Peptide synthesis with a proteinase from the extremely thermophilic organism Thermus Rt41A

A proteinase isolated from Thermus RT41a was immobilized to controlled pore glass beads and was used in the free and immobilized forms for peptide synthesis. The observed maximum yield was the same in both cases. a number of dipeptides were produced from amino acid esters and amides. The best acyl components, from those tested, were found to be Ac-Phe-OEt and Bz-Ala-OMe. Tur-NH(2), Trp-NH(2), Leu-pNA, and Val-pNA were all reactive nucleophiles.The kinetically controlled synthesis of Bz-ala-Tyr-NH(2) was optimized by studying the effect of pH, temperature, solvent concentration, ionic strength, and nucleophile and acyl donor concentration, ionic strength, and nucleophile and acyl donor concentration on the maximum yield. The initial conditions used were 25 mM Bz-ala-OMe, 25 mM Tyr-NH(2), 70 degrees C, pH 8.0, and 10% v/v dimethylformamide (DMF). The optimum conditions were 90% v/v DMF using 80 mM bz-Ala-OMe and 615 mM Tyr-NH(2) at 40 degrees C and pH 10. These conditions increased the maximum conversion from 0.75% to 26% (of the original ester concentration). In a number of other cosolvents, the best peptide yields were observed with acetonitrile and ethyl acetate. In 90% acetonitrile similar yields were observed to those in 90% DMF under optimized conditions except that the acyl donor and nucleophile concentrations could be reduced to 25 mM and 100mM, respectively. The effect of the blocking group on the nucleophile was also investigated; -betaNA and -pNA as blocking groups improved the yields markedly. The blocking and leaving groups of the acyldonor had no effect on the dipeptide yield. (c) 1994 John Wiley & Sons, Inc.     

Metabolic activation of acetylenes. Covalent binding of [1,2-14C]octyne to protein, DNA and haem in vitro and the protective effects of certain thiol compounds

[1,2-14C]Oct-l-yne was used to investigate metabolic activation of the ethynyl substituent in vitro. Activation of octyne by liver microsomal cytochrome P-450-dependent enzymes gave intermediate(s) that bound covalently to protein, DNA and to haem. The time course and extent of covalent binding of octyne to haem and to protein were similar. However, two different activating mechanisms are probably involved. Whereas covalent binding to protein or to DNA was inhibited by nucleophiles such as N-acetylcysteine, that to haem was little affected. When N-acetylcysteine was included in the reaction mixtures, two major octyne-N-acetylcysteine adducts were isolated and purified by high-pressure liquid chromatography. G.l.c.-mass spectrometry and n.m.r. suggest that these are the cis-trans isomers of S-3-oxo-oct-1-enyl-N-acetylcysteine. Oct-1-yn-3-one reacted non-enzymically with N-acetylcysteine at pH 7.4 and 37 degrees C with a t1/2 of about 6 s also to yield S-3-oxo-oct-l-enyl-N-acetylcysteine. The same product was formed when microsomal fractions were incubated with oct-1-yn-3-ol, N-acetylcysteine and NAD(P)+. Octyn-3-one did not appear to react with haem or protoporphyrin IX. 5. A mechanism for the metabolic activation of oct-1-yne is proposed, consisting in (a) microsomal hydroxylation of the carbon atom alpha to the acetylenic bond and (b) oxidation to yield octyn-3-one as the reactive species.     

DNA strand transfer catalyzed by vaccinia topoisomerase: peroxidolysis and hydroxylaminolysis of the covalent protein-DNA intermediate

