Porphyrin binding to jacalin is facilitated by the inherent plasticity of the carbohydrate-binding site: novel mode of lectin–ligand interaction

The crystal structure of the complex of meso-tetrasulfonatophenylporphyrin (H(2)TPPS) with jack fruit (Artocarpus integriflora) agglutinin (jacalin) has been determined at 1.8 A resolution. A porphyrin pair is sandwiched between two symmetry-related jacalin monomers in the crystal, leading to a cross-linking network of protein molecules. Apart from the stacking interactions, H(2)TPPS also forms hydrogen bonds, some involving water bridges, with jacalin at the carbohydrate-binding site. The residues that are involved in rendering galactopyranoside specificity to jacalin undergo conformational adjustments in order to accommodate the H(2)TPPS molecule. The water molecules at the carbohydrate-binding site of jacalin cement the jacalin-porphyrin interactions, optimizing their complementarity. Interactions of porphyrin with jacalin are relatively weak compared with those observed between galactopyranoside and jacalin, perhaps because the former largely involves water-mediated hydrogen bonds. While H(2)TPPS binds to jacalin at the carbohydrate-binding site as in the case of ConA, its mode of interaction with jacalin is very different. H(2)TPPS does not enter the carbohydrate-binding cavity of jacalin. Instead, it sits over the binding site. While the porphyrin binding is mediated by replicating the hydrogen-bonding network of mannopyranoside through the sulfonate atoms in the case of ConA, the plasticity associated with the carbohydrate-binding site accommodates the pluripotent porphyrin molecule in the case of jacalin through an entirely different set of interactions.

Interaction of membrane-spanning proteins with peripheral and lipid-anchored membrane proteins: perspectives from protein-lipid interactions (Review)

Studies of lipid-protein interactions in double-reconstituted systems involving both integral and peripheral or lipid-anchored proteins are reviewed. Membranes of dimyristoyl phosphatidylglycerol containing either myelin proteolipid protein or cytochrome c oxidase were studied. The partner peripheral proteins bound to these membranes were myelin basic protein or cytochrome c, respectively. In addition, the interactions between the myelin proteolipid protein and avidin that was membrane-anchored by binding to N-biotinyl phosphatidylethanolamine were studied in dimyristoyl phosphatidylcholine membranes. Steric exclusion plays a significant role when sizes of the peripheral protein and transmembrane domain of the integral protein are comparable. Even so, the effects on avidin-linked lipids are different from those induced by myelin basic protein on freely diffusible lipids, both interacting with the myelin proteolipid protein. Both the former and the cytochrome c/cytochrome oxidase couple evidence a propagation of lipid perturbation out from the intramembrane protein interface that could be a basis for formation of microdomains.

Specific surface association of avidin with N-biotinylphosphatidylethanolamine membrane assemblies: effect on lipid phase behavior and acyl-chain dynamics

The interaction of avidin with aqueous dispersions of N-biotinylphosphatidylethanolamines, of acyl chain lengths C(14:0), C(16:0), and C(18:0), was studied by using spin-label electron spin resonance (ESR) spectroscopy, (31)P nuclear magnetic resonance ((31)P NMR) spectroscopy, differential scanning calorimetry, and chemical binding assays. In neutral buffer containing 1 M NaCl, binding of avidin is due to specific interaction with the biotinyl lipid headgroup because avidin presaturated with biotin does not bind. Saturation binding of the protein corresponds to a ratio of 50 lipid molecules per tetrameric avidin. Phospholipid probes spin-labeled at various positions between C-4 and C-14 in the sn-2 chain were used to characterize the effects of avidin binding on the lipid chain dynamics. In the fluid phase, protein binding results in a decrease of chain mobility at all positions of labeling while the flexibility gradient characteristic of a liquid-crystalline lipid phase is maintained. There is no evidence from the spin-label ESR spectra for penetration of the protein into the hydrophobic interior of the membrane. At temperatures corresponding to the gel phase, the lipid chain mobility increases on binding protein. The near constancy in mobility found with chain position, however, suggests that in the gel phase the lipid chains remain interdigitated upon binding avidin. Binding of increasing amounts of avidin results in a gradual decrease of the lipid chain-melting transition enthalpy with only small change in the transition temperature. At saturation binding, the calorimetric enthalpy is reduced to zero. (31)P NMR spectroscopy indicates that protein binding increases the surface curvature of dispersions of all three biotin lipids. The C(14:0) biotin lipid yields isotropic (31)P NMR spectra in the presence of avidin at all temperatures between 10 and 70 degrees C, in contrast to dispersions of the lipid alone, which give lamellar spectra at low temperature that become isotropic at the chain-melting temperature. In the presence of avidin, the C(16:0) and C(18:0) biotin lipids yield primarily lamellar (31)P NMR spectra at low temperature with a small isotropic component; the intensity of the isotropic component increases with temperature, and the spectra narrow and become totally isotropic at high temperature, in contrast to dispersions of the lipids alone, which give lamellar spectra in the fluid phase. The binding of avidin therefore reduces the cooperativity of the biotin lipid packing, regulates the mobility of the lipid chains, and enhances the surface curvature of the lipid aggregates. These effects may be important for both lateral and transbilayer communication in the membrane.

Thermodynamic and kinetic analysis of porphyrin binding to Trichosanthes cucumerina seed lectin

The interaction of several metallo-porphyrins with the galactose-specific lectin from Trichosanthes cucumeirna (TCSL) has been investigated. Difference absorption spectroscopy revealed that significant changes occur in the Soret band region of the porphyrins upon binding to TCSL and these changes have been monitored to obtain association constants (Ka) and stoichiometry of binding (n). The dimeric lectin binds two porphyrin molecules and the presence of the specific saccharide lactose did not affect porphyrin binding significantly, indicating that the sugar and the porphyrin bind at different sites. The Ka values obtained for the binding of different porphyrins with TCSL at 25 degrees C were in the range of 2 x 10(3)-5 x 10(5) m(-1). Association constants for meso-tetra(4-sulphonatophenyl)porphyrinato copper(II) (CuTPPS), a porphyrin bearing four negative charges and meso-tetra(4-methylpyridinium)porphyrinato copper(II) (CuTMPyP), a porphyrin with four positive charges, were determined at several temperatures; from the temperature dependence of the association constants, the thermodynamic parameters change in enthalpy (DeltaH degrees ) and change in entropy (DeltaS degrees ) associated with the binding process were estimated. The thermodynamic data indicate that porphyrin binding to TCSL is driven largely by a favourable entropic contribution; the enthalpic contribution is very small, suggesting that the binding process is governed primarily by hydrophobic forces. Stopped-flow spectroscopic measurements show that binding of CuTMPyP to TCSL takes place by a single-step process and at 20 degrees C, the association and dissociation rate constants were 1.89 x 10(4) m(-1).s(-1) and 0.29 s(-1), respectively.

