Steric course of the hydrolysis of alpha,alpha-trehalose and alpha-D-glucosyl fluoride catalyzed by pig kidney trehalase

We are unable to confirm the report of Labat et al.3 that pig kidney trehalase hydrolyzes alpha,alpha-trehalose to form solely alpha-D-glucose. Highly purified trehalase from pig renal cortex was found, in reactions monitored by 1H-n.m.r. spectra, to hydrolyze alpha,alpha-trehalose with the formation of both alpha- and beta-D-glucose. That the beta anomer constitutes the enzymically mobilized glucosyl residue is indicated by the further finding that beta-D-glucose is the product formed on hydrolysis of alpha-D-glucosyl fluoride by the enzyme. Present results show the stereochemical behavior of pig kidney trehalase in hydrolyzing alpha,alpha-trehalose to be indistinguishable from that reported by ourselves and others for trehalase preparations from a range of biological sources including rabbit renal cortex.

Formation of highly ordered cross-linked lattices between asparagine-linked oligosaccharides and lectins observed by electron microscopy

The interaction of asparagine-linked carbohydrates (N-linked) with carbohydrate binding proteins called lectins has been demonstrated to be involved in a variety of cellular recognition processes. Certain N-linked carbohydrates have been shown to be multivalent and capable of binding, cross-linking, and precipitating lectins (Bhattacharyya, L., Ceccarini, C., Lorenzoni, P., and Brewer, C. F. (1987) J. Biol. Chem. 262, 1288-1293; Bhattacharyya, L., Haraldsson, M., and Brewer, C. F. (1987) J. Biol. Chem. 262, 1294-1299; Bhattacharyya, L., Haraldsson, M., and Brewer, C. F. (1988) Biochemistry 27, 1034-1041). Recent data have further suggested that certain oligomannose and bisected hybrid-type N-linked glycopeptides form homogeneous cross-linked lattices with concanavalin A (Bhattacharyya, L., Khan, M. I., and Brewer, C. F. (1988) Biochemistry 27, 8762-8767). In the present study, evidence has been obtained from electron microscopy for the formation of highly ordered and distinct lattices for two bivalent complex type oligosaccharides cross-linked with soybean lectin (Glycine max) and isolectin A from Lotus tetragonolobus, respectively. The results indicate a new source of specificity for interactions of N-linked carbohydrates with lectins, namely their ability to form highly ordered homogeneous aggregates.

Alpha-secondary tritium kinetic isotope effects for the hydrolysis of alpha-D-glucopyranosyl fluoride by exo-alpha-glucanases

alpha-Secondary tritium kinetic isotope effects ranging from 1.17 to 1.26 were measured for the hydrolysis of alpha-D-glucopyranosyl fluoride (forming beta-D-glucose) catalyzed by several glucoamylases and a glucodextranase. These results indicate that cleavage of the C-F bond is slow and that the enzymic transition state has significant oxo-carbonium ion character. Strong support for this conclusion is provided by the agreement found in the case of Rhizopus niveus glucoamylase (alpha-TV/K 1.26; Km 26 mM) between measured values of the alpha-secondary deuterium kinetic isotope effects (alpha-DV/K 1.16; alpha-DV 1.20) and those calculated from the tritium isotope effect. The data are consistent with the promotion of an intramolecular elimination of fluoride by the present exo-alpha-glucanases based on their ability to stabilize, perhaps with a counter ion, the development of a carbonium ion-like transition state. Although the oxo-carbonium ion is formally denoted as an intermediate it could represent a transition state along a reaction pathway to a covalent glucosyl intermediate.

Interactions of concanavalin A with asparagine-linked glycopeptides. Structure/activity relationships of the binding and precipitation of oligomannose and bisected hybrid-type glycopeptides with concanavalin A

We have recently demonstrated that certain oligomannose and bisected hybrid-type glycopeptides are bivalent for concanavalin A (ConA) binding and that they can precipitate the lectin [Bhattacharyya, L., Ceccarini, C., Lorenzoni, P & Brewer, C. F. (1987) J. Biol. Chem. 262, 1288-1293]. Two protein-binding sites on each glycopeptide were identified: one on the alpha(1-6) arm of the core beta-mannose residue which binds with high affinity (primary site); the other on the alpha(1-3) arm of the core beta-mannose residue which binds with lower affinity (secondary site). In the present study, we have investigated the relationship between the structures of the primary sites of oligomannose-type glycopeptides and their affinities for ConA. Two mechanisms of binding at the primary sites of oligomannose-type glycopeptides have been identified which account for the 3000-fold increase in affinity of a Man9 glycopeptide relative to that of methyl alpha-D-mannopyranoside. Changes in the structures and affinities of both the primary and secondary sites are observed to influence the precipitation activities of the glycopeptides. These findings have important consequences for the specificity of ConA binding in solutions containing mixtures of the carbohydrates.

Synthesis of (Z)-3,7-anhydro-1,2-dideoxy-2-deuterio-D-gluco-oct-2- enitol, a prochiral substrate for probing the catalytic functioning of of glucosylases

Synthesis of the title compound provides a prochiral, glycosyl-donor substrate well suited for use as a probe of the catalytic functioning of D-glucosyl-mobilizing enzymes, because the full stereochemistry of enzymic reactions at its double bond may be unambiguously determined by examining the reaction products. The starting material for the synthesis was 2,6-anhydro-D-glycero-D-gulo-heptonic acid, from which 3,7-anhydro-4,5,6,8-tetra-O-benzyl-1-deoxy-D-glycero-D-gulo-2- octulose was prepared in eight steps. Reduction with lithium aluminum deuteride, and conversion of the resulting diastereomeric alcohols into (Z)-3,7-anhydro-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio-D- gluco-oct-2-enitol (11) and 3,7-anhydro-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio-D- glycero-D-gulo-oct-1-enitol (16), was carried out. By-products were 3,7-anhydro-2-O-benzoyl-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio -D-erythro-L-galacto-octitol and 3,7-anhydro-2-O-benzoyl-4,5,6,8-tetra-O-benzyl-1,2-dideoxy-2-deuterio -D-erythro-L-talo-octitol, which could, like compound 16, be recycled. On debenzylation the oct-2-enitol 11 yielded (Z)-3,7-anhydro-1,2-dideoxy-2-deuterio-D-gluco-oct-2-enitol.

Interactions of concanavalin A with asparagine-linked glycopeptides: formation of homogeneous cross-linked lattices in mixed precipitation systems

We have previously shown that certain oligomannose and bisected hybrid type glycopeptides are bivalent for binding to concanavalin A (Con A) [Bhattacharyya, L., Ceccarini, C., Lorenzoni, P., & Brewer, C. F. (1987) J. Biol. Chem. 262, 1288-1293]. Each glycopeptide gives a quantitative precipitation profile with the protein which consists of a single peak that corresponds to the binding stoichiometry of glycopeptide to protein monomer (1:2). We have shown that the affinities of the primary and secondary sites of the glycopeptides influence their extent of precipitation with the lectin [Bhattacharyya, L., & Brewer, C. F. (1988) Eur. J. Biochem. (in press)]. In the present study, we demonstrate that equimolar mixtures of any two of the glycopeptides result in a quantitative precipitation profile which shows two protein peaks. Using radiolabeled glycopeptides, the precipitation profiles of the individual glycopeptides were determined. The results show that each glycopeptide forms its own precipitation profile with the protein which is independent of the profile of the other glycopeptide. For mixtures containing an equimolar ratio of two glycopeptides, the glycopeptide with lower affinity shows a precipitation maximum at a lower concentration than the one with higher affinity. However, this can be reversed by increasing the ratio of the lower affinity glycopeptide in the mixture. Thus, the relative precipitation maxima of the glycopeptides are determined by mass-action equilibria involving competitive binding of the two carbohydrates to the protein. These equilibria, in turn, are sensitive to the relative amounts and affinities of the carbohydrates at both their primary and secondary sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Lectin-carbohydrate interactions. Studies of the nature of hydrogen bonding between D-galactose and certain D-galactose-specific lectins, and between D-mannose and concanavalin A