Vaccinia topoisomerase forms a covalent DNA-(3'-phosphotyrosyl)-enzyme intermediate at sites containing the sequence 5'-CCCTT downward arrow. The covalently bound topoisomerase can religate the CCCTT strand to a 5'-OH-terminated polynucleotide or else transfer the strand to a non-DNA nucleophile such a water or glycerol. Here, we report that vaccinia topoisomerase also catalyzes strand transfer to hydrogen peroxide. The observed alkaline pH-dependence of peroxidolysis is consistent with enzyme-mediated attack by peroxide anion on the covalent intermediate. The reaction displays apparent first-order kinetics. From a double-reciprocal plot of k(obs) versus [H(2)O(2)] at pH 10, we determined a rate constant for peroxidolysis of 6.3 x 10(-)(3) s(-)(1). This rate is slower by a factor of 200 than the rate of topoisomerase-catalyzed strand transfer to a perfectly aligned 5'-OH DNA strand but is comparable to the rate of DNA strand transfer across a 1-nucleotide gap. Strand transfer to 2% hydrogen peroxide is 300 times faster than strand transfer to 20% glycerol and approximately 2000 times faster than topoisomerase-catalyzed hydrolysis of the covalent intermediate. Hydroxylamine is also an effective nucleophile in topoisomerase-mediated strand transfer (k(obs) = 6.4 x 10(-)(4) s(-)(1)). The rates of the peroxidolysis, hydroxylaminolysis, glycerololysis, and hydrolysis reactions catalyzed by the mutant enzyme H265A were reduced by factors of 100-700, in accordance with the 100- to 400-fold rate decrements in DNA cleavage and religation by H265A. We surmise that vaccinia topoisomerase catalyzes strand transfer to DNA and non-DNA nucleophiles via a common reaction pathway in which His-265 stabilizes the scissile phosphate in the transition state rather than acting as a general acid or base.     

Factor I co-factor activity of CR1 overcomes the protective effect of IgG on covalently bound C3b residues

We have shown previously that C3b resides in a protected site when it is covalently bound to IgG (C3b-IgG). Such C3b displays a reduced affinity for factor H, with consequent enhanced survival in the presence of factors H and I and increased capacity for promoting alternative pathway consumption of C3. Because erythrocyte CR1 may be a major co-factor for factor I-mediated inactivation of immune complex-borne C3b in blood, we have examined the effect of covalently bound IgG on the C3b-CR1 interaction. Binding of monomeric C3b and C3b-IgG to human erythrocyte CR1 demonstrates identical ionic strength dependence for both species. Identical numbers of binding sites with indistinguishable affinities are detected by both ligands. Cleavage of the alpha'-chain of C3b and the alpha'-heavy chain of C3b-IgG proceeds at the same rate when erythrocyte CR1 serves as co-factor for factor I. Unlike factor H, CR1 supports a second cleavage of fluid-phase iC3b alpha'1 chain (free or bound to IgG) that generates C3c and a 33,000 m.w. fragment, which bears antigenic markers characteristic of C3g. Inactivation of C3b and C3b-IgG by CR1 and factor I also occurs at physiologic ionic strength, but proceeds very slowly relative to rates attainable with sub-physiologic inputs of factor H. CR1 does not recognize IgG-bound C3b as being in a protected site but, because of low binding affinity at physiologic ionic strength, is probably highly dependent on multivalent ligand-receptor interactions to efficiently exert its co-factor functions. Thus, inactivation of C3b-IgG heterodimers or small immune complexes bearing limited numbers of C3b residues may remain largely factor H-dependent in vivo, with resultant enhanced C3b survival.     

Comparative metabolism, covalent binding and toxicity of BHT congeners in rat liver slices.

The metabolism, covalent binding and hepatotoxicity of butylated hydroxytoluene (BHT, 4-methyl-2,6-di-t-butylphenol) and two congeners (E-BHT, 4-ethyl-2,6-di-t-butylphenol; I-BHT, 4-isopropyl-2,6-di-t-butylphenol) were compared using precision-cut liver slices prepared from phenobarbital (PB)-treated male Sprague-Dawley rats. At equimolar concentrations (1 mM) BHT was the most toxic of the three compounds, causing an 80% decrease in cell viability over a 6 h incubation period. E-BHT was intermediate in toxicity while the isopropyl derivative was relatively nontoxic. Intracellular glutathione levels decreased prior to the onset of cytotoxicity. The cytochrome P450 inhibitor metyrapone completely inhibited the toxicity of all three compounds. The rates of metabolism of the three compounds to glutathione conjugates were compared in both PB-treated microsomes and PB-induced liver slices. In both models, the rate of formation was greatest for BHT, followed by E-BHT and I-BHT. Synthetic quinone methides (QMs) were prepared from each parent phenol and the rates of reactivity with three nucleophiles (water, methanol and glutathione) were compared. With each nucleophile, BHTQM was the most reactive, while I-BHTQM was the least reactive. Finally, covalent binding to protein was assessed in two ways. First, alkylation of an isolated model protein (bovine insulin) was measured in a microsomal enzyme activation system by mass spectrometry. Incubations with BHT produced the greatest extent of protein alkylation, followed by E-BHT, while no alkylation was observed with I-BHT. In the second system, covalent binding to cellular protein was assessed in rat liver PB microsomes and tissue slices by Western blotting using an antibody specific for the tert-butylphenol portion of the compounds. Binding was greatest for BHT, intermediate for E-BHT and could not be detected for I-BHT. The alkylation pattern for E-BHT was strikingly similar to that of BHT, suggesting that both compounds bound similar proteins. In summary, our results suggest that for hindered phenols such as BHT, increasing the length of the 4-alkyl substituent retards the rate of formation of reactive intermediates, significantly reduces the electrophilicity of the reactive intermediate, and greatly reduces the amount but not the selectivity of covalent binding to cellular protein, thereby reducing the toxicity of the parent compound.     