Functional equality in the absence of structural similarity: an added dimension to molecular mimicry

The crystal structure of meso-tetrasulfonatophenylporphyrin complexed with concanavalin A (ConA) was determined at 1.9 A resolution. Comparison of this structure with that of ConA bound to methyl alpha-d-mannopyranoside provided direct structural evidence of molecular mimicry in the context of ligand receptor binding. The sulfonatophenyl group of meso-tetrasulfonatophenylporphyrin occupies the same binding site on ConA as that of methyl alpha-d-mannopyranoside, a natural ligand. A pair of stacked porphyrin molecules stabilizes the crystal structure by end-to-end cross-linking with ConA resulting in a network similar to that observed upon agglutination of cells by lectins. The porphyrin binds to ConA predominantly through hydrogen bonds and water-mediated interactions. The sandwiched water molecules in the complex play a cementing role, facilitating favorable binding of porphyrin. Seven of the eight hydrogen bonds observed between methyl alpha-d-mannopyranoside and ConA are mimicked by the sulfonatophenyl group of porphyrin after incorporating two water molecules. Thus, the similarity in chemical interactions was manifested in terms of functional mimicry despite the obvious structural dissimilarity between the sugar and the porphyrin.

Membrane insertion and lipid-protein interactions of bovine seminal plasma protein PDC-109 investigated by spin-label electron spin resonance spectroscopy

The interaction of the major acidic bovine seminal plasma protein, PDC-109, with dimyristoylphosphatidylcholine (DMPC) membranes has been investigated by spin-label electron spin resonance spectroscopy. Studies employing phosphatidylcholine spin labels, bearing the spin labels at different positions along the sn-2 acyl chain indicate that the protein penetrates into the hydrophobic interior of the membrane and interacts with the lipid acyl chains up to the 14th C atom. Binding of PDC-109 at high protein/lipid ratios (PDC-109:DMPC = 1:2, w/w) results in a considerable decrease in the chain segmental mobility of the lipid as seen by spin-label electron spin resonance spectroscopy. A further interesting new observation is that, at high concentrations, PDC-109 is capable of (partially) solubilizing DMPC bilayers. The selectivity of PDC-109 in its interaction with membrane lipids was investigated by using different spin-labeled phospholipid and steroid probes in the DMPC host membrane. These studies indicate that the protein exhibits highest selectivity for the choline phospholipids phosphatidylcholine and sphingomyelin under physiological conditions of pH and ionic strength. The selectivity for different lipids is in the following order: phosphatidylcholine approximately sphingomyelin > or = phosphatidic acid (pH 6.0) > phosphatidylglycerol approximately phosphatidylserine approximately and rostanol > phosphatidylethanolamine > or = N-acyl phosphatidylethanolamine >> cholestane. Thus, the lipids bearing the phosphocholine moiety in the headgroup are clearly the lipids most strongly recognized by PDC-109. However, these studies demonstrate that this protein also recognizes other lipids such as phosphatidylglycerol and the sterol androstanol, albeit with somewhat reduced affinity.

Spin-label electron paramagnetic resonance studies on the interaction of avidin with dimyristoyl-phosphatidylglycerol membranes

The interaction of avidin--a basic protein from hen egg-white--with dimyristoyl-phosphatidylglycerol membranes was investigated by spin-label electron paramagnetic resonance spectroscopy. Phosphatidylcholines, bearing the nitroxide spin label at different positions along the sn-2 acyl chain of the lipid were used to investigate the effect of protein binding on the lipid chain-melting phase transition and acyl chain dynamics. Binding of the protein at saturating levels results in abolition of the chain-melting phase transition of the lipid and accompanying perturbation of the lipid acyl chain mobility. In the fluid phase region, the outer hyperfine splitting increases for all phosphatidylcholine spin-label positional isomers, indicating that the chain mobility is decreased by binding avidin. However, there was no evidence for direct interaction of the protein with the lipid acyl chains, clearly indicating that the protein does not penetrate the hydrophobic interior of the membrane. Selectivity experiments with different spin-labelled lipid probes indicate that avidin exhibits a preference for negatively charged lipid species, although all spin-labelled lipid species indirectly sense the protein binding. The interaction with negatively charged lipids is relevant to the use of avidin in applications such as the ultrastructural localization of biotinylated lipids in histochemical studies.

Crystallization and preliminary X-ray studies of snake gourd lectin: homology with type II ribosome-inactivating proteins

The lectin from the seeds of snake gourd (Trichosanthes anguina) has been crystallized in two forms using the hanging-drop method. Both the forms are hexagonal, with the asymmetric unit containing one subunit consisting of two polypeptide chains linked through disulfide bridges. Intensity data from one of the forms were collected at room temperature as well as at low temperature to 3 A resolution. Molecular-replacement studies indicate that the lectin is homologous to type II ribosome-inactivating proteins. Partial refinement confirms this conclusion.

Interaction of N-myristoyldimyristoylphosphatidylethanolamine with dimyristoylphosphatidylcholine investigated by differential scanning calorimetry: binary phase diagram

The temperature-composition phase diagram was derived for hydrated, binary mixtures of N-myristoyldimyristoylphosphatidylethanolamine (N-14 DMPE) and dimyristoylphosphatidylcholine by high sensitivity differential scanning calorimetry. Gel phase immiscibility was detected in mixtures containing up to 20 mol% N-14 DMPE and there was no evidence for compound formation between the two components. In the fluid phase nearly complete miscibility is indicated by the calorimetric data. These results are relevant to understanding the role of N-acylphosphatidylethanolamines in the stress combating responses of organisms and in their application to developing liposome-based drug delivery systems.

Thermodynamic analysis of saccharide binding to snake gourd (Trichosanthes anguina) seed lectin. Fluorescence and absorption spectroscopic studies

The interaction of different saccharides with the snake gourd (Trichosanthes anguina) seed lectin (SGSL) was investigated by fluorescence spectroscopy. Binding of 4-methylumbelliferyl-beta-D-galactopyranoside (MeUmb beta Gal) to SGSL resulted in a significant increase in the fluorescence emission intensity of the sugar at 376 nm, and this change was used to estimate the association constants for the binding interaction. Interestingly, the increase in emission intensity changed with a change in temperature, increasing from 19.2% at 20 degrees C to 80.2% at 40 degrees C. At 20 degrees C the association constant, K(a), for the MeUmb beta Gal-SGSL interaction was found by fluorescence titration to be 5.8 x 10(4) M(-1). From the temperature dependence of the association constants, the changes in enthalpy (Delta H) and entropy (Delta S) associated with binding of MeUmb beta Gal to SGSL were estimated to be -80.85 kJ.mol(-1) and -184.0 J.mol(-1).K(-1), respectively. Binding of unlabeled sugars was investigated by monitoring the decrease in fluorescence intensity when they were added to a mixture of SGSL and MeUmb beta Gal. The Ka values for different sugars were determined at several temperatures, and Delta H and Delta S were determined from the van't Hoff plots. Enthalpy-entropy compensation was noticed in all cases. The results indicate that saccharide binding to SGSL is enthalpy-driven and the negative contribution from entropy is, in general, quite high.