The binding of galactose-specific lectins from Erythrina indica (EIL), Erythrina arborescens (EAL), Ricinus communis (agglutinin; RCA-I), Abrus precatorius (agglutinin; APA), and Bandeiraea simplicifolia (lectin I; BSL-I) to fluoro-, deoxy-, and thiogalactoses were studied in order to determine the strength of hydrogen bonds between the hydroxyl groups of galactose and the binding sites of the proteins. The results have allowed insight into the nature of the donor/acceptor groups in the lectins that are involved in hydrogen bonding with the sugar. The data indicate that the C-2 hydroxyl group of galactose is involved in weak interactions as a hydrogen-bond acceptor with uncharged groups of EIL and EAL. With RCA-I, the C-2 hydroxyl group forms two weak hydrogen bonds in the capacity of a hydrogen-bond acceptor and a donor. On the other hand, there is a strong hydrogen bond between the C-2 hydroxyl group of galactose, which acts as a donor, and a charged group on BSL-I. The C-2 hydroxyl group of the sugar is also a hydrogen-bond donor to APA. The lectins are involved in strong hydrogen bonds through charged groups with the C-3 and C-4 hydroxyl groups of galactose, with the latter serving as hydrogen-bond donors. The C-6 hydroxyl group of the sugar is weakly hydrogen bonded with neutral groups of EIL, EAL, and APA. With BSL-I, however, a strong hydrogen bond is formed at this position with a charged group of the lectin. The C-6 hydroxyl groups is a hydrogen-bond acceptor for EIL and EAL, a hydrogen-bond donor for APA and BSL-I, and appears not to be involved in binding to RCA-I. The data with the thiosugars indicate the involvement of the C-1 hydroxyl group of galactose in binding to EIL, EAL, and BSL-I, but not to RCA-I and APA. We have also performed a similar analysis of the binding data of fluoro- and deoxysugars to concanavalin A [Poretz, R. D. and Goldstein, I. J. (1970) Biochemistry 9, 2890-2896]. This has allowed comparison of the donor/acceptor properties and free energies of hydrogen bonding of the hydroxyl groups of methyl alpha-D-mannopyranoside to concanavalin A with the results in the present study. On the basis of this analysis, new assignments are suggested for amino acid residues of concanavalin A [corrected] that may be involved in hydrogen bonding to the sugar.

Formation of homogeneous cross-linked lattices between oligomannose type glycopeptides and concanavalin A

Certain oligomannose type glycopeptides have previously been shown to be bivalent for binding to concanavalin A, and to give quantitative precipitation profiles with the protein that consist of single peaks which correspond to the binding stoichiometry of glycopeptide to protein monomer (1:2) (Bhattacharyya, L., Ceccarini, C., Lorenzoni, P., and Brewer, C.F. (1987) J. Biol. Chem. 262, 1288-1293). In the present study, equimolar mixtures of two oligomannose type glycopeptides, a Man-6 and a Man-9 glycopeptide, gives a quantitative precipitation profile which shows two protein peaks. Each glycopeptide was radiolabelled with 3H or 14C, and the the precipitation profiles of the individual glycopeptides in the mixture determined. The results show that the radioactivity profile of the Man-6 glycopeptide corresponds to the first protein peak, while the radioactivity profile of the Man-9 glycopeptide corresponds to the second protein peak. The results indicate that each glycopeptide forms a unique homogeneous cross-linked lattice with the lectin which excludes the lattice of the other glycopeptide.

Binding and precipitation of lectins from Erythrina indica and Ricinus communis (agglutinin I) with synthetic cluster glycosides

We recently reported that tri- and tetraantennary complex type oligosaccharides with nonreducing terminal galactose residues and the triantennary asialofetuin glycopeptide can bind and precipitate certain galactose specific lectins (L. Bhattacharyya, and C.F. Brewer (1986) Biochem. Biophys. Res. Commun. 141, 963-967; L. Bhattacharyya, M. Haraldsson, and C.F. Brewer (1988) Biochemistry 27, 1034-1041). The present study investigates the binding interactions of two of these lectins, those from Erythrina indica and Ricinus communis (Agglutinin I), with mono-, bi-, and triantennary synthetic cluster glycosides, which have little structural resemblance to complex type oligosaccharides other than they possess nonreducing terminal galactose residues (R.T. Lee, P. Lin, and Y.C. Lee (1984) Biochemistry 23, 4255-4261). The enhanced affinities of the bi- and triantennary glycosides relative to the monoantennary glycoside for the two lectins are consistent with an increase in the probability of binding due to multiple binding residues in the multiantennary glycosides. The triantennary glycoside is capable of precipitating the two lectins, and quantitative precipitation data indicate that it is a trivalent ligand. The results show that the binding and precipitation activities of complex type oligosaccharides with these lectins is due solely to the presence of multiple terminal galactose residues and not to the overall structures of the oligosaccharides.

Steric course of the hydration of D-gluco-octenitol catalyzed by alpha-glucosidases and by trehalase

Crystalline Aspergillus niger alpha-glucosidase and highly purified preparations of rice alpha-glucosidase II and Trichoderma reesei trehalase were found to catalyze the hydration of [2-(2)H]-D-gluco-octenitol, i.e., (Z)-3,7-anhydro-1,2-dideoxy-[2-2H]-D-gluco-oct-2-enitol, to yield 1,2-dideoxy-[2-2H]-D-gluco-octulose. In each case, the stereochemistry of the reaction was elucidated by examining the newly formed centers of asymmetry at C-2 and C-3 of the hydration product. The C-1 to C-3 fragment of each isolated [2-2H]-D-gluco-octulose product was recovered as [2-2H]propionic acid and identified by its positive optical rotatory dispersion as the S isomer, showing that each enzyme had protonated the octenitol (at C-2) from above its re face. 1H NMR spectra of enzyme/D-gluco-octenitol digests in D2O showed that the alpha-anomer of [2-2H]-D-gluco-octulose was exclusively produced by each alpha-glucosidase, whereas the beta-anomer was formed by action of the trehalase. The trans hydration catalyzed by the alpha-glucosidases was found to be very strongly inhibited by the substrate; the cis hydration reaction catalyzed by the trehalase showed no such inhibition. Special importance is attached to the finding that in hydrating octenitol each enzyme creates a product of the same anomeric form as in hydrolyzing an alpha-D-glucosidic substrate. This result adds substantially to the growing evidence that individual glycosylases create the configuration of their reaction products by a means that is independent of donor substrate configuration, that is, by a means other than "retaining" or "inverting" substrate configuration.

