The physical chemistry of Hemes II. Electron paramagnetic resonance study of the interaction of some iron(III)-porphyrins with fluoride ion in dimethylformamide

The interaction of F⁻ with high and low spin ferric deuteroporphyrin IX dimethyl ester and a low spin model compound, bis(histidine methyl ester) deuterohemin IX has been studied in dimethylformamide solution by low-temperature EPR. The reaction of F⁻ with these complexes leads to high spin compounds. The structure of the EPR line at g = 2 is due to superhyperfine interactions with axial fluoride ligands. It allows their identification as mono- or difluoride complexes. Their optical absorption spectra are reported. In the particular cases of bis(imidazole) deuterohemin IX dimethyl ester and of the model compound, the variations of the EPR spectra as functions of concentration of ionic ligand are reported. Three new low spin complexes are thus obtained. They are characterized by a specific interaction of F⁻ with the NH group of the imidazole ring. This is proved following a second independent study in which we report the changes in g tensor principal values of low spin ferric porphyrins with the basicity (pK_a) of various nitrogenous bases.     

Chemo-selectivity of the N,O-enzymatic acylation in organic media and in ionic liquids

The chemo-selectivity and the efficiency of the enzymatic acylation of 6-amino-1-hexanol have been studied in organic solvents distinct by their nature and their dissociation power, in solvent-free systems corresponding to free fatty acid or ethyl ester media and in different ionic liquids. In organic solvents and fatty acid ester media, a sequential reaction allowed the major production of the diacylated derivative at the equilibrium state. Conversely, the use of a solvent-free system with free fatty acid orientated the reaction exclusively towards the O-acylation by modifying the ionization state of the amino group and decreased the reaction time to reach the equilibrium state. Ionic liquids as 1-butyl-3-methyl imidazolium cation coupled with anions of low nucleophilicity significantly improved the efficiency of the reaction (substrate conversion and initial rate) and also led to the N,O-diacyl product. The nature of the reaction medium was shown to influence the ionization state of functional groups, then their capacity to react, and finally, the efficiency of the reaction.     

Structure and enzymic activity of ribonuclease-A esterified at glutamic acid-49 and aspartic acid-53

The dimethyl ester of bovine pancreatic ribonuclease-A (dimethyl RNAase-A), the initial product of esterification of RNAase-A in anhydrous methanolic HCl, was isolated in a homogeneous form. The two carboxy functions esterified in this derivative are those of glutamic acid-49 and aspartic acid-53. There were no changes in the u.v.-absorption spectral characteristics, the accessibility of the methionine residues, the resistance of the protein to proteolysis by trypsin and the antigenic behaviour of RNAase-A as a result of the esterification of these two carboxy groups. Dimethyl RNAase-A exhibited only 65% of the specific activity of RNAase-A, but still had the same K_m value for both RNA and 2′:3′-cyclic CMP. However, the V_max. was decreased by about 35%. On careful hydrolysis of the methyl ester groups at pH9.5, dimethyl RNAase-A was converted back into RNAase-A. Limited proteolysis of dimethyl RNAase-A by subtilisin resulted in the formation of an active RNAase-S-type derivative, namely dimethyl RNAase-S, which was chromatographically distinct from dimethyl RNAase-A and had very nearly the same enzymic activity as dimethyl RNAase-A. Fractionation of dimethyl RNAase-S by trichloroacetic acid yielded dimethyl RNAase-S-protein and dimethyl RNAase-S-peptide, both of which were inactive by themselves but regenerated dimethyl RNAase-S when mixed together. Dimethyl RNAase-A-peptide was identical with RNAase-S-peptide. RNAase-S-protein could be generated from dimethyl RNAase-S-protein by careful hydrolysis of the methyl ester groups at pH9.5. The interaction of dimethyl RNAase-S-protein with RNAase-S-peptide appears to be about 4-fold weaker than that between the RNAase-S-protein and RNAase-S-peptide. Conceivably, the binding of the S-peptide `tail' of dimethyl RNAase-A with the remainder of the molecule is similarly weaker than that in RNAase-A, and this brings about subtle changes in the geometrical orientation of the active-site amino acid residues of these modified methyl ester derivatives. It is suggested that these changes could be responsible for the generation of the catalytically less-efficient RNAase-A and RNAase-S molecules (dimethyl RNAase-A and dimethyl RNAase-S respectively).     