Spin-label electron spin resonance studies on the mode of anchoring and vertical location of the N-acyl chain in N-acylphosphatidylethanolamines

Electron spin resonance (ESR) studies have been performed on N-myristoyl dimyristoylphosphatidylethanolamine (N-14-DMPE) membranes using both phosphatidylcholines spin-labeled at different positions in the sn-2 acyl chain and N-acyl phosphatidylethanolamines spin-labeled in the N-acyl chain to characterize the location and mobility of the N-acyl chain in the lipid membranes. Comparison of the positional dependences of the spectral data for the two series of spin-labeled lipids suggests that the N-acyl chain is positioned at approximately the same level as the sn-2 chain of the phosphatidylcholine spin-label. Further, similar conclusions are reached when the ESR spectra of the N-acyl PE spin-labels in dimyristoylphosphatidylcholine (DMPC) or dimyristoylphosphatidylethanolamine (DMPE) host matrixes are compared with those of phosphatidylcholine spin-labels in these two lipids. Finally, the chain ordering effect of cholesterol has also been found to be similar for the N-acyl PE spin-label and PC spin-labels, when the host matrix is either DMPC and cholesterol or N-14-DMPE and cholesterol at a 6:4 mole ratio. In both cases, the gel-to-liquid crystalline phase transition is completely abolished but cholesterol perturbs the gel-phase mobility of N-14-DMPE more readily than that of DMPC. These results demonstrate that the long N-acyl chains are anchored firmly in the hydrophobic interior of the membrane, in an orientation that is parallel to that of the O-acyl chains, and are located at nearly the same vertical position as that of the sn-2 acyl chains in the lipid bilayer. There is a high degree of dynamic compatibility between the N-acyl chains and the O-acyl chains of the lipid bilayer core, although bilayers of N-acyl phosphatidylethanolamines possess a more hydrophobic interior than phosphatidylcholine bilayers. These results provide a structural basis for rationalizing the biological properties of NAPEs.

Binding of porphyrins by the tumor-specific lectin, jacalin [Jack fruit (Artocarpus integrifolia) agglutinin]

Jacalin (Artocarpus integrifolia agglutinin) specifically recognizes the tumor-associated T-antigenic disaccharide structure, Gal beta13GalNAc. Porphyrins and their derivatives are currently used as photosensitizers in photodynamic therapy to treat malignant tumors. In this study, the interaction of several free base porphyrins and their metal derivatives with jacalin is investigated by absorption and fluorescence spectroscopy. Each lectin subunit was found to bind one porphyrin molecule and the association constants were estimated to be in the range of 2.4 x 10(3) M(-1) to 1.3 x 10(5) M(-1) at room temperature for the interaction of different porphyrins with jacalin. These values are in the same range as those obtained for the interaction of monosaccharides to jacalin. Both free lectin and lectin saturated with the specific saccharide were found to bind different porphyrins with comparable binding strength indicating that porphyrin binding takes place at a site different from the sugar binding site. Further, both anionic and cationic porphyrins were found to interact with the lectin with comparable affinity, clearly indicating that the charge on the porphyrin does not play any role in the binding process and that most likely the interaction is mediated by hydrophobic forces. These results suggest that jacalin and other lectins may potentially be useful for targeted delivery of porphyrins to tumor tissues in photodynamic therapy.

Fluorescence and absorption spectroscopic studies on the interaction of porphyrins with snake gourd (Trichosanthes anguina) seed lectin

The interaction of several free-base porphyrins and their corresponding copper(II) and zinc(II) derivatives with the galactose-specific lectin from snake gourd (Trichosanthes anguina) seeds has been investigated by absorption and fluorescence spectroscopic techniques. The lectin dimer contains two apparently equivalent binding sites for the porphyrins. Association constants obtained for the interaction of various porphyrins with the lectin are in the range 1.7 x 10(4)-6.2 x 10(5) M(-1), with the metalloporphyrins being seen to have higher affinity for the lectin compared with the free-base analogues. Both positively charged and negatively charged porphyrins bind to snake gourd seed lectin (SGSL) with comparable affinities, suggesting that binding occurs primarily via hydrophobic interactions. Further, binding of porphyrins is found to be largely unaffected by the presence of the sugar ligand, lactose, indicating that the binding sites for the carbohydrate and porphyrin are different. This study thus suggests that the lectin may serve as a receptor for some endogenous non-carbohydrate, hydrophobic ligand in vivo, in addition to the saccharide ligands. It also opens up the possibility of employing the T. anguina lectin in applications such as photodynamic therapy, which involve the use of porphyrins.

Derivatised lipids in membranes. Physico-chemical aspects of N-biotinyl phosphatidylethanolamines, N-acyl phosphatidylethanolamines and N-acyl ethanolamines

The physical properties of N-biotinyl phosphatidylethanolamines, N-acyl phosphatidylethanolamines and of N-acyl ethanolamines, in aqueous dispersions, are reviewed. Emphasis is placed on the calorimetric (i.e. chain melting) properties, the thermotropic phase behaviour, certain aspects of the structure and dynamics, and the miscibility with other membrane lipids. In the case of N-biotinyl phosphatidylethanolamines, the specific binding of avidin, and in the case of N-acyl ethanolamines, the function of the third chain, is also considered. All of these properties are relevant to the role of these rather unusual lipids in membranes.

Interactions between lipid-anchored and transmembrane proteins. Spin-label ESR studies on avidin-biotinyl phosphatidylethanolamine in membrane recombinants with myelin proteolipid proteins

Interactions between lipid-anchored and transmembrane proteins are relevant to the intracellular membrane sorting of glycosyl phosphatidylinositol-linked proteins. We have studied the interaction of a spin-labeled biotinyl diacyl phospholipid, with and without specifically bound avidin, with the myelin proteolipid protein (or the DM-20 isoform) reconstituted in dimyristoylphosphatidylcholine. Tetrameric avidin bound to the N-biotinyl lipid headgroup is a surface-anchored protein, and the myelin proteolipid is an integral protein containing four transmembrane helices. The electron spin resonance (ESR) spectrum of N-biotinyl phosphatidylethanolamine spin-labeled at the C-14 position of the sn-2 chain consists of two components in fluid-phase membranes of dimyristoylphosphatidylcholine containing the proteolipid. In the absence of avidin, this is characteristic of lipid-protein interactions with integral transmembrane proteins. The more motionally restricted component represents the lipid population in direct contact with the intramembranous surface of the integral protein, and the more mobile component corresponds to the bulk fluid lipid environment of the bilayer. In the presence of avidin, the biotin-lipid chains have reduced mobility because of the binding to avidin, even in the absence of the proteolipid [Swamy, M. J., and Marsh, D. (1997) Biochemistry 36, 7403-7407]. In the presence of the proteolipid, the major fraction of the avidin-anchored chains is further restricted in its mobility by interaction with the transmembrane protein. At a biotin-lipid concentration of 1 mol %, approximately 80% of the avidin-linked chains are restricted in membranes with a phosphatidylcholine:proteolipid molar ratio of 37:1. This relatively high stoichiometry of interaction can be explained when allowance is made for the closest interaction distance between the lipid-anchored avidin tetramer and the transmembrane proteolipid hexamer, without any specific interaction between the two types of membrane-associated proteins. The interaction is essentially one of steric exclusion, but the lipid chains are rendered more sensitive to interaction with the integral protein by being linked to avidin, even though they are removed from the immediate intramembrane protein-lipid interface. This could have implications for the tendency of lipid-anchored chains to associate with membrane domains with reduced lipid mobility.