Stereochemical studies of D-glucal hydration by alpha-glucosidases and exo-alpha-glucanases: indications of plastic and conserved phases in catalysis by glycosylases

Alpha-Glucosidases from Aspergillus niger, pig serum, ungerminated rice, buckwheat, and sugar beet seeds (but not from brewers' yeast or honeybee) were found to catalyze the hydration of D-glucal. Each reactive alpha-glucosidase, incubated with D-glucal in D2O, was shown to protonate (deuteriate) this prochiral substrate from above its re face, i.e., from a direction opposite that assumed for protonating alpha-D-glucosidic substrates. At the same time, D-glucal hydration catalyzed by three of the alpha-glucosidases that acted rapidly enough in D2O to determine product configuration was found to yield 2-deoxy-D-glucose of the same specific (alpha-) configuration as the D-glucose produced from alpha-D-glucosidic substrates. These findings substantially extend those reported earlier for the hydration of D-glucal by one (Candida tropicalis) alpha-glucosidase preparation. Together with other recent results, they suggest that the process of catalysis by alpha-glucosidases (and perhaps glycosylases in general) may comprise two separate and separately controlled parts, namely, a "plastic" phase concerned with substrate protonation and a substrate-unrelated "conserved" phase concerned with the creation of product configuration. In contrast to the alpha-glucosidases, three "inverting" exo-alpha-glucanases (Arthrobacter globiformis glucodextranase; Rhizopus niveus and Paecilomyces varioti glucoamylase) were found to protonate D-glucal from below its si face. Further, whereas the catalysis of D-glucal hydration by the alpha-glucosidases was intensively inhibited by excess substrate, that promoted by the exo-glucanases showed no detectable substrate inhibition.

Precipitation of galactose-specific lectins by complex-type oligosaccharides and glycopeptides: studies with lectins from Ricinus communis (agglutinin I), Erythrina indica, Erythrina arborescens, Abrus precatorius (agglutinin), and Glycine max (soybean)

We have recently demonstrated that certain oligomannose and bisected hybrid type glycopeptides and bisected complex type oligosaccharides are bivalent for binding to concanavalin A and can precipitate the lectin [Bhattacharyya, L., Ceccarini, C., Lorenzoni, P., & Brewer, C.F. (1987) J. Biol. Chem. 262, 1288-1293; Bhattacharyya, L., Haraldsson, M., & Brewer, C.F. (1987) J. Biol. Chem. 262, 1294-1299]. The present results show that tri- and tetraantennary complex type oligosaccharides containing nonreducing terminal galactose residues, and a related triantennary glycopeptide, precipitate the D-galactose-specific lectins from Ricinus communis (agglutinin I) (RCA-I), Erythrina indica (EIL), Erythrina arborescens (EAL), and Glycine max (soybean) (SBA). Nonbisected and bisected biantennary complex type oligosaccharides can precipitate SBA, which is a tetrameric lectin, but not RCA-I, EIL, or EAL, which are dimeric lectins. The relative affinities of the oligosaccharides and glycopeptide were determined by hemagglutination inhibition measurements and their valencies by quantitative precipitin analyses. The equivalence points of the precipitin curves indicate that the tri- and tetraantennary oligosaccharides are tri- and tetravalent, respectively, for EIL, EAL, and SBA binding. However, the oligosaccharides are all trivalent for RCA-I binding due apparently to the larger size of the monomeric subunit of the lectin. The triantennary glycopeptide was also trivalent for RCA-I and EIL binding. Biantennary oligosaccharides with adequate chain lengths were found to be bivalent for binding to SBA; those with shorter chains did not precipitate the lectin.(ABSTRACT TRUNCATED AT 250 WORDS)

Circular dichroism and 1H NMR studies of Co2+- and Ni2+-substituted concanavalin A and the lentil and pea lectins

Visible absorption, circular dichroism (CD) and magnetic circular dichroism spectra have been recorded for the Ca2+-Co2+ derivatives of the lentil (CCoLcH) and pea (CCoPSA) lectins (Co2+ at the S1 sites and Ca2+ at the S2 sites) and shown to be very similar for both proteins. The visible absorption and magnetic circular dichroism spectra indicate similar octahedral geometries for high spin Co2+ at S1 in both proteins, as found in the Ca2+-Co2+ complex of concanavalin A (CCoPL) (Richardson, C. E., and Behnke, W. D. (1976) J. Mol. Biol. 102, 441-451). The visible CD data, however, indicate differences in the environment around S1 of CCoLcH and CCoPSA compared to CCoPL. 1H NMR spectra at 90 MHz of the Co2+ and Ni2+ derivatives of the lectins show a number of isotropically shifted signals which arise from protons in the immediate vicinity of the S1 sites. Analysis of the spectra of the Co2+ derivatives in H2O and D2O has permitted resonance assignments of the side chain ring protons of the coordinated histidine at S1 in the lectins. Differences are observed in the H-D exchange rate of the histidine NH proton at S1 in concanavalin A compared to the lentil and pea lectins. NMR data of the Ni2+-substituted proteins, together with spectra of the Co2+ derivatives, also indicate that the side chains of a carboxylate ligand and of the histidine residue at S1 are positioned differently in concanavalin A than in the other two lectins. These results appear to account, in part, for the differences observed in the visible CD spectra of the Co2+-substituted proteins. In addition, binding of monosaccharides does not significantly perturb the spectra of the lectins. An unusual feature in the 1H NMR spectra of all three Co2+-substituted lectins is the presence of two exchangeable downfield shifted resonances which appear to be associated with the two protons of a slowly exchanging water molecule coordinated to the Ca2+ ion at S2. T1 measurements of CCoLcH have provided an estimation of the distances from the Co2+ ion to these two protons of 3.7 and 4.0 A.

Electron paramagnetic resonance and magnetic susceptibility studies of dimanganese concanavalin A. Evidence for antiferromagnetic exchange coupling

The double Mn2+ complex of concanavalin A with bound saccharide (SMMPL) was examined by electron paramagnetic resonance (EPR) spectroscopy and magnetic susceptibility measurements. A room temperature X-band (9 GHz) EPR spectrum of SMMPL revealed a relatively weak, broad resonance in contrast to the spectrum with a six-line hyperfine-split pattern observed for the mononuclear, high-spin Mn2+ complex found in Ca2+-Mn2+-concanavalin A with saccharide present (SCMPL). The EPR spectrum of SMMPL at 77 K, however, consisted of a series of overlapping patterns of 11 hyperfine-split lines near g = 2.0 with members of each pattern separated by 47 G, half the value of the hyperfine splitting of SCMPL. These 11-line patterns are preserved at Q-band (35 GHz), indicating that the manganese ions in SMMPL form a spin-coupled, binuclear center. As expected for an exchange-coupled system, the EPR signal of SMMPL at 77 K saturates at a higher microwave power than those for SCMPL or Mn2+ aquoion. There is also a marked loss of EPR signal intensity for SMMPL between 4.2 and 1.4 K, which supports the view that the pair of manganese ions is exchanged-coupled. The temperature dependence of both the magnetic susceptibility and the low-temperature EPR spectral intensity can be explained by a model in which the two high-spin Mn2+ ions of SMMPL are antierromagnetically exchanged-coupled with an isotropic coupling constant J = 1.8 cm-1 (for the spin Hamiltonian Hex = JS1.S2). Zero-field splitting D' was estimated to be 375 G from the EPR spectrum.(ABSTRACT TRUNCATED AT 250 WORDS)

Catalytic versatility of Bacillus pumilus beta-xylosidase: glycosyl transfer and hydrolysis promoted with alpha- and beta-D-xylosyl fluoride

Bacillus pumilus beta-xylosidase, an enzyme considered restricted to hydrolyzing a narrow range of beta-D-xylosidic substrates with inversion of configuration, was found to catalyze different stereochemical, essentially irreversible, glycosylation reactions with alpha- and beta-D-xylopyranosyl fluoride. The enzyme promoted the hydrolysis of beta-D-xylopyranosyl fluoride at a high rate, V = 6.25 mumol min-1 mg-1 at 0 degrees C, in a reaction that obeyed Michaelis-Menten kinetics. In contrast, its action upon alpha-D-xylopyranosyl fluoride was slow and characterized by an unusual relation between the rate of fluoride release and the substrate concentration, suggesting the possible need for two substrate molecules to be bound at the active center in order for reaction to occur. Moreover, 1H NMR spectra of a digest of alpha-D-xylosyl fluoride showed the substrate to be specifically converted to alpha-D-xylose by the enzyme. The observed retention of configuration is not consistent with direct hydrolysis by this "inverting" enzyme but is strongly indicative of the occurrence of two successive inverting reactions: xylosyl transfer from alpha-D-xylosyl fluoride to form a beta-D-xylosidic product, followed by hydrolysis of the latter to produce alpha-D-xylose. The transient intermediate product formed enzymically from alpha-D-xylosyl fluoride in the presence of [14C]xylose was isolated and shown by its specific radioactivity and 1H NMR spectrum as well as by methylation and enzymic analyses to be 4-O-beta-D-xylopyranosyl-D-xylopyranose containing one [14C]xylose residue.(ABSTRACT TRUNCATED AT 250 WORDS)