Nucleophilic activation by positioning in phosphoryl transfer catalyzed by nucleoside diphosphate kinase

The nonenzymatic reaction of ATP with a nucleophile to generate ADP and a phosphorylated product proceeds via a dissociative transition state with little bond formation to the nucleophile. Consideration of the dissociative nature of the nonenzymatic transition state leads to the following question: To what extent can the nucleophile be activated in enzymatic phosphoryl transfer? We have addressed this question for the NDP kinase reaction. A mutant form of the enzyme lacking the nucleophilic histidine (H122G) can be chemically rescued for ATP attack by imidazole or other exogenous small nucleophiles. The ATP reaction is 50-fold faster with the wild-type enzyme, which has an imidazole nucleophile positioned for reaction by a covalent bond, than with H122G, which employs a noncovalently bound imidazole nucleophile [(kcat/KM)ATP]. Further, a 4-fold advantage for imidazole positioned in the nucleophile binding pocket created by the mutation is suggested from comparison of the reaction of H122G and ATP with an imidazole versus a water nucleophile, after correction for the intrinsic reactivities of imidazole and water toward ATP in solution. X-ray structural analysis shows no detectable rearrangement of the residues surrounding His 122 upon mutation to Gly 122. The overall rate effect of approximately 10(2)-fold for the covalent imidazole nucleophile relative to water is therefore attributed to positioning of the nucleophile with respect to the reactive phosphoryl group. This is underscored by the more deleterious effect of replacing ATP with AlphaTauPgammaS in the wild-type reaction than in the imidazole-rescued mutant reaction, as follows. For the wild-type, AlphaTauPgammaS presumably disrupts positioning between nucleophile and substrate, resulting in a large thio effect of 300-fold, whereas precise alignment is already disrupted in the mutant because there is no covalent bond to the nucleophile, resulting in a smaller thio effect of 10-fold. In summary, the results suggest a catalytic role for activation of the nucleophile by positioning in phosphoryl transfer catalyzed by NDP kinase.     

Characterization of four covalently-linked yeast cytochrome c/cytochrome c peroxidase complexes: Evidence for electrostatic interaction between bound cytochrome c molecules

Four covalent complexes between recombinant yeast cytochrome c and cytochrome c peroxidase (rCcP) were synthesized via disulfide bond formation using specifically designed protein mutants (Papa, H. S., and Poulos, T. L. (1995) Biochemistry 34, 6573-6580). One of the complexes, designated V5C/K79C, has cysteine residues replacing valine-5 in rCcP and lysine-79 in cytochrome c with disulfide bond formation between these residues linking the two proteins. The V5C/K79C complex has the covalently bound cytochrome c located on the back-side of cytochrome c peroxidase, approximately 180 degrees from the primary cytochrome c-binding site as defined by the crystallographic structure of the 1:1 noncovalent complex (Pelletier, H., and Kraut J. (1992) Science 258, 1748-1755). Three other complexes have the covalently bound cytochrome c located approximately 90 degrees from the primary binding site and are designated K12C/K79C, N78C/K79C, and K264C/K79C, respectively. Steady-state kinetic studies were used to investigate the catalytic properties of the covalent complexes at both 10 and 100 mM ionic strength at pH 7.5. All four covalent complexes have catalytic activities similar to those of rCcP (within a factor of 2). A comprehensive study of the ionic strength dependence of the steady-state kinetic properties of the V5C/K79C complex provides evidence for significant electrostatic repulsion between the two cytochromes bound in the 2:1 complex at low ionic strength and shows that the electrostatic repulsion decreases as the ionic strength of the buffer increases.     