Molecular packing and intermolecular interactions in N-acylethanolamines: crystal structure of N-myristoylethanolamine

N-Acylethanolamines elicited much interest in recent years owing to their occurrence in biological membranes under conditions of stress as well as under normal conditions. The molecular conformation, packing properties and intermolecular interactions of N-myristoylethanolamine (NMEA) have been determined by single crystal X-ray diffraction analysis. The lipid crystallized in the space group P21/a with unit cell dimensions: a=9.001, b=4.8761, c=39. 080. There are four symmetry-related molecules in the monoclinic unit cell. The molecules are organized in a tail-to-tail fashion, similar to the arrangement in a bilayer membrane. The hydrophobic acyl chain of the NMEA molecule is tilted with respect to the bilayer normal by an angle of 37 degrees. Each hydroxy group forms two hydrogen bonds, one as a donor and the other as an acceptor, with the hydroxy groups of molecules in the opposing leaflet. These O-H...O hydrogen bonds form an extended, zig-zag type network along the b-axis. In addition, the N-H and C=O groups of adjacent molecules are involved in N-H...O hydrogen bonds, which also connect adjacent molecules along the b-axis.

Fluorescence quenching and time-resolved fluorescence studies on Momordica charantia (bitter gourd) seed lectin

Chemical modification studies implicated tryptophan (Trp) residues in the sugar binding activity of Momordica charantia lectin (MCL) [Mazumdar, T., Gaur, N. & Surolia, A. (1981) Eur. J. Biochem. 113, 463-470]. In the present study, the accessibility and environment of Trp residues in MCL were investigated by intrinsic fluorescence quenching and time-resolved fluorescence. The emission lamda max of native MCL in the absence as well as in the presence of 0.1 M lactose was around 335 nm, which shifted to 365 nm in the presence of 8 M urea, suggesting that the Trp residues which are predominantly buried in the hydrophobic core of the native lectin get exposed to the aqueous environment upon denaturation. At a quencher concentration of 0.5 M, the extent of quenching observed for the native MCL with acrylamide, I- and Cs+ was 46%, 17% and 12%, respectively. In the presence of 0.1 M lactose this quenching was smaller, suggesting that the sugar ligand provides a partial protection to the Trp residues. In time-resolved fluorescence measurements, the decay curves could be fitted well to a biexponential function with the estimated life times 0.92 ns and 4.64 ns for the native protein and 1.15 ns and 5.1 ns in the presence of 0.1 M lactose. All these results are consistent with the involvement of Trp residues in the sugar-binding activity of MCL.

Identification of histidine residues in the sugar binding site of snake gourd (Trichosanthes anguina) seed lectin

Chemical modification studies have been carried out on the galactose-specific lectin (SGSL) purified from snake gourd (Trichosanthes anguina) seeds. Modification of the imidazole side chains of histidine residues with ethoxyformic anhydride resulted in a complete loss of activity of the lectin. A total of 9.5 (+/- 0.7) histidine residues were modified per dimer of M(r) 55,000 when the reaction was carried out for 2 hours. A partial protection was observed when the modification was done in the presence of 0.1 M galactose, indicating that histidine residues are directly involved in the sugar-binding activity of the lectin. Complete recovery of the lectin activity was observed when the modification was reversed by treatment with hydroxylamine. In immunodiffusion experiments, the histidine-modified lectin reacted with rabbit antiserum raised against the native SGSL forming a precipitin line, indicating that the loss of activity upon modification was not due to changes in the overall conformation of the lectin. Modification of the side chains of lysine, cysteine and tyrosine residues did not result in any change in the activity of SGSL.

Differential scanning calorimetry of chain-melting phase transitions of N-acylphosphatidylethanolamines

Phosphatidylethanolamines in which the polar headgroup is N-acylated by a long-chain fatty acid (N-acyl PEs) are present in many plasma membranes under normal conditions, and their content increases dramatically in response to membrane stress in a variety of organisms. The thermotropic phase behavior of a homologous series of saturated N-acyl PEs, in which the length of the N-acyl chain is equal to that of the O-acyl chains attached at the glycerol backbone, has been investigated by differential scanning calorimetry (DSC). All fully hydrated N-acyl PEs with even chain lengths from C-12 to C-18 exhibit sharp endothermic chain-melting phase transitions in the absence of salt and in 1 M NaCl. Cooperative chain-melting is demonstrated directly by the temperature dependence of the electron spin resonance spectra from probe phospholipids bearing a spin label group in the acyl chain. The calorimetric transition enthalpy and the transition entropy obtained from DSC depend approximately linearly on the chain length with incremental values per CH2 group that exceed those of normal diacyl phosphatidylethanolamines, but to an extent that underrepresents the additional N-acyl chain. A thermodynamic model is constructed for the chain-length dependences and end effects of the calorimetric quantities, which includes a deficit proportional to the difference in O-acyl and N-acyl chain lengths for nonmatched chains, as is found and justified structurally for mixed-chain diacyl phospholipids. From data on the chain-length dependence of N-acyl diC16PEs, it is then deduced that the N-acyl chains are less well packed than the O-acyl chains and, from the data on the matched-chain N-acyl PEs, that the O-acyl chain packing is similar to that in normal diacyl PEs. The gel-to-fluid phase transition temperatures of the N-acyl PEs in the absence of salt are practically the same as those of the normal diacyl PEs of the corresponding chain lengths, although the transition enthalpies and entropies are appreciably greater, indicating entropy-enthalpy compensation. In 1 M NaCl, the transition temperatures are 3-4.5 degrees higher than in the absence of salt, representing the contribution of the electrostatic surface potential of the N-acyl PEs.

Differential scanning calorimetric studies on the thermotropic phase transitions of dry and hydrated forms of N-acylethanolamines of even chainlengths

N-acylethanolamines (NAEs) have attracted the attention of researchers in the last two decades due to their occurrence in biological membranes under conditions of stress as well as under normal conditions. Differential scanning calorimetric studies have been carried out on dry and hydrated samples of a homologous series of N-acylethanolamines containing saturated acyl chains of even number of carbon atoms (n = 8-20). In both cases a major sharp endothermic transition was observed which occurs at the melting point for the dry NAEs whereas for the hydrated samples it occurs at considerably lower temperatures. The enthalpies and entropies corresponding to this transition could be fitted, in each case, to a straight line suggesting that the transition enthalpy and transition entropy consist of a fixed component from the polar head group and the terminal methyl group, whereas the contribution of the methylene groups, (CH2)n, is linearly proportional to the number of carbon atoms in it. The contributions of each methylene unit to the transition enthalpy and transition entropy of NAEs were found to be deltaH(inc) = 0.82 (+/-0.02) and 0.96 (+/-0.06) kcal mol(-1), and deltaS(inc) = 2.01 (+/- 0.06) and 2.37 (+/-0.17) cal mol(-1) K(-1), respectively, for the dry and hydrated samples of NAEs, whereas the end contributions arising from the head group and the terminal methyl group were determined to be deltaH(o) = -0.10 (+/-0.26) and -0.52 (+/-0.82) kcal mol(-1) and deltaS(o) = 2.12 (+/-0.71) and 3.1 (+/-2.3) cal mol(-1) K(-1), respectively, for the dry and hydrated samples of NAEs. These results are relevant to an understanding of the thermodynamics of the phase properties of NAEs in membranes.