Nuclear magnetic resonance investigation of cadmium 113 substituted pea and lentil lectins

The lentil (LcH) and pea (PSA) lectins, which are members of the class of D-glucose/D-mannose binding lectins, are Ca2+ X Mn2+ metalloproteins that require the metal ions for their saccharide binding and biological activities. We have prepared a variety of Cd2+ derivatives of PSA and LcH, with Cd2+ in either the transition metal (S1) or calcium (S2) sites, or in both. Thus, Cd2+ X Zn2+, Cd2+ X Mn2+, and Ca2+ X Cd2+ derivatives were prepared, in addition to the Cd2+ X Cd2+ derivatives which we have recently reported. This is the first report of stable mixed metal Cd2+ complexes of lectins. The physical and saccharide binding properties of the Cd2+ derivatives of both lectins were characterized by a variety of physiochemical techniques and found to be the same as those of the corresponding native proteins. 113Cd NMR spectra of mono- and disubstituted 113Cd2+ complexes of LcH and PSA were recorded and compared with 113Cd NMR data for concanavalin A (ConA) (Palmer, A.R., Bailey, D.B., Behnke, W.D., Cardin, A.D., Yang, P.P., and Ellis, P.D. (1980) Biochemistry 19, 5063-5070). The data for the PSA and LcH derivatives were found to be very similar, indicating close homology of their metal ion binding sites. 113Cd resonances at 44.6 ppm and -129.4 ppm for 113Cd2+ X 113Cd2+ X LcH, and at 46.6 and -130.4 for the corresponding PSA derivative, are chemical shifts very similar to those observed for 113Cd2+ X 113Cd2+ X ConA. Assignment of the resonances to the transition metal (S1) and calcium (S2) sites were unambiguous since the Ca2+ X 113Cd2+ and 113Cd2+ X Zn2+ derivatives of both lectins showed single resonances characteristic of the S1 and S2 sites, respectively. The results indicate that, unlike ConA, 113Cd2+ binds tightly to PSA and LcH. Binding of monosaccharide to both lectins induce small (2 ppm) upfield shifts in their S2 113Cd resonances, in contrast to the larger shift (8 ppm) observed in ConA. The 113Cd2+ X Mn2+ complexes of PSA and LcH fail to show a 113Cd resonance characteristic of these derivatives, which provides evidence for the close proximity of the metal ions in the two proteins. The present findings indicate that the coordinating ligand atoms to the metal ions at the S1 and S2 sites in LcH, PSA, and ConA are the same.

Concanavalin A interactions with asparagine-linked glycopeptides. Bivalency of bisected complex type oligosaccharides

In the preceding paper (Bhattacharyya, L., Ceccarini, C., Lorenzoni, P., and Brewer, C.F. (1987) J. Biol. Chem. 262, 1288-1293), we have demonstrated that certain high mannose and bisected hybrid type glycopeptides are bivalent for concanavalin A (ConA) binding. In the present study, we have investigated the interactions of ConA with a series of synthetic nonbisected and bisected complex type oligosaccharides and related glycopeptides. The modes of binding of the carbohydrates were studied by nuclear magnetic relaxation dispersion techniques, and their affinities were determined by hemagglutination inhibition measurements. We find that certain bisected complex type oligosaccharides are capable of binding and precipitating the lectin. The corresponding nonbisected analogs, however, bind but do not precipitate the protein. The stoichiometries of the precipitin reactions were investigated by quantitative precipitation analyses. The equivalence zones (regions of maximum precipitation) of the precipitin curves indicate that the bisected complex type oligosaccharides are bivalent for lectin binding. Data for the nonbisected analogs are consistent with their being univalent. The nuclear magnetic relaxation dispersion and precipitation data indicate that nonbisected and bisected complex type carbohydrates bind with different mechanisms and conformations. The former class binds by extended site interactions with the protein involving the 2 alpha-mannose residues on the alpha(1-6) and alpha(1-3) arms of the core beta-mannose residue. The latter class binds by only 1 of these 2 mannose residues, which leaves the other mannose residue free to bind to a second ConA molecule. The role of the bisecting GlcNAc residue in affecting the binding properties of complex type carbohydrates to ConA is discussed, and the results are related to the possible structure-function properties of complex type glycopeptides on the surface of cells.

Concanavalin A interactions with asparagine-linked glycopeptides. Bivalency of high mannose and bisected hybrid type glycopeptides

We have previously reported that concanavalin A (ConA) is precipitated by a high mannose type glycopeptide (Brewer, C. F. (1979) Biochem. Biophys. Res. Commun. 90, 117-122; Bhattacharyya, L., and Brewer, C. F. (1986) Biochem. Biophys. Res. Commun. 137, 670-674). In the present study, we have investigated the ability of a series of high mannose and bisected hybrid type glycopeptides to bind and precipitate the lectin. The modes of binding of the glycopeptides were studied by nuclear magnetic relaxation dispersion (NMRD) techniques, and their affinities were determined by hemagglutination inhibition measurements. The stoichiometries of the precipitation reactions were investigated by quantitative precipitation analysis. The equivalence zones (regions of maximum precipitation) of the precipitin curves indicate that certain high mannose and bisected hybrid type glycopeptides are bivalent for lectin binding. From the NMRD and precipitation data, we have identified two protein binding sites on each glycopeptide: one site on the alpha(1-6) arm of the core beta-mannose residue involving a trimannosyl moiety which binds with high affinity (primary site); and the other site on the alpha(1-3) arm of the core beta-mannose residue involving an alpha-mannose residue(s), which binds with lower affinity (secondary site). These two types of sites bind to ConA by different mechanisms. Certain bisected hybrid type glycopeptides were found to possess only the primary ConA binding sites, but not the secondary sites, and hence were able to bind but not precipitate the lectin. Other related glycopeptides have only the secondary type sites and thus exhibit low affinity and are unable to precipitate the protein. The results are related to the possible structure-function properties of cell-surface glycopeptides.

Precipitation of the D-galactose specific lectin from Erythrina indica by a triantennary complex type oligosaccharide

We have previously demonstrated that a high mannose type glycopeptide is bivalent for binding Concanavalin A (Con A) and can precipitate the lectin (Bhattacharyya L. and Brewer, C.F. (1986) Biochem. Biophys. Res. Commun. 137, 670-674). The present results show that a triantennary complex type oligosaccharide containing nonreducing terminal galactose residues can precipitate the D-galactose/N-acetyl-D-galactosamine specific lectin from Erythrina indica (EIL). The interactions of the oligosaccharide with EIL was investigated by quantitative precipitin analysis. The equivalence point of the precipitin curve indicated that the glycopeptide is trivalent for EIL binding. These results indicate that each arm of the oligosaccharide can independently bind separate lectin molecules leading to precipitation of the complex. These findings are discussed in terms of the possible biological structure-function properties of complex type oligosaccharides.