Phospholipid membranes affect tertiary structure of the soluble cytochrome b5 heme-binding domain

The influence of charged phospholipid membranes on the conformational state of the water-soluble fragment of cytochrome b5 has been investigated by a variety of techniques at neutral pH. The results of this work provide the first evidence that aqueous solutions with high phospholipid/protein molar ratios (pH 7.2) induce the cytochrome to undergo a structural transition from the native conformation to an intermediate state with molten-globule like properties that occur in the presence of an artificial membrane surface and that leads to binding of the protein to the membrane. At other phospholipid/protein ratios, equilibrium was observed between cytochrome free in solution and cytochrome bound to the surface of vesicles. Inhibition of protein binding to the vesicles with increasing ionic strength indicated for the most part an electrostatic contribution to the stability of cytochrome b5-vesicle interactions at pH 7.2. The possible physiological role of membrane-induced conformational change in the structure of cytochrome b5 upon the interaction with its redox partners is discussed.     

The binding of human complement component C4 to antibody-antigen aggregates.

The binding of human complement component C4 to antibody-antigen aggregates and the nature of the interaction have been investigated. When antibody-antigen aggregates with optimal C1 bound are incubated with C4, the C4 is rapidly cleaved to C4b, but only a small fraction (1-2%) is bound to the aggregates, the rest remaining in the fluid phase as inactive C4b. It has been found that C4b and th antibody form a very stable complex, due probably to the formation of a covalent bond. On reduction of the C4b-immunoglobulin G (IgG) complex, the beta and gamma chains, but not the alpha' chain, of C4b are released together with all the light chain, but only about half of the heavy chain of IgG. The reduced aggregates contain two main higher-molecular-weight complexes, one shown by the use of radioactive components to contain both IgG and C4b and probably therefore the alpha' chain of C4b and the heavy chain of IgG, and the other only C4b and probably an alpha' chain dimer. The aggregates with bound C1 and C4b show maximal C3 convertase activity, in the presence of excess C2, when the alpha'-H chain component is in relatively highest amounts. When C4 is incubated with C1s in the absence of aggregates, up to 15% of a C4b dimer is formed, which on reduction gives an alpha' chain complex, probably a dimer. The apparent covalent interaction between C4b and IgG and between C4b and other C4b molecules cannot be inhibited by iodoacetamide and hence cannot be catalysed by transglutaminase (factor XIII). The reaction is, however, inhibited by cadaverine and putrescine and 14C-labelled putrescine is incorporated into C4, again by a strong, probably covalent, bond. It is suggested that a reactive group, possibly an acyl group, is generated when C4 is activated by C1 and that this reactive group can react with IgG, with another C4 molecule, or with water.     

Physical characteristics of hyaluronate binding to the surface of simian virus 40-transformed 3T3 cells

The binding of labeled hyaluronate to the surface of Simian virus 40-transformed 3T3 cells was studied as a function of 1) pH, 2) ionic strength, 3) temperature, and 4) molecular weight of the hyaluronate. Binding occurred over a wide range of pH values with optima at pH 7 and at less than pH 4. Binding at low pH was eliminated at high ionic strength whereas that at physiological pH was enhanced, with a maximum at 0.5 M NaCl. The enhancement of binding at pH 7 was reversible and independent of the particular salt used. Scatchard plot analysis showed that increasing the ionic strength resulted in both a decrease in the dissociation constant (Kd) and an increase in the amount bound at saturation (Bmax). Temperature also influenced the binding of hyaluronate to the cell surface. The amount bound at low temperatures (0 degrees C) was 3 to 5 times that bound at high temperatures (40 degrees C) with a sharp transition occurring at 18 degrees C, the temperature of phase transition of the plasma membrane. The temperature effect was primarily a change in the Bmax and was reversible. Finally the molecular weight of the ligand influenced the binding. High molecular weight preparations of hyaluronate had a higher binding affinity (lower Kd) and a lower Bmax than did smaller molecular weight preparations.     