Spin-label studies on the anchoring and lipid-protein interactions of avidin with N-biotinylphosphatidylethanolamines in lipid bilayer membranes

The specific binding of hen egg white avidin to phosphatidylcholine lipid membranes containing spin-labeled N-biotinylphosphatidylethanolamines (biotin-PESLs) was investigated by using ESR spectroscopy. Spin-labeled biotin-PEs were prepared with the nitroxide group at position C-5, C-8, C-10, C-12, or C-14 of the sn-2 chain and were incorporated at 1 mol % in lipid bilayer membranes of dimyristoylphosphatidylcholine. Binding of avidin produced a strong and selective restriction of the biotin-PE lipid mobility at all positions of chain labeling, as shown by the ESR spectra recorded in the fluid lipid phase. The spectral components of the fraction of the biotin-PESLs that were not complexed by avidin indicated that the mobility of the bulk membrane lipids was unperturbed by binding avidin, as demonstrated by difference spectroscopy. Comparison of the positional profiles and temperature dependences of the outer hyperfine splittings from the biotin-PESLs suggests that the C-12 and C-14 positions of the avidin-bound biotin-PEs are in register with the C-5 and C-7/C-6 positions, respectively, of the chains of the bulk membrane lipids. This indicates that the biotin-PEs are partially withdrawn from the membrane, with a vertical displacement of ca. 7-8 A, on complexation with avidin. In addition, the specific lipid-protein interaction with avidin results in a selective reduction in the rates of lipid chain motion, as shown by the increased ESR line widths. These data define the way in which avidin is anchored to lipid membranes containing biotin-PEs.

Fourier-transform infrared spectroscopic studies on avidin secondary structure and complexation with biotin and biotin-lipid assemblies

Fourier-transform infrared studies have been carried out to investigate the secondary structure and thermal stability of hen egg white avidin and its complexes with biotin and with a biotinylated lipid derivative, N-biotinyl dimyristoyl phosphatidylethanolamine (DMBPE) in aqueous dispersion. Analysis of the amide I stretching band of avidin yielded a secondary structural content composed of approximately 66% beta-sheet and extended structures, with the remainder being attributed to disordered structure and beta-turns. Binding of biotin or specific association with the biotinylated lipid DMBPE did not result in any appreciable changes in the secondary structure content of the protein, but a change in hydrogen bond stability of the beta-sheet or extended chain regions was indicated. The latter effect was enhanced by surface interactions in the case of the biotin-lipid assemblies, as was demonstrated by electrostatic binding to a nonspecific negatively charged lipid. Difference spectra of the bound biotin implicated a direct involvement of the ureido moiety in the ligand interaction that was consistent with hydrogen bonding to amino acid residues in the avidin protein. It was found that complexation with avidin leads to a decrease in bond length of the biotin ureido carbonyl group that is consistent with a reduction of sp3 character of the C-O bond when it is hydrogen bonded to the protein. Studies of the temperature dependence of the spectra revealed that for avidin alone the secondary structure was unaltered up to approximately 75 degrees C, above which the protein undergoes a highly cooperative transition to an unfolded state with concomitant loss of ordered secondary structure. The complexes of avidin with both biotin and membrane-bound DMBPE lipid assemblies display a large increase in thermal stability compared with the native protein.

Purification in high yield and characterisation of the galactose-specific lectin from the seeds of snake gourd (Trichosanthes anguina)

The galactose-specific lectin present in the seeds of snake gourd (Trichosanthes anguina) was purified in high yield by affinity chromatography on cross-linked guar gum. The purified snake gourd seed lectin (SGSL) yielded a single symmetrical peak on gel filtration with an M(r) of 62 kDa and gave a single band in PAGE under non-denaturing conditions. In SDS-PAGE, SGSL gave a single band of M(r) 53 kDa in the absence of beta-mercaptoethanol, and two bands of M(r) 32 and 23 kDa in its presence, indicating that the lectin is a heterodimer in which the subunits are linked by a disulphide bridge. The lectin gave a single precipitin line in immunodiffusion experiments with antiserum raised against the purified SGSL. No cross-reactivity was found between SGSL and antiserum raised against the Momordica charantia lectin and vice versa, suggesting that the two lectins are antigenically dissimilar. Haemagglutination-inhibition data show that Me beta D-Gal is the best monosaccharide inhibitor of SGSL and indicate that an equatorial hydroxyl at C-2 and axial hydroxyl at C-4 in the pyranose form are important binding loci for the lectin.

Thermodynamics of interdigitated phases of phosphatidylcholine in glycerol

Comparison of the electron spin resonance spectra of phosphatidylcholines spin-labeled in the sn-2 chain at a position close to the polar region and close to the methyl terminus indicate that symmetrical saturated diacyl phosphatidylcholines with odd and even chain lengths from 13 to 20 C-atoms (and probably also 12 C-atoms) have gel phases in which the chains are interdigitated when dispersed in glycerol. The chain-length dependences of the chain-melting transition enthalpies and entropies are similar for phosphatidylcholines dispersed in glycerol and in water, but the negative end contributions are smaller for phosphatidylcholines dispersed in glycerol than for those dispersed in water: d delta Ht/dCH2 = 1.48 (1.43) kcal.mol-1, d delta St/dCH2 = 3.9 (4.0) cal.mol-1K-1, and delta H o = -12.9 (-15.0) kcal.mol-1, delta S o = -29 (-40) cal.mol-1K-1, respectively, for dispersions in glycerol (water). These differences reflect the interfacial energetics in glycerol and in water, and the different structure of the interdigitated gel phase.

Phase polymorphism, molecular interactions, and miscibility of binary mixtures of dimyristoyl-N-biotinylphosphatidylethanolamine with dimyristoylphosphatidylcholine

The phase diagram of hydrated binary mixtures of dimyristoyl-N-biotinylphosphatidylethanolamine with dimyristoylphosphatidylcholine in 1 M NaCl has been established by differential scanning calorimetry. Identification of the structures of the phases involved has been made by using X-ray diffraction, spin label ESR spectroscopy, and 31P NMR spectroscopy. On the composition axis, the phase diagram is divided into three regions corresponding to formation of compounds in the gel phase with biotinyl lipid to phosphatidylcholine stoichiometries of approximately 1:1 and 1:3 mol/mol. For the first two regions (up to 75 mol % phosphatidylcholine), the lipids in the gel phase have interdigitated chains (L beta i), whereas in the third region the gel phase is not interdigitated (L beta' or L beta). For the first region (up to 50 mol % phosphatidylcholine), the fluid phase is of the novel isotropic type (IMI) composed of aggregated normal micelles that is characteristic of shorter chainlength biotinylated lipids [Swamy, M.J., Würz, U., & Marsh, D. (1993) Biochemistry 32, 9960-9967], whereas for the other two regions a normal fluid lamellar (L alpha) phase obtains. The equimolar mixture, which lies at a stoichiometric phase boundary, melts isothermally and then undergoes a transition from the isotropic IMI structure to the lamellar L alpha structure with increasing temperature in the fluid phase.