Precipitation of concanavalin A by a high mannose type glycopeptide

The interactions of a high mannose type glycopeptide with Concanavalin A has been investigated by quantitative precipitation analysis. The equivalence points of the precipitin curves indicate that the glycopeptide is bivalent for lectin binding. These results and others demonstrate that there are two lectin binding sites per molecule of the glycopeptide: one site on the alpha (1-6) arm of the core beta-mannose residue involving a trimannosyl moiety, and another site on the alpha (1-3) arm of the core beta-mannose residue involving an alpha (1-2) mannobiosyl group. The two sites are unequal in their affinities, and bind by different mechanisms. These results are related to the possible structure-function properties of high mannose type of glycopeptides on the surface of cells.

Specificity of concanavalin A binding to asparagine-linked glycopeptides. A nuclear magnetic relaxation dispersion study

We have investigated the binding of a series of high affinity asparagine-linked glycopeptides, including high mannose type and a bisected hybrid type, and several related synthetic oligosaccharides, to Ca2+- Mn2+-concanavalin A (ConA), using solvent proton nuclear relaxation dispersion (NMRD) measurements. We find that binding of the glycopeptides induces a common smaller decrease in the NMRD profile of ConA compared to that induced by monosaccharide binding. This effect is also observed with a synthetic analog of complex-type carbohydrates, hepta, which also shows enhanced affinity for the protein relative to monosaccharide binding. The high affinity of the glycopeptides and hepta, and their unique effects on the NMRD profile, are mimicked by binding of the trimannosyl oligosaccharide, 3,6-di-O-(alpha-D-mannopyranosyl)-D-mannose, which is present as a structural element in all of the glycopeptides and synthetic oligosaccharides. However, adding a so-called bisecting N-acetyl-D-glucosamine residue to the trimannosyl oligosaccharide greatly reduces its binding affinity and produces a decrease in the NMRD profile of the protein similar to that observed for monosaccharide binding. These results indicate that the trimannosyl oligosaccharide is a unique moiety recognized by the lectin for high affinity and extended site binding, and the presence of a bisecting N-acetyl-D-glucosamine residue in the trimannosyl oligosaccharide eliminates this type of interaction. The results also demonstrate that ConA primarily binds to the outer trimannosyl regions of high mannose and bisected hybrid-type glycopeptides compared to the central trimannosyl region of complex glycopeptides. Two mechanisms of enhanced affinity binding of saccharides and glycopeptides to ConA are discussed.

Circular dichroism studies of cobalt substituted lentil lectin

A recent method has been developed to effect metal ion substitution at the Mn2+ site in the lentil lectin (Bhattacharyya et al. (1984) Biochem. Biophys. Res. Commun. 124, 857-862). We report here the preparation of cobalt substituted lentil lectin, containing Co2+ at the S1 site and Ca2+ at the S2 site. The cobalt derivative possesses full saccharide binding activity and can be used for spectroscopic studies. The near UV and visible CD spectra of the derivative are shown, and its spectral properties are compared with various cobalt complexes of concanavalin A.

Hydration of cellobial by exo- and endo-type cellulases: evidence for catalytic flexibility of glycosylases

New insight has been obtained into the catalytic capabilities of cellulase. Essentially homogeneous preparations of exo- (or Avicelase-) type and endo- (or CMCase-) type cellulases from Irpex lacteus and Aspergillus niger, respectively, were shown to hydrate the enolic bond of cellobial to form 2-deoxycellobiose. The A. niger enzyme also synthesized a small amount of a 2-deoxycellobiosyl-transfer product from cellobial. By use of digests conducted in deuterated buffer and 1H NMR spectra for product analysis, both cellulases were found to protonate (deuterate) the double bond of cellobial from below the si face of the D-glucal moiety, i.e., from a direction opposite that assumed for protonation of the beta-D-glycosidic linkages of cellulose and cellodextrins. The exo enzyme, which hydrolyzes the latter substrates primarily to cellobiose, rapidly catalyzed cellobial hydration to produce the beta-anomer of beta-D-glucopyranosyl(1----4)-2-deoxy-D-glucose-2(e)-d. The A. niger cellulase produced the same 2-deoxycellobiose-d from cellobial, though too slowly for its configuration to be determined. However, evidence was obtained for the formation of a beta-2-deoxycellobiosyl-d-D-glucose-transfer product by the enzyme. Thus, it is likely that all of the observed reactions with cellobial represent trans additions at the double bond. In any case, the anomeric configuration of products is created de novo. Separate mechanisms are described for the reaction of cellobial hydration and for the stereochemically different reaction of cellulose hydrolysis catalyzed by the present enzymes, assuming an arrangement of their catalytic groups analogous to that found in lysozyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Catalytic flexibility of glycosylases. The hydration of maltal by beta-amylase to form 2-deoxymaltose

Crystalline, alpha-glucosidase-free sweet potato beta-amylase was found to catalyze hydration of the enolic bond of maltal (alpha-D-glucopyranosyl-(1----4)-2-deoxy-D-glucal) to form 2-deoxymaltose (alpha-D-glucopyranosyl-(1----4)-2-deoxy-D-glucose). The reaction at pH 5.0 showed Vmax 0.082 mumol/min/mg and km 94.5 mM. An exceptionally large solvent deuterium isotope effect, VH/VD = 8, was observed from pH(pD) 4.2 to 5.4; and at pH(pD) 5.0 the effect was found to be directly related to the mole fraction of 2H. The hydration product, isolated from a beta-amylase/maltal digest in acetate-d4/D2O buffer (pD 5.4) was identified through its 1H NMR spectrum as alpha-D-glucopyranosyl-(1----4)-2-deoxy-D-[2(a)-2H]glucose. beta-Amylase in 2H2O thus catalyzes deuteration of the double bond of maltal from a direction opposite that assumed for protonation of the glycosidic oxygen atoms of starch chains and maltosaccharides. This finding confirms the functional flexibility of the enzyme's catalytic groups first demonstrated in studies of the reactions catalyzed with alpha- and beta-maltosyl fluoride (Hehre, E. J., Brewer, C. F., and Genghof, D. S. (1979) J. Biol. Chem. 254, 5942-5950). A possible mechanism of the maltal hydration by beta-amylase involves protonation of substrate from above as the first and rate-limiting step, followed by formation of a transient carbonium ion-enzyme intermediate. Although other possible mechanisms cannot be ruled out, it is clear that this hydration reaction differs from reactions catalyzed with amylaceous substrates and with alpha- and beta-maltosyl fluoride. The ability of beta-amylase to catalyze different types of reactions with different substrates is discussed with respect to observations with other enzymes that, likewise, strongly support the view (Hehre et al.) that the catalytic groups of glycosylases in general may be functionally flexible beyond requirements of the principle of microscopic reversibility.

Catalytic versatility of trehalase: synthesis of alpha-D-glucopyranosyl alpha-D-xylopyranoside from beta-D-glucosyl fluoride and alpha-D-xylose

Trehalase was previously shown (see ref. 5) to hydrolyze alpha-D-glucosyl fluoride, forming beta-D-glucose, and to synthesize alpha, alpha-trehalose from beta-D-glucosyl fluoride plus alpha-D-glucose. Present observations further define the enzyme's separate cosubstrate requirements in utilizing these nonglycosidic substrates. alpha-D-Glucopyranose and alpha-D-xylopyranose were found to be uniquely effective in enabling Trichoderma reesei trehalase to catalyze reactions with beta-D-glucosyl fluoride. As little as 0.2mM added alpha-D-glucose (0.4mM alpha-D-xylose) substantially increased the rate of enzymically catalyzed release of fluoride from 25mM beta-D-glucosyl fluoride at 0 degrees. Digests of beta-D-glucosyl fluoride plus alpha-D-xylose yielded the alpha, alpha-trehalose analog, alpha-D-glucopyranosyl alpha-D-xylopyranoside, as a transient (i.e., subsequently hydrolyzed) transfer-product. The need for an aldopyranose acceptor having an axial 1-OH group when beta-D-glucosyl fluoride is the donor, and for water when alpha-D-glucosyl fluoride is the substrate, indicates that the catalytic groups of trehalose have the flexibility to catalyze different stereochemical reactions.