The physical chemistry of hemes and hemopeptides III. Synthesis and physicochemical properties of cytochrome c-related heme-pentapeptide soluble in organic solvents

Synthesis is described of a N_α- and C-protected pentapeptide (Cys—Gly—Gly—Cys—His) which corresponds to the segment 14–18 in the primary structure of cytochrome c. The preparation of hemipentapeptide which contains peptide covalently linked to 2.4 side chains of the tetrapyrrolic ring is described.The hemipentapeptide is soluble in organic solvents and can be reduced in heterogeneous solvent mixture. In benzene its absorption and EPR spectra show that the iron ion is a high spin state. The titration of model compound in the oxidized and reduced forms was followed by optical absorption and it indicates a one-base fixation to form hexacoordinated complexes showing the presence of histidine residue liganded to the iron ion in the hemichrome and hemochrome obtained. The stability constants are 1.3 · 10³ and 1.7 · 10³ l/mole, respectively.From these results and by analogy with the absorption spectra of biological systems or others models, the presence of histidine residue on iron ion in the model compound is discussed.     

Structures and inclusion properties of racemic and enantiopure dimethyl and diphenyl 9,9'-bianthryl-2,2'-dicarboxylates

The authors prepared the dimethyl and diphenyl esters of 9,9'-bianthryl-2,2'-dicarboxylic acid in racemic and enantiopure (M) forms. The enantiopure dimethyl ester forms inclusion compounds with various organic compounds such as benzene, methanol, phenol, and aniline whereas the racemic form does this only with benzene. No guest molecules are included by the racemic and enantiopure diphenyl esters. These effects of substituents and homochirality on the inclusion properties are discussed on the basis of X-ray structures of some inclusion and guest-free compounds.     

Pyruvoyl-dependent histidine decarboxylase from Lactobacillus 30a. Covalent modifications of aspartic acid 191, lysine 155, and the pyruvoyl group

The pyruvoyl-dependent histidine decarboxylase from Lactobacillus 30a is rapidly inactivated by incubation with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and glycine ethyl ester. On 90% of inactivation, 1.3 residues of [14C]glycine ethyl ester are incorporated per alpha subunit; nearly 60% of this is linked to the beta-carboxyl group of Asp-191. Histamine, a competitive inhibitor, protects against this inactivation. The KM value of the modified enzyme for histidine (6.2 mM) is much higher than that of the unmodified enzyme (KM = 0.4 mM); catalytic activity is reduced but not eliminated. Thus, Asp-191 is the most reactive accessible carboxyl group under these conditions and is close to the substrate-binding site, but apparently is not essential for catalysis. At pH 8.0, fluorodinitrobenzene inactivates histidine decarboxylase completely with the incorporation of two dinitrophenyl residues/alpha subunit; the modified residues are Lys-155 and Cys-228. Urocanic acid, a competitive inhibitor, protects against inactivation. Treatment with mercaptoethanol restores the free -SH of Cys-228 but does not restore activity. Conversion of Cys-228 to its cyano derivative slows but does not prevent dinitrophenylation of Lys-155; the resulting derivative is catalytically inactive. Thus, Lys-155 is located within the active site and may play an essential role in catalysis. Finally, histidine methyl ester was shown to inhibit this decarboxylase by forming a Schiff's base with the essential pyruvoyl group.     