Thermodynamic analysis of biotin binding to avidin. A high sensitivity titration calorimetric study

Titration calorimetric studies on the binding of biotin to avidin were performed in phosphate buffered saline, pH 7.4. From the temperature dependence of the binding enthalpy (delta H), the delta Cp value was determined. While the delta H value of -23.4 kcal/mol at 25 degrees C is in close agreement with the previously determined value of -22.5 kcal/mol (Suurkusk, J. & Wadso, I. (1972) Eur. J. Biochem. 28, 438-441), the delta Cp value of -461 cal/mol biotin/K is significantly at variance with the value of -237 cal/mol biotin/K obtained in the previous study. A comparison of the thermodynamic data obtained for the avidin-biotin system with that of the streptavidin-biotin system revealed that the higher binding affinity of avidin for biotin is due to a smaller (negative) entropy of binding.

Spin-label electron spin resonance studies on the dynamics of the different phases of N-biotinylphosphatidylethanolamines

The chain dynamics and phase behavior of a homologous series of diacyl-N-biotinylphosphatidylethanolamines of chain lengths from C(12:0) to C(20:0) were investigated by spin-label electron spin resonance (ESR) spectroscopy using both phosphatidylcholine and N-biotinylphosphatidylethanolamine spin-label probes. Chain-melting phase transition temperatures determined from the ESR spectral measurements, for all the five lipids in the presence as well as in the absence of 1 M NaCl, correlate with the endothermic phase transition temperatures determined by differential scanning calorimetry [Swamy, M. J., Angerstein, B., & Marsh, D. (1994) Biophys. J. 66, 31-39], confirming that the latter correspond to chain-melting transitions. ESR spectra obtained in the gel phase from phosphatidylcholine probes with the spin-label near the terminal methyl of the hydrocarbon chain showed a similar degree of immobilization (as reflected by the outer hyperfine splittings) to that for spin-labels positioned close to the glycerol backbone, indicating that the biotin-lipids of chain lengths from 14 to 20 carbon atoms form interdigitated gel phases in the presence of salt, as also do those of chain lengths between 16 and 20 carbon atoms in the absence of salt. For dispersions of the C(16:0) chain length biotin-lipid in 1 M NaCl, continuous monitoring of the central ESR peak intensity as a function of temperature detects a cooperative decrease in mobility in the fluid phase at ca. 65 degrees C with a spin-label in the lipid head group and an accompanying increase in mobility at the same temperature with spin-labels positioned toward the terminal methyl of the hydrocarbon chain.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential scanning calorimetry of thermotropic phase transitions in vitaminylated lipids: aqueous dispersions of N-biotinyl phosphatidylethanolamines

The thermotropic phase behavior of a homologous series of saturated diacyl phosphatidylethanolamines in which the headgroup is N-derivatized with biotin has been investigated by differential scanning calorimetry. In 1 M NaCl, derivatives with acyl chainlengths from C(12:0) to C(20:0) all exhibit sharp chain-melting phase transitions, which are reversible with a hysteresis of 1.5 degrees or less, except for the C(12:0) lipid which has a transition temperature below 0 degree C. The transition enthalpy and the transition entropy depend approximately linearly on the lipid chainlength, with incremental values per CH2 group that are very similar to those obtained for the corresponding underivatized phosphatidylethanolamines in aqueous dispersion. The chainlength-independent contribution to the transition enthalpy is significantly smaller than that for the underivatized phosphatidylethanolamines, and that for the transition entropy is much smaller; the latter suggesting that the N-biotinylated phosphatidylethanolamine headgroups are differently hydrated from those of the underivatized lipids. The gel-to-fluid phase transition temperatures of the N-biotinylated lipids are lower than those of the parent phosphatidylethanolamines, and their chainlength dependence conforms well with that predicted by assuming that the transition enthalpy and entropy are linearly dependent on chainlength. Although the chain-melting phase behavior is generally similar to that of the parent phosphatidylethanolamines, the gel phases (and the fluid phases in the case of chainlengths C(12:0) to C(16:0)) have a different lyotropic structure in the two cases, and this is reflected in the chainlength-independent contributions to the thermodynamic parameters. In the absence of salt, the thermotropic phase behavior of aqueous dispersions of the N-biotinyl phosphatidylethanolamines is considerably more complex. The transition temperatures are consistently lower than those in 1 M NaCI, but the transitions are broader, contain multiple peaks and exhibit a much larger hysteresis between heating and cooling scans. Additionally, the lipids with shorterchainlengths exhibit metastability in the absence of salt, converting from a micellar solution to a lamellar gel phase only after incubation at low temperature with freeze-thaw cycling.

Structure of vitaminylated lipids in aqueous dispersion: X-ray diffraction and 31P NMR studies of N-biotinylphosphatidylethanolamines

The structures of the phases formed in excess buffer (at pH 7.4) by a homologous series of saturated diacylphosphatidylethanolamines in which the headgroup is N-derivatized with biotin have been investigated for chain lengths of C(12:0) to C(20:0), using both 31P nuclear magnetic resonance (NMR) spectroscopy and small-angle X-ray diffraction. In 1 M NaCl, all lipids display 31P NMR spectra characteristic of a lamellar gel phase at low temperature. In the fluid phase, the lipids of C(12:0) and C(14:0) chain lengths display isotropic 31P NMR spectra, corresponding to aggregated phases with high surface curvature, whereas those with C(18:0) and C(20:0) chain lengths display sharp axial powder patterns characteristic of a lamellar (L alpha) phase. The lipid of intermediate C(16:0) chain length displays a more complex temperature dependence of the 31P NMR spectra in the fluid phase. The spectra convert from an axial powder pattern of unusually low chemical shift anisotropy to one characteristic of a fluid lamellar (L alpha) phase with increasing temperature. The small-angle X-ray diffraction patterns of the lipids in 1 M NaCl have lamellar repeat spacings in the gel phase which increase linearly with chain length and are consistently lower than those in the fluid phase [for chain lengths of C(16:0) to C(20:0)]. In addition, the gradient in long spacing with chain length in the gel phase is approximately half that expected for a gel phase with untilted, all-trans chains, indicating that the lipid chains are interdigitated in the gel phase (L beta i).(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of avidin with spin-labelled N-biotinyl phosphatidylethanolamine in a lipid membrane

N-Biotinyl phosphatidylethanolamine spin labelled at the C-14 position of the sn-2 chain has been incorporated at a level of 1 mol% in bilayers of dimyristoyl phosphatidylcholine, and the effects on the chain mobility of binding avidin to the biotin lipid headgroup have been studied by electron spin resonance spectroscopy. In the fluid phase, avidin causes a large and selective restriction in the chain motion of the biotin lipids to which it is attached, without perturbing appreciably the mobility of the bulk lipid chains. This specific type of lipid-protein interaction is different in kind from that observed both with integral and peripheral membrane proteins and may be involved in transmembrane communication on ligand binding to lipid headgroups, as well as lateral communication (at high packing densities) between proteins with covalent lipid anchors.