Preparation and properties of metal ion derivatives of the lentil and pea lectins

Lentil lectin (LcH) and pea lectin (PSA) belong to the class of D-glucose/D-mannose binding lectins and resemble concanavalin A (Con A) closely in physicochemical, structural, and biological properties. LcH and PSA, like Con A, are Ca2+-Mn2+ metalloproteins that require the metal ions for their saccharide binding and biological activities. Studies of the relationship between the metal ions binding and saccharide binding activity in LcH and PSA have been difficult due to the problem of metal ion replacement in these proteins. We now report a method of metal ion replacement in both lectins that allows substitution of the Mn2+ in the native proteins with a variety of transition metal ions, as well as substitution of the Ca2+ with Cd2+ in a particular complex. The following metal ion derivatives of both LcH and PSA have been prepared: Ca2+-Zn2+, Ca2+-Co2+, Ca2+-Ni2+, and Cd2+-Cd2+. All of these derivatives are as active as the native lectins, as demonstrated by precipitation with specific polysaccharides, saccharide inhibition of precipitation, and hemagglutination assays. The yields of these derivatives are good (generally greater than 70%), and the degree of metal ion incorporation is high (generally greater than 90%). The method of preparation is quite different from that for metal ion substitution in Con A, which proceeds via the apoprotein. In contrast, the apoproteins of LcH and PSA are unstable, aggregate above pH 4.0, and cannot be remetallized once formed.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton and deuteron nuclear magnetic relaxation dispersion studies of Ca2+-Mn2+-concanavalin A: evidence for two classes of exchanging water molecules

We have measured the magnetic field dependence of the nuclear magnetic relaxation rates (NMRD profiles) of solvent protons and deuterons in solutions of Ca2+-Mn2+-concanavalin A (Con A) with and without saccharide present. Data were obtained over the range -8 to 35 degrees C; the extension to the lowest temperature was made possible by the presence of 5 M salt. Since previous theoretical analyses, using accepted relaxation theories of 1H NMRD profiles alone, led to unsatisfactory conclusions, we have attempted to take advantage of the fact that the residence lifetime of a water ligand of the metal ions can influence the relaxation behavior of protons and deuterons differently. From a comparison of the present proton and deuteron results, we find that Ca2+-Mn2+-Con A has two classes of binding sites: one, associated with the inner coordiation sphere of the Mn2+ ions, having a resident lifetime for solvent water of approximately 10(-5) s that is reduced by the presence of saccharide and another having a lifetime of approximately 5 X 10(-9) s, located with the protons of the bound waters approximately 4.4 A from the Mn2+ ions (assuming two equivalent water molecules in this class), which is well beyond the coordination environment of the Mn2+ ions. The relaxation contribution of these more distant sites is unaffected by saccharide. The conclusions are corroborated by measurements of the temperature dependences of the proton NMRD profiles, which show quite clearly that the profiles are composite, containing two contributions with opposite dependences on temperature.(ABSTRACT TRUNCATED AT 250 WORDS)

Proton and deuteron nuclear magnetic relaxation dispersion studies of Ca2+-Mn2+-lentil lectin and Ca2+-Mn2+-pea lectin: evidence for a site of solvent exchange in common with concanavalin A

Measurements of the magnetic field dependence of the longitudinal magnetic relaxation rates (NMRD profiles) of solvent protons and deuterons led to the discovery of two classes of solvent binding sites in Ca2+-Mn2+-concanavalin A (CMPL) [Koenig, S. H., Brown, R. D., III, & Brewer, C. F. (1985) Biochemistry (second of three papers in this issue)]. In this paper, we compare proton and deuteron NMRD profiles of Ca2+-Mn2+-lentil lectin (CMLcH) and Ca2+-Mn2+-pea lectin (CMPSA) with those of CMPL. All three metalloproteins are D-mannose/D-glucose-specific lectins that have a high degree of structural similarity and require the metal ions for their biological activities. We have developed a method for the preparation of fully active metal ion derivatives of lentil lectin (LcH) and pea lectin (PSA), including the diamagnetic derivatives Ca2+-Zn2+-LcH and Ca2+-Zn2+-PSA [Bhattacharyya, L., Brewer, C. F., Brown, R. D., III, & Koenig, S. H.(1984) Biochem. Biophys. Res. Commun. 124, 857-862]. The behavior of these two lectins with regard to their NMRD profiles is essentially identical, for both the paramagnetic and diamagnetic forms. Together with CMPL, all three lectins have a common paramagnetic contribution with a negative temperature dependence of the rates, while CMPL contributes an additional component with a positive temperature dependence. The common contribution derives from the class of fast exchanging water molecules observed in the proton NMRD profile of CMPL (Koenig et al., 1985); their protons are calculated to be relatively remote from the Mn2+ ions (4.4 A for CMPL and 5.5 A for LcH and PSA).(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of the spectroscopic and saccharide binding properties of lentil and pea isolectins

The lentil isolectins, CMLcH A and CMLcH B, and pea isolectins, CMPSA A and CMPSA B, are compared in terms of their spectroscopic and saccharide binding properties. The paramagnetic contribution to the solvent proton magnetic relaxation dispersion profiles of solutions of the isolectins of each protein are found to be essentially identical. Electron paramagnetic resonance spectra suggest a high degree of octahedral symmetry at the Mn2+ site for both pairs of isolectins. The near-ultraviolet absorption spectra of CMLcH A and CMLcH B are identical, as are the spectra of CMPSA A and CMPSA B. Carbohydrate binding activities of the isolectins of each protein are compared using hemagglutination, precipitation, and precipitation-inhibition assays, and are found to be identical, although the activities of CMLcH and CMPSA differ somewhat. These results demonstrate that the spectroscopic and saccharide binding properties of the isolectins of CMLcH are essentially identical, as are those of the isolectins of CMPSA, and suggest that native mixtures of the isolectins may be treated as single proteins in further studies.

Magnetic field dependence of solvent proton relaxation induced by Gd3+ and Mn2+ complexes

The effect of solute paramagnetic ions on the longitudinal magnetic relaxation rate 1/T1 of solvent water protons depends on magnetic field strength and on the chemical environment of the ions. The variation of 1/T1 with field has been measured for solutions of Gd3+ and Mn2+ ions in three grossly different environments near physiological pH: the hydrated aquoion; chelated by EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepentaacetic acid); and bound to the protein concanavalin A. It is demonstrated that over the field range at which NMR imaging is currently being done, the chemical environment can alter 1/T1 of solvent protons by more than an order of magnitude. The relevance of these results to the potential utility of these ions as agents for enhancement of contrast in NMR images is discussed.

