples of molecular weight < or = approximately 25 kDa. A C15N/13C-labeled potato spindle tuber viroid T1 RNA domain is used as an example to illustrate this procedure.     

High-yield enzymatic bioconversion of hydroquinone to α-arbutin, a powerful skin lightening agent, by amylosucrase

α-Arbutin (α-Ab) is a powerful skin whitening agent that blocks epidermal melanin biosynthesis by inhibiting the enzymatic oxidation of tyrosine and L-3,4-dihydroxyphenylalanine (L-DOPA). α-Ab was effectively synthesized from hydroquinone (HQ) by enzymatic biotransformation using amylosucrase (ASase). The ASase gene from Deinococcus geothermalis (DGAS) was expressed and efficiently purified from Escherichia coli using a constitutive expression system. The expressed DGAS was functional and performed a glycosyltransferase reaction using sucrose as a donor and HQ as an acceptor. The presence of a single HQ bioconversion product was confirmed by thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC). The HQ bioconversion product was isolated by silica gel open column chromatography and its chemical structure determined by 1H and 13C nuclear magnetic resonance (NMR). The product was determined to be hydroquinone-O-α-D-glucopyranoside with a glucose molecule linked to HQ through an α-glycosidic bond. However, the production yield of the transfer reaction was significantly low (1.3%) due to the instability of HQ in the reaction mixture. The instability of HQ was considerably improved by antioxidant agents, particularly ascorbic acid, implying that HQ is labile to oxidation. A maximum yield of HQ transfer product of 90% was obtained at a 10:1 molar ratio of donor (sucrose) and acceptor (HQ) molecules in the presence of 0.2 mM ascorbic acid.