Primary structure of a Thomsen-Friedenreich-antigen-specific lectin, jacalin [Artocarpus integrifolia (jack fruit) agglutinin]. Evidence for the presence of an internal repeat

Jacalin [Artocarpus integrifolia (jack fruit) agglutinin] is made up of two types of chains, heavy and light, with M(r) values of 16,200 +/- 1200 and 2090 +/- 300 respectively (on the basis of gel-permeation chromatography under denaturing conditions). Its complete amino acid sequence was determined by manual degradation using a 4-dimethylaminoazobenzene 4'-isothiocyanate double-coupling method. Peptide fragments for sequence analysis were obtained by chemical cleavages of the heavy chain with CNBr, hydroxylamine hydrochloride and iodosobenzoic acid and enzymic cleavage with Staphylococcus aureus proteinase. The peptides were purified by a combination gel-permeation and reverse-phase chromatography. The light chains, being only 20 residues long, could be sequenced without fragmentation. Amino acid analyses and carboxypeptidase-Y-digestion C-terminal analyses of the subunits provided supportive evidence for their sequence. Computer-assisted alignment of the jacalin heavy-chain sequence failed to show sequence similarity to that of any lectin for which the complete sequence is known. Analyses of the sequence showed the presence of an internal repeat spanning residues 7-64 and 76-130. The internal repeat was found to be statistically significant.

Further characterization of the saccharide specificity of peanut (Arachis hypogaea) agglutinin

2-Dansylamino-2-deoxy-D-galactose (GalNDns) has been shown to bind to peanut (Arachis hypogaea) agglutinin (PNA) in a saccharide-specific manner. This binding was accompanied by a five-fold increase in the fluorescence of GalNDns. The interaction was characterized by an association constant of 0.15 mM at 15 degrees and delta H and delta S values of -57.04 kJ.mol-1 and -118.1J.mol-1.K-1, respectively. Binding of a variety of other mono-, di- and oligo-saccharides to PNA, studied by monitoring their ability to dissociate the PNA GalNDns complex, revealed that PNA interacts with several T-antigen-related structures, such as beta-D-Galp-(1----3)-D-GalNAc, beta-D-Galp-(1----3)-alpha-D-GalpNAcOMe, and beta-D-Galp-(1----3)-alpha-D-GalpNAc-(1----3)-Ser, as well as the asialo-GM1 tetrasaccharide, with comparable affinity, thus showing that this lectin does not discriminate between saccharides in which the penultimate sugar of the beta-D-Galp-(1----3)-D-GalNAc unit is the alpha or beta anomer, in contrast to jacalin (Artocarpus integrifolia agglutinin), another anti T-lectin which preferentially binds to beta-D-Galp-(1----3)-alpha-D-GalNAc and does not recognize beta-D-Galp-(1----3)-beta-D-GalNAc or the related asialo-GM1 oligosaccharide. These studies also indicated that, in the extended combining region of PNA which accommodates a disaccharide, the primary subsite (subsite A) is highly specific for D-galactose, whereas the secondary subsite (subsite B) is less specific and can accommodate various structures, such as D-galactose, 2-acetamido-2-deoxy-D-galactose, D-glucose, and 2-acetamido-2-deoxy-D-glucose.

On the metabolism of GM3 ganglioside in cultured human foreskin fibroblasts

The metabolism of GM3 ganglioside in cultured human foreskin fibroblasts was investigated by labeling cultured cells with [1-3H]-galactose for 48 hours, followed by a 48 hour chase. More than 80% of the radioactivity associated with GM3 was found in the hexose portion of the carbohydrate chain, whereas approximately 12% of the radioactivity was observed in the sialic acid moiety. The hexose and sialic acid residues lost 42% and 53% of their initial radioactivity, respectively, during the chase period, indicating an active metabolism of these sugar residues of GM3 in growing cultures.

Studies on the tryptophan residues of soybean agglutinin. Involvement in saccharide binding

Modification of tryptophan side chains of soybean agglutinin (SBA) with N-bromosuccinimide results in a loss of the hemagglutinating and carbohydrate binding activities of the protein. One residue/subunit is probably essential for the binding activity. Modification leads to a large decrease in the fluorescence of the protein accompanied by a blue shift. Iodide ion quenching of the protein fluorescence shows that saccharide binding results in a decreased accessibility of some of the tryptophan side chains. These results strongly point towards the involvement of tryptophan residues in the active site of SBA.

Analysis of dynamics and mechanism of ligand binding to Artocarpus integrifolia agglutinin. A 13C and 19F NMR study

Binding of 13C-labeled N-acetylgalactosamine (13C-GalNAc) and N-trifluoroacetylgalactosamine (19F-GalNAc) to Artocarpus integrifolia agglutinin has been studied using 13C and 19F nuclear magnetic resonance spectroscopy, respectively. Binding of these saccharides resulted in broadening of the resonances, and no change in chemical shift was observed, suggesting that the alpha- and beta-anomers of 13C-GalNAc and 19F-GalNAc experience a magnetically equivalent environment in the lectin combining site. The alpha- and beta-anomers of 13C-GalNAc and 19F-GalNAc were found to be in slow exchange between free and protein bound states. Binding of 13C-GalNAc was studied as a function of temperature. From the temperature dependence of the line broadening, the thermodynamic and kinetic parameters were evaluated. The association rate constants obtained for the alpha-anomers of 13C-GalNAc and 19F-GalNAc (k+1 = 1.01 x 10(5) M-1.s-1 and 0.698 x 10(5) M-1.s-1, respectively) are in close agreement with those obtained for the corresponding beta-anomers (k+1 = 0.95 x 10(5) M-1.s-1 and 0.65 x 10(5) M-1.s-1, respectively), suggesting that the two anomers bind to the lectin by a similar mechanism. In addition these values are several orders of magnitude slower than those obtained for diffusion controlled processes. The dissociation rate constants obtained are 49.9, 56.9, 42, and 43 s-1, respectively, for the alpha- and beta-anomers of 13C-GalNAc and 19F-GalNAc. A two-step mechanism has been proposed for the interaction of 13C-GalNAc and 19F-GalNAc with A. integrifolia lectin in view of the slow association rates and high activation entropies. The thermodynamic parameters obtained for the association and dissociation reactions suggest that the binding process is entropically favored and that there is a small enthalpic contribution.

Studies on tryptophan residues of Abrus agglutinin. Stopped-flow kinetics of modification and fluorescence-quenching studies

The presence of two essential tryptophan residues/molecule was implicated in the binding site of Abrus agglutinin [Patanjali, Swamy, Anantharam, Khan & Surolia (1984) Biochem. J. 217, 773-781]. A detailed study of the stopped-flow kinetics of the oxidation of tryptophan residues revealed three classes of tryptophan residues in the native protein. A discrete reorganization of tryptophan residues revealed three classes of tryptophan residues in the native protein. A discrete reorganization of tryptophan residues into two phases was observed upon ligand binding. The heterogeneity of tryptophan exposure was substantiated by quenching studies with acrylamide, succinimide and Cs+. Our study revealed the microenvironment of tryptophan residues to be hydrophobic, and also the presence of acidic amino acid residues in the vicinity of surface-localized tryptophan residues.