Preparation and characterization of Ca2+-Zn2+-derivatives of lentil and pea lectins and comparison with the native forms

Ca2+-Zn2+-derivatives of lentil and pea lectins were prepared for the first time by a unique method involving dialysis of the native Ca2+-Mn2+-lectins against large excesses of metal ions in pH 4.0 buffer. Each derivative contained about 1.5 g atoms of Ca2+ and about 1 g atom of Zn2+ per monomer. The derivatives were found to be identical to their respective native forms, both in molecular weight and carbohydrate binding activities. Solvent proton relaxation dispersion measurements were used to characterize both the Ca2+-Zn2+- and Ca2+-Mn2+-complexes of the lentil lectin.

van der Waals' induced 13C NMR shifts in crystalline amino acids and peptides

Recent reports in the literature of solid-state 13C nuclear magnetic resonance (NMR) spectra of crystalline L-alanine [Naito, A., Ganapathy, S., Akasaka, K., and McDonnell, C. A. (1981) J. Chem. Phys. 74, 3190-3197] and L-leucine [Frey, M. H. and Opella, S. J. (1980) J. Chem. Soc. Chem. Commun., 474-475], recorded with cross-polarization and magic-angle spinning (CP-MAS), show downfield resonance shifts of several parts per million in their side-chain methyl groups, relative to their resonance positions in aqueous solution. Similar findings are reported here for crystalline aliphatic amino acids and L-alanine peptides, including tetra(L-alanine), which show similar, specific downfield shifts in their side-chain methyl resonances. Coupled with X-ray crystallographic data of these compounds, and previous gas and solution-phase 13C NMR studies, the CP-MAS 13C NMR data indicate that these downfield shifts are a result of van der Waals' interactions. This group have reported similar van der Waals' induced shifts of the same magnitude for 13C resonances of the side-chain methyl groups of 13C-enriched tetra(L-alanine) upon binding to high-affinity Fab' fragments of heterogeneous sheep anti-[poly(L-alanine)] antibodies in aqueous solution [Geller, S., Wei, S. C., Shkuda, G. K., Marcus, D. M., and Brewer, C. F. (1980) Biochemistry 19, 3614-3623]. The above findings show that van der Waals' induced 13C NMR shifts of similar magnitudes can be detected in specific antibody-hapten complexes and the side chains of crystalline aliphatic amino acids and peptides. The results also indicate that water possesses relatively little attractive van der Waals' interactions with aliphatic molecules.

Metal ion binding and conformational transitions in concanavalin A: a structure-function study

The affinity of the lectin Concanavalin A (Con A) for saccharides, and its requirement for metal ions such as Mn2+ and Ca2+, have been known for about 50 years. However the relationship between metal ion binding and the saccharide binding activity of Con A has only recently been examined in detail. Brown et al. (Biochemistry 16, 3883 (1977)) showed that Con A exists as a mixture of two conformational states: a "locked" form and an "unlocked" form. The unlocked form of the protein weakly binds metal ions and saccharide, and is the predominate conformation of demetallized Con A (apo-Con A) at equilibrium. The locked form binds two metal ions per monomer with the resulting complex(es) possessing full saccharide binding activity. Brown and coworkers measured the kinetics of the transition of the unlocked form to the fully metallized locked conformation containing Mn2+ and Ca2+. They also demonstrated that Mn2+ alone could form a locked ternary complex with Con A, and that rapid removal of the ions resulted in a metastable form of apo-Con A in the locked conformation which slowly (hours at 25 degrees C) reverted back to (predominantly) the unlocked conformation. The ability to form either conformation in the absence or presence of metal ions has thus allowed us to explore the relationship between metal ion binding and conformational transitions in Con A as determinants of the saccharide binding activity of the lectin. Based on the kinetics of the transition of unlocked apo-Con A to fully metallized locked Con A, and X-ray crystallographic data, it appears that the transition between the two conformations of Con A involves a cis-trans isomerization of an Ala-Asp peptide bond in the backbone of the protein, near one of the two metal ion binding sites. The relatively large activation energy for the transition (approximately 22 kcal M-1) results in relatively slow interconversions between the conformations (from minutes to days), whereas the equilibria with metal ions and saccharide are rapid. Thus, many metastable complexes can be formed and a variety of transition pathways between the two conformations studied. We have identified and characterized binary, ternary, and quaternary complexes of both conformations of Con A containing Mn2+ and saccharide, and have determined both metal ion and saccharide dissociation constants for all of them, as well as equilibrium and kinetic values for the conformational transitions between them.(ABSTRACT TRUNCATED AT 400 WORDS)

Synthesis and 19F spectra of tetra-L-alanine analogs containing selectively incorporated 3-fluoro-L-alanine residues

The synthesis of analogs of tetra-L-alanine containing 3-fluoro-L-alanine selectively incorporated at each position is described. The standard procedures in the literature used to couple L-alanine peptides together were often found to lead to undesired products, or elimination reactions when corresponding 3-fluoro-L-alanine peptide analogs were used. Several modified procedures have thus been developed for the synthesis of fluorine-substituted analogs. In addition, the pH-dependence of 19F n.m.r. spectra of 3-fluoro-L-alanine and the tetrapeptide analogs is presented.

13C n.m.r. study of L-alanine peptides

The di-, tri-, and tetrapeptides of L-alanine have been studied in aqueous solution by 13C n.m.r. spectroscopy at 25 and 50 MHz. By using selectively 13C enriched analogs containing either 90% 13C methyl or carbonyl carbons and measurements as a function of pH, assignment of the chemical shifts of the peptides has been made. T1 and NOE measurements of the peptides in their cationic, anionic, and zwitterionic states have been recorded as a function of concentration. The results show considerable segmental motion along the backbone carbons of the peptides, with only small changes occurring in the dynamic motions of the peptides as their charge states are altered. The lack of concentration dependence of the chemical shift and T1 values, as well as the similarity of T1 values for individual peptides in the three charge states, indicate that the peptides do not self-associate in aqueous solution.

Stoichiometry of manganese and calcium ion binding to concanavalin A

Using measurements of solvent nuclear (proton) magnetic relaxation dispersion (NMRD), we have previously shown that concanavalin A (Con A) can exist in two conformational forms and that, in the absence of Ca2+, Mn2+ can bind to both the S1 and S2 sites of each monomer of Con A of at least one conformer [Brown, R.D., III, Brewer, C.F., & Koenig, S.H. (1977) Biochemistry 16, 3883-3896]. Recently other investigators have claimed that the stoichiometry of Mn2+ binding to Con A is only 1:1 for this conformational state, both in the absence and presence of saccharide; the same was claimed for Ca2+ under similar conditions. We now present titration and equilibrium dialysis experiments, both in the absence and presence of saccharide, using NMRD and atomic absorption spectroscopy, to investigate the stoichiometry of Mn2+ and Ca2+ binding to Con A. We have extended the NMRD method to include the determination of the total concentration of Mn2+ in samples of Con A. This, coupled with our previous use of NMRD to measure the concentration of free Mn2+ in protein solutions as well as the distribution of bound Mn2+ among different sites, allows us to measure the stoichiometry of binding with precision. We reconfirm that, at equilibrium in the presence of excess Mn2+, the binding stoichiometry of Mn2+ to Con A is 2:1, both in the absence and presence of saccharide. Addition of Ca2+ to a solution of Mn2+-Con A results in stoichiometric displacement of Mn2+ from the S2 site under the conditions investigated. Under nonequilibrium conditions, Mn2+ forms a metastable binary complex with the protein that persists for days at 5 degrees C. We also report, for the first time, values for all of the dissociation constants of binary and ternary complexes of Mn2+ with both conformations of Con A in solution. Atomic absorption measurements also indicate that Ca2+, in the absence of Mn2+, binds to both S1 and S2 sites in the absence and presence of saccharides.

Kinetics of conformational transitions of demetalized Concanavalin A

Demetalized Concanavalin A exists in two conformational states, known as locked (PL) and unlocked (P) [Brown et al., Biochemistry 16, 3883 (1977)]. The equilibrium ratio [PL]/[P] is 0.14 +/- 0.01 at 25 degrees C, pH 6.4 [Brown et al., Biochemistry 21, 465 (1982)]. We now report values of the rate constants for the P in equilibrium; k1 = (33 +/- 4 h)-1 and k-1 = (4.6 +/- 0.6 h)-1 for the P leads to PL and PL leads to P transitions, respectively, at 25 degrees C, pH 6.4. The experiments utilize the fact that saccharide binds to PL [Koenig et al., Biochemistry 17, 4251 (1978)], producing a time-dependent increase in the total concentration of locked forms at equilibrium, and use a new technique for measuring this concentration.