Covalent binding of acetaminophen to mouse hemoglobin. Identification of major and minor adducts formed in vivo and implications for the nature of the arylating metabolites

When hepatotoxic doses of [ring-U-14C]acetaminophen ([ring-U-14C]APAP) were administered to mice, radioactivity became bound irreversibly to hemoglobin as well as to proteins in the liver and kidney. The covalent binding to hemoglobin was dose-dependent, and in phenobarbital-pretreated mice occurred to the extent of approximately 8% of the corresponding binding to liver proteins. Degradation of the modified globin by acid hydrolysis yielded 3-cystein-S-yl-4-hydroxyacetanilide as the major radioactive product, accounting for approximately 70% of protein-bound drug residues. This finding is consistent with the view that the majority of covalent binding of APAP to proteins is mediated by N-acetyl-p-benzoquinone imine (NAPQI), a reactive metabolite which preferentially arylates cysteinyl thiol residues. However, after administration of [acetyl-3H]APAP to mice, it was found that approximately 20% of the drug bound to hemoglobin had lost the N-acetyl side-chain, indicating the existence of a second type of APAP-protein adduct. One minor component of the globin hydrolysate was identified as S-(2,5-dihydroxyphenyl)-cysteine, which most likely arises from binding to hemoglobin of p-benzoquinone, a hydrolysis product of NAPQI. The two adducts reported represent the first identified examples of arylating drugs binding to hemoglobin. Experiments on the influence of different cytochrome P-450 inducing agents on the ratio of drug bound to hemoglobin versus hepatic proteins suggested that the reactive metabolites of APAP are formed in the liver and migrate to the erythrocyte, rather than being produced by hemoglobin-catalyzed oxidation of APAP. These findings imply that the reactive metabolites of APAP escape from hepatocytes in some latent forms, which then participate in the arylation of protein thiols in red blood cells and, possibly, at other remote sites.     

Binding of Clostridium botulinum neurotoxin to gangliosides

The binding characteristics of Clostridium botulinum neurotoxins of types B, C1, and F to gangliosides was studied by thin layer chromatography plate and microtiter plate methods at low (10 mM NaCl in 10 mM Tris-HCl buffer, pH 7.2) or high (150 mM NaCl in 10 mM Tris-HCl buffer, pH 7.2) ionic strengths and at 0 or 37 degrees C. The three types of toxins bound exclusively to three kinds of gangliosides, GD1a, GD1b, and GT1b, in both the thin layer chromatography plate and the microtiter plate methods. Type C1 toxin bound to the three gangliosides under all the conditions, while type B and F toxins bound only at low ionic strength and 37 degrees C. At low ionic strength, the binding kinetics for the three toxins was monophasic in Scatchard plots, and the association constants obtained in the microtiter plate system were 2-4 X 10(8) M-1. In contrast, the binding kinetics of type C1 toxin in high ionic strength was biphasic in the Scatchard plot, and two association constants were obtained in the microtiter plate system. The heavy chain facilitated the binding of the toxin to the gangliosides. These results indicate that different types of botulinum toxins bind to the gangliosides under different optimal conditions and that gangliosides may not be the common receptor for all types of botulinum toxins. The gangliosides may bind to type C1 toxin together with other potential receptor(s) on synaptosomal membranes.     

Reactions of electrophiles with nucleophilic thiolate sites: relevance to pathophysiological mechanisms and remediation

Electrophiles are electron-deficient species that form covalent bonds with electron-rich nucleophiles. In biological systems, reversible electrophile–nucleophile interactions mediate basal cytophysiological functions (e.g. enzyme regulation through S-nitrosylation), whereas irreversible electrophilic adduction of cellular macromolecules is involved in pathogenic processes that underlie many disease and injury states. The nucleophiles most often targeted by electrophiles are side chains on protein amino acids (e.g. Cys, His, and Lys) and aromatic nitrogen sites on DNA bases (e.g. guanine N7). The sulfhydryl thiol (RSH) side chain of cysteine residues is a weak nucleophile that can be ionized in specific conditions to a more reactive nucleophilic thiolate (RS^?). This review will focus on electrophile interactions with cysteine thiolates and the pathophysiological consequences that result from irreversible electrophile modification of this anionic sulfur. According to the Hard and Soft, Acids and Bases (HSAB) theory of Pearson, electrophiles and nucleophiles can be classified as either soft or hard depending on their relative polarizability. HSAB theory suggests that electrophiles will preferentially and more rapidly form covalent adducts with nucleophiles of comparable softness or hardness. Application of HSAB principles, in conjunction with in vitro and proteomic studies, have indicated that soft electrophiles of broad chemical classes selectively form covalent Michael-type adducts with soft, highly reactive cysteine thiolate nucleophiles. Therefore, these electrophiles exhibit a common mechanism of cytotoxicity. As we will discuss, this level of detailed mechanistic understanding is a necessary prerequisite for the rational development of effective prevention and treatment strategies for electrophile-based pathogenic states.     