Isolation, macromolecular properties, and combining site of a chito-oligosaccharide-specific lectin from the exudate of ridge gourd (Luffa acutangula)

A lectin specific for chito-oligosaccharides from the exudate of ridge gourd (Luffa acutangula) fruits has been purified to homogeneity by affinity chromatography. The lectin has a molecular weight of 48,000, an S(0)20,w of 4.06 S and a Stokes radius of 2.9 nm. Upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis, a single band corresponding to Mr of 24,000 was observed both in the presence and absence of beta-mercaptoethanol. The subunits in this dimeric lectin are, therefore, held together solely by noncovalent interactions. The lectin is not a glycoprotein, and secondary structure analysis by CD measurements showed 31% alpha-helix. The hemagglutinating activity of L. acutangula agglutinin was not inhibited by any of the monosaccharides tested. Among the disaccharides only di-N-acetylchitobiose was inhibitory. The inhibitory potency of chito-oligosaccharides increased dramatically with their size up to penta-N-acetylchitopentaose. The lectin has two binding sites for saccharides. The affinity of chito-oligosaccharides for L. acutangula lectin, as monitored by titrating the changes in the near UV-CD spectra and intrinsic fluorescence, increased strikingly with the number of GlcNAc units in them. The values of delta G, delta H, and delta S for the binding process showed a pronounced dependence on the size of the chito-oligosaccharides, indicating that the binding of higher oligomers is progressively more favored thermodynamically than di-N-acetylchitobiose. The thermodynamic data are consistent with an extended binding site in this lectin, which accommodates a tetrasaccharide.

Analysis of saccharide binding to Artocarpus integrifolia lectin reveals specific recognition of T-antigen (beta-D-Gal(1----3)D-GalNAc)

The binding of Artocarpus integrifolia lectin to N-dansylgalactosamine (where dansyl is 5-dimethylaminonaphthalene-1-sulfonyl) leads to a 100% increase in dansyl fluorescence with a concomitant blue shift in the emission maximum by 10 nm. This binding is carbohydrate-specific and has an association constant of 1.74 X 10(4) M-1 at 20 degrees C. The lectin has two binding sites for N-dansylgalactosamine. The values of -delta H and -delta S for the binding of N-dansylgalactosamine are in the range of values reported for several lectin-monosaccharide interactions, indicating an absence of nonpolar interaction of the dansyl moiety of the sugar with the combining region of the protein. Dissociation of the bound N-dansylgalactosamine from its complex with the lectin and the consequent change in its fluorescence on addition of nonfluorescent sugars allowed evaluation of the association constant for competing ligands. The thermodynamic parameters for the binding of monosaccharides suggest that the OH groups at C-2, C-3, C-4, and C-6 in the D-galactose configuration are important loci for interaction with the lectin. The acetamido group at C-2 of 2-acetamido-2-deoxygalactopyranose and a methoxyl group at C-1 of methyl-alpha-D-galactopyranoside are presumably also involved in binding through nonpolar and van der Waals' interactions. The T-antigenic disaccharide Gal beta 1----3GalNAc binds very strongly to the lectin when compared with methyl-beta-D-galactopyranoside, the beta(1----3)-linked disaccharides such as Gal beta 1----3GlcNAc, and the beta(1----4)-linked disaccharides, N-acetyllactosamine and lactose. The major stabilizing force for the avid binding of T-antigenic disaccharide appears to be a favorable enthalpic contribution. The combining site of the lectin is, therefore, extended. These data taken together suggest that the Artocarpus lectin is specific toward the Thomsen-Friedenreich (T) antigen. There are subtle differences in the overall topography of its combining site when compared with that of peanut (Arachis hypogaea) agglutinin. The results of stopped flow spectrometry for the binding of N-dansylgalactosamine tot he Artocarpus lectin are consistent with a simple single-step bimolecular association and unimolecular dissociation rate processes. The value of K+1 and K-1 at 21 degrees C are 8.1 X 10(5) M-1 s-1 and 50 s-1, respectively. The activation parameters indicate an enthalpy-controlled association process.

Thermodynamic and kinetic studies on saccharide binding to soya-bean agglutinin

The fluorescence of N-dansylgalactosamine [N-(5-dimethylaminonaphthalene-1-sulphonyl)galactosamine] was enhanced 11-fold with a 25 nm blue-shift in the emission maximum upon binding to soya-bean agglutinin (SBA). This change was used to determine the association constants and thermodynamic parameters for this interaction. The association constant of 1.51 X 10(6) M-1 at 20 degrees C indicated a very strong binding, which is mainly due to a relatively small entropy value, as revealed by the thermodynamic parameters: delta G = -34.7 kJ X mol-1, delta H = -37.9 kJ X mol-1 and delta S = -10.9 J X mol-1 X K-1. The specific binding of this sugar to SBA shows that the lectin can accommodate a large hydrophobic substituent on the C-2 of galactose. Binding of non-fluorescent ligands, studied by monitoring the fluorescence changes when they are added to a mixture of SBA and N-dansylgalactosamine, indicates that a hydrophobic substituent at the anomeric position increases the affinity of the interaction. The C-6 hydroxy group also stabilizes the binding considerably. Kinetics of binding of N-dansylgalactosamine to SBA studied by stopped-flow spectrofluorimetry are consistent with a single-step mechanism and yielded k+1 = 2.4 X 10(5) M-1 X s-1 and k-1 = 0.2 s-1 at 20 degrees C. The activation parameters indicate an enthalpicly controlled association process.

Arrangement of subunits in peanut lectin. Rotation function and chemical cross-linking studies

X-ray intensity data from the native orthorhombic crystals of peanut lectin have been collected using oscillation photography. Rotation function studies using data up to a resolution of 4.5 A indicate that the four subunits in the molecule, which constitute the asymmetric unit in the crystals, are related to one another by three mutually perpendicular noncrystallographic 2-fold axes. Chemical cross-linking experiments in solution followed by sodium dodecyl sulfate gel electrophoresis, carried out in parallel, suggest that there is more than one type of intersubunit approach in the molecule. Rotation function and cross-linking studies thus show that the tetrameric molecule of peanut lectin is a dimer of a dimer. The two monomers in a dimer are related by a 2-fold axis. The two dimers are in turn related by another 2-fold axis perpendicular to the one that relates the two monomers in the dimer, endowing the molecule with 222 (D2) symmetry.

Chemical modification studies on Abrus agglutinin. Involvement of tryptophan residues in sugar binding

The galactose-binding lectin from the seeds of the jequirity plant (Abrus precatorius) was subjected to various chemical modifications in order to detect the amino acid residues involved in its binding activity. Modification of lysine, tyrosine, arginine, histidine, glutamic acid and aspartic acid residues did not affect the carbohydrate-binding activity of the agglutinin. However, modification of tryptophan residues carried out in native and denaturing conditions with N-bromosuccinimide and 2-hydroxy-5-nitrobenzyl bromide led to a complete loss of its carbohydrate-binding activity. Under denaturing conditions 30 tryptophan residues/molecule were modified by both reagents, whereas only 16 and 18 residues/molecule were available for modification by N-bromosuccinimide and 2-hydroxy-5-nitrobenzyl bromide respectively under native conditions. The relative loss in haemagglutinating activity after the modification of tryptophan residues indicates that two residues/molecule are required for the carbohydrate-binding activity of the agglutinin. A partial protection was observed in the presence of saturating concentrations of lactose (0.15 M). The decrease in fluorescence intensity of Abrus agglutinin on modification of tryptophan residues is linear in the absence of lactose and shows a biphasic pattern in the presence of lactose, indicating that tryptophan residues go from a similar to a different molecular environment on saccharide binding. The secondary structure of the protein remains practically unchanged upon modification of tryptophan residues, as indicated by c.d. and immunodiffusion studies, confirming that the loss in activity is due to modification only.