Factors determining steric course of enzymic glycosylation reactions: glycosyl transfer products formed from 2,6-anhydro-1-deoxy-D-gluco-hept-1-enitol by alpha-glucosidases and an inverting exo-alpha-glucanase

Glycosyl transfer products were formed from 2,6-anhydro-1-deoxy-D-gluco-hept-1-enitol (heptenitol) by purified alpha-glucosidases from Candida tropicalis and rice and by an inverting exo-alpha-glucanase (glucodextranase) from Arthrobacter globiformis. The products were structurally defined through 1H and 13C NMR (nuclear magnetic resonance) spectra of their crystalline per-O-acetates in comparison with those of authentic methyl 1-deoxy-alpha- and methyl 1-deoxy-beta-D-gluco-heptuloside. 1-Deoxy-alpha-D-gluco-heptulosyl-(2 leads to 7)-heptenitol and 1-deoxy-alpha-D-gluco-heptulosyl-(2 leads to 7)-D-gluco-heptulose were produced by both the Candida alpha-glucosidase and the glucodextranase; 1-deoxy-alpha-D-gluco-heptulosyl-(2 leads to 5)- and 1-deoxy-alpha-D-gluco-heptulosyl-(2 leads to 7)-D-gluco-heptuloses by the rice alpha-glucosidase. These results, together with our earlier findings of sterospecific hydration of heptenitol catalyzed by the same enzymes [Hehre, E. J., Brewer, C. F., Uchiyama, T., Schlesselmann, P., & Lehmann, J. (1980) Biochemistry 19, 3557-3564], show the inadequacy of the long-accepted notion that carbohydrase-catalyzed reactions always lead to retention (or always lead to inversion) of substrate configuration. In particular, the finding that glucodextranase forms transfer products of alpha configuration and a hydration product of beta configuration from the same substrate provides a clear example of the functioning of acceptors rather than donor substrates in selecting the steric course of reactions catalyzed by a glycosylase. The circumstances under which acceptor cosubstrates might be expected to show this significant effect are discussed. The opportunity presumably would exist whenever carbonium ion mediated reactions are catalyzed by glycosylases that provide oppositely oriented approaches of different acceptors to the catalytic center.

Synthesis of the diastereoisomers of 1,2-dipalmitoyl-sn-glycero-3-thiophosphorylethanolamine and their stereospecific hydrolysis by phospholipases A2 and C

A convenient three-step synthesis of the phosphorothioate analogue of phosphatidylethanolamine is described. The reaction pathway involves the conversion of a 1,2-diacyl-sn-glycerol to its corresponding thiophosphoric acid dichloride by using PSCl3 in the presence of a tertiary base. Treatment of the dichloride with ethanolamine results in the formation of a cyclic thiophosphoramidate which, upon acidification, undergoes P--N cleavage, giving rise to 1,2-diacyl-sn-glycero-3-thiophosphorylethanolamine. 31P NMR reveals that both diastereoisomers are present in equivalent amounts. It is not possible, however, to separate the two isomers by high-pressure liquid chromatography. 31P NMR amd high-pressure liquid chromatography are used to show that phospholipases A2 and C exhibit absolute and opposite stereoselectivity in the hydrolysis of the pair of diastereoisomers.

Trehalase: stereocomplementary hydrolytic and glucosyl transfer reactions with alpha- and beta-D-glucosyl fluoride

A new understanding has been obtained of the catalytic capabilities of trehalase, an enzyme heretofore held to be strictly specific for hydrolyzing alpha, alpha-trehalose and devoid of transglycosylative ability. Highly purified rabbit renal cortical trehalase and a partly purified Candida tropicalis yeast trehalase were found to utilize both alpha- and beta-D-glucosyl fluoride as substrates. In each case, the reactions were competitively inhibited by alpha, alpha-trehalose. Both enzymes catalyzed rapid hydrolysis of alpha-D-glucosyl fluoride to form beta-D-glucose (also, of alpha, alpha-trehalose to form equimolar alpha- and beta-D-glucose). In addition, digests of beta-D-glucosyl fluoride plus alpha-D-[14C]-glucopyranose with either trehalase (but not controls of enzyme with alpha-D-[14C]glucopyranose alone) yielded small amounts of radioactive trehalose (alpha-D-glucopyranosyl alpha-D-[14C]glucopyranoside) which does not accumulate since it is rapidly hydrolyzed. Trehalase thus catalyzes two stereocomplementary types of glycosylation reactions: (I) alpha-D-glucosyl fluoride (or alpha, alpha-trehalose) + H2O leads to beta-D-glucose + HF (or alpha-D-glucose); (II) beta-D-glucosyl fluoride + alpha-D-glucopyranose leads to alpha, alpha-trehalose + HF. Such behavior shows that the catalytic groups of trehalase, as recently found for other glycosylases, are functionally flexible. The results illustrate the inadequacy of conventional views of carbohydrase specificity and the rigor, as a basic guiding principle, of the concept that glycoside hydrolases and glycosyltransferases form a class of glycosylases effecting glycosyl/proton interchange.

Conformational equilibrium of demetalized concanavalin A

Concanavalin A (Con A) is known to exist in two conformations [Brown, R. D., III, Brewer, C. F., & Koenig, S. H. (1977) Biochemistry 16, 3883-3896] that differ in their metal ion and saccharide binding properties. The conformation that binds metal ions tightly, and which is associated with saccharide binding, has been designated as "locked" and that which binds metal ions only weakly as "unlocked". In the presence of excess metal ions, such as Mn2+ and Ca2+, essentially 100% of the protein is in the locked conformation. The scheme proposed to explain these effects [Koenig, S. H., Brewer, C. F., & Brown, R. D., III (1978) Biochemistry 17, 4251-4260] predicts an equilibrium between these conformations for the apoprotein. By monitoring the solvent proton relaxation dispersion as equimolar concentrations of Mn2+ and Ca2+ are titrated, at 5 degrees C, into an apo-Con A solution that had been equilibrated at 25 degrees C, we find that 12.5% of the apoprotein is in the locked conformation, corresponding to an energy separation of 1.2 kcal mol-1. We also show that these conformations can be separated by column chromatography at 5 degrees C and that the 100% unlocked form prepared in this way returns to the expected equilibrium mixture when kept at 25 degrees C.

Solution structure of 5-keto-D-fructose: relevance to the specificity of hexose kinases

5-Keto-D-fructose (5KF) is isolated from cultures of Gluconobacter cerinus growing on D-fructose as the sole carbon source. 5KF is a substrate for hexokinase, fructokinase, and several polyol dehydrogenases. 1H and 13C nuclear magnetic resonance studies show that 5KF exists in different forms in anhydrous dimethyl-d6 sulfoxide and D2O. In dimethyl-d6 sulfoxide, 5KF exists as a spirane dimer with linked furanose and pyranose rings, similar to the structure reported for crystalline 5KF [Hassen, L., Hordvik, A., & Hove, R. (1976) J. Chem. Soc., Chem. Commun., 572-. In D2O, 5KF exists predominantly (greater than 95%) in a beta-pyranose form with the 5-keto group hydrated to form a gem-diol. 13C--1H coupling patterns, 13C relaxation measurements, and 13C deuterium-induced differential isotope shifts confirm this structure of 5KF. The phosphorylation of 5KF by fructokinase can be accounted for by an approximately 2% proportion of the beta-furanose form in solution at 25 degrees C. Both the beta-pyranose and beta-furanose forms of 5KF are proposed to be substrates for yeast hexokinase.