Reactivity and binding of benzo(a)pyrene diol epoxide to poly(dG-dC).(dG-dC) and poly(dG-m5dC).(dG-m5dC) in the B and Z forms

The physical and covalent binding of the carcinogen benzo(a)pyrene-7,8-diol-9,10-oxide (BaPDE) to poly(dG-dC).(dG-dC) and poly(dG-m5dC).(dG-m5dC) in the B and Z forms were studied utilizing absorbance, fluorescence and linear dichroism techniques. In the case of poly(dG-dC).(dG-dC) the decrease in the covalent binding of BaPDE with increasing NaCl concentration (0.1-4 M) as the B form is transformed to the Z form is attributed to the effects of high ionic strengths on the reactivity and physical binding of BaPDE to the polynucleotides; these effects tend to obscure differences in reactivities with the B and Z forms of the nucleic acids. In the case of poly(dG-m5dC).(dG-m5dC) the B-to-Z transition is induced at low ionic strength (2 mM NaCl + 10 microM Co(NH3)6Cl3) and the covalent binding is found to be 2-3-times lower to the Z form than to the B form. Physical binding of BaPDE by intercalation, which precedes the covalent binding reaction, is significantly lower in the Z form than in the B form, thus accounting, in part, for the lower covalent binding. The linear dichroism characteristics of BaPDE covalently bound to the Z and B forms of poly(dG-m5dC).(dG-m5dC) are consistent with nonintercalative, probably external conformations of the aromatic pyrenyl residues.     

Reexamination of the carbohydrate binding stoichiometry of lima bean lectin

The carbohydrate binding stoichiometry of lima bean lectin component III was reexamined using equilibrium dialysis and quantitative affinity chromatography following limited chemical modification. Equilibrium dialysis employing methyl[2-14C]benzamido-2-deoxy-alpha-D-galactopyranoside as ligand demonstrated that the lectin tetramer bound 4 mol of sugar with Kassoc = 1.44 +/- 0.13 X 10(3) M-1 (T = 5 degrees C, pH 7.0, ionic strength 0.1). The previous report of two sites/tetramer [Bessler, W. and Goldstein, I. J. (1974) Arch. Biochem. Biophys. 165, 444] appears to be the result of partial inactivation of the lectin due to oxidation of essential thiol groups. Following limited chemical modification of the thiol groups by methyl methanethiosulfonate, multiple intermediate forms with reduced affinity for Synsorb A were obtained. The number and hemagglutinating activities of these intermediates provided further support for the presence of four carbohydrate binding sites on lima bean lectin component III.     

The interaction between Beta-lactoglobulin and sodium N-dodecyl sulphate.

1. The binding of sodium n-dodecyl sulphate to beta-lactoglobulin was studied in the pH range 3.5-7.0 by equilibrium dialysis, ultracentrifugation and microcalorimetry. 2. At low binding concentrations (less than 30 bound surfactants anions per protein molecule) the complexes formed aggregates in solution. 3. At higher binding concentrations aggregation does not occur at low ionic strength (0.01 mol/litre), but continues at high ionic strength (0.1 mol/litre). 4. At 25 degrees C the enthalpy of interaction of sodium n-dodecyl sulphate with beta-lactoglobulin can be interpreted as the sum of the enthalpies of formation of a complex with 2 bound surfactant anions, with an enthalpy change of -9.5 kJ-mol-1 of bound surfactant, and complexes containing at least 22 bound surfactant anions, with limiting enthalpies per bound surfactant anion of -12.4 kJ-mol-1 at pH 3.5 and -3.25 kJ-mol-1 at pH 5.5. 5. The binding of surfactant and the enthalpy of interaction at pH 3.5 ARE NOT SIGNIFICANTLY AFFECTED BY THE ADDITION Of 8 M-urea. 6. The data indicate that at low binding concentrations the interaction is of an ionic nature, and is accompanied by a conformational change in the protein.     

Interaction of C3b, B, and D in the alternative pathway of complement activation.

The interaction of ZXd2, an insoluble intermediate of the alternative pathway on zymosan (Z5), with factor B and the enzyme D proceeds in a two-step reaction: 1) B binds in the presence of Mg++ to ZXd2 to form the intermediate ZXd2B, 2) B bound to ZXd2 is subsequently activated enzymatically by D to yield the complex ZXd2B which cleaves C3. Evidence was obtained that C3b, which is present on ZXd2, is required for ZXd2B formation. Studies of the functional role of C3b for ZXd2B formation revealed that C3b is involved in the first reaction step i.e., binding of B to ZXd2 to yield ZXd2B. Formation of ZXd2B is inhibited by pretreatment of ZXd2 with either anti-C3 Fab or with C3b-INA. Low ionic strength of about 2 mS was found to favor the interaction of the C3b with B. Mg++ concentrations from 1 to 31 mM as well as variation of pH in the range from 6.2 to 8.5 did not greatly influence the reaction of B with ZXd2. For the enzymatic activation of B only C3b on ZXd2 and factor D are required. This is concluded from the finding that fluid phase C3b is sufficient for the activation of B in the presence of D. This does not exclude the fact that other proteins present on ZXd2 may help to stabilize the intermediate ZXd2B or the enzymatically active complex AXd2B, or both of them.     

Modelling complexation and electrostatic attraction in heavy metal biosorption by Sargassum biomass

Biosorption, the passive accumulation of metal ions by biomass, can be used for purifying metal bearing wastewater. Seaweeds represent a readily available source of biosorbent material that possesses a high metal binding capacity. For example, Sargassum can accumulate 2 mequiv of Cd per gram of biomass i.e. 10% of its dry weight. Binding of Cd and Cu by Sargassum is an ion exchange process involving both covalent and ionic bonds. The amount of cations bound covalently or by complexation can be predicted using multi-component sorption isotherms involving 2 types of binding sites, carboxyl and sulphate. A Donnan model was used to account for the effect of ionic strength and electrostatic attraction. The use of a multi-component isotherm that included one term for Na binding was less appropriate than the Donnan model for modelling ionic strength effects. It was possible to predict metal and proton binding as a function of the pH value, metal concentration and ionic strength of the solution. This revised version was published online in June 2006 with corrections to the Cover Date.     

Chloroacetates of 2- and 3-demethylthiocolchicine: specific covalent interactions with tubulin with preferential labeling of the beta-subunit.

We synthesized two chemically reactive A ring modified analogs of colchicine, 2-chloroacetyl-2-demethylthiocolchicine (2-CTC) and 3-chloroacetyl-3-demethylthiocolchicine (3-CTC). Both are similar to colchicine as inhibitors of tubulin polymerization and act as competitive inhibitors of colchicine binding (apparent Ki values, 3 microM). [14C]-labeled 2-CTC and 3-CTC bound to tubulin at 37 degrees C but not at 0 degree C, and bound drug formed covalent bond(s) with tubulin. The binding and covalent reactions were inhibited by podophyllotoxin. About 60% of the bound 3-CTC rapidly formed a covalent bond with tubulin. With 2-CTC the covalent reaction was slower than the binding reaction, and only one-third of the bound 2-CTC reacted covalently with tubulin. The ratio of radiolabel in beta-tubulin to that in alpha-tubulin was about 4:1 with both 2-CTC and 3-CTC.     

Structural basis of biological nitrile reduction

The enzyme QueF catalyzes the reduction of the nitrile group of 7-cyano-7-deazaguanine (preQ(0)) to 7-aminomethyl-7-deazaguanine (preQ(1)), the only nitrile reduction reaction known in biology. We describe here two crystal structures of Bacillus subtilis QueF, one of the wild-type enzyme in complex with the substrate preQ(0), trapped as a covalent thioimide, a putative intermediate in the reaction, and the second of the C55A mutant in complex with the substrate preQ(0) bound noncovalently. The QueF enzyme forms an asymmetric tunnel-fold homodecamer of two head-to-head facing pentameric subunits, harboring 10 active sites at the intersubunit interfaces. In both structures, a preQ(0) molecule is bound at eight sites, and in the wild-type enzyme, it forms a thioimide covalent linkage to the catalytic residue Cys-55. Both structural and transient kinetic data show that preQ(0) binding, not thioimide formation, induces a large conformational change in and closure of the active site. Based on these data, we propose a mechanism for the activation of the Cys-55 nucleophile and subsequent hydride transfer.