Structure and cooperativity of a T-state mutant of histidine decarboxylase from Lactobacillus 30a

Histidine decarboxylase (HDC) from Lactobacillus 30a converts histidine to histamine, a process that enables the bacteria to maintain the optimum pH range for cell growth. HDC is regulated by pH; it is active at low pH and inactive at neutral to alkaline pH. The X-ray structure of HDC at pH 8 revealed that a helix was disordered, resulting in the disruption of the substrate-binding site. The HDC trimer has also been shown to exhibit cooperative kinetics at neutral pH, that is, histidine can trigger a T-state to R-state transition. The D53,54N mutant of HDC has an elevated Km, even at low pH, indicating that the enzyme assumes the low activity T-state. We have solved the structures of the D53,54N mutant at low pH, with and without the substrate analog histidine methyl ester (HME) bound. Structural analysis shows that the apo-D53,54N mutant is in the inactive or T-state and that binding of the substrate analog induces the enzyme to adopt the active or R-state. A mechanism for the cooperative transition is proposed.     

1H-, 13C-, and 113Cd-nmr study of the Cd(II) complex of a blocked peptide,Z-Cys-Ala-Pro-His-OMe,in organic solvents

The Cd(II) complex of a peptide, Z-Cys-Ala-Pro-His-OMe was prepared and characterized by absorption, CD, ¹H-, ¹³C-, and ¹¹³Cd-nmr, and nuclear Overhauser effect spectroscopy (NOESY) spectra to show the coordination of cysteine thiolate and histidine imizazole to Cd(II) ion. The NOESY spectra in dimethyl formamide showed that the cysteine residue was in proximity to the histidine residue. These results reveal the dictation of Z-Cys-Ala-Pro-His-OMe to Cd(II) ion in solution. Temperature-dependent dissociation equilibrium of histidine imidazole in solution was observed in this complex. Structural features of the chelating peptide are discussed. © 1995 John Wiley & Sons, Inc.     

The alphastat hypothesis in respiratory control and acid-base balance

This selective review 1) evaluates recent interpretations that broaden the definition of the alphastat hypothesis, 2) proposes that central chemoreception and acid-base regulation via ion transport involve proteins conforming to the alphastat hypothesis, and 3) describes, using recent evidence, possible candidates for these proteins. The alphastat hypothesis states that proteins that contain appropriate function-determining titratable groups maintain a constant charge state and unaltered function with temperature-dependent pH changes but can be very sensitive to isothermal pH changes. Appropriate groups, e.g., imidazole histidine, are determined by the pK and the effect of temperature on the pK. The hypothesis explains how protein structure and function can be conserved among a diversity of vertebrate and invertebrate pH values. It also suggests a mechanism for sensing or regulating temperature-independent pH changes, e.g., in central chemosensitivity and transmembrane ion exchange. Possible candidates for such alphastat-conforming proteins include two, the glutamate receptor and the Na(+)-H+ antiporter, for which recent evidence indicates the presence of numerous histidines at probable function-determining sites and demonstrates pH sensitivity inhibitable by the histidine blocker diethylpyrocarbonate (DEPC).     

Pyrrolnitrin and phenazine production by Pseudomonas cepacia,strain 5.5B,a biocontrol agent of Rhizoctonia solani

Rhizoctonia stem rot of poinsettia caused by Rhizoctonia solani is controlled by strain 5.5B of Pseudomonas cepacia when poinsettia cuttings are rooted in polyfoam rooting cubes. Experiments were conducted to isolate and characterize secondary metabolites from strain 5.5B that were inhibitory towards R. solani. Inhibitory compounds were detected in fractions processed from liquid cultures of strain 5.5B. The most inhibitory compound isolated was pyrrolnitrin. A purple pigment consistently produced in culture by strain 5.5B was isolated and identified as 4,9-dihydroxyphenazine-1,6-dicarboxylic acid dimethyl ester, a phenazine. In vitro inhibition of R. solani occurred with the phenazine.     

Molecular modelling of human 5-hydroxytryptamine receptor (5-HT2A) and virtual screening studies towards the identification of agonist and antagonist molecules

The serotonin receptors, also known as 5-hydroxytryptamine (5-HT) receptors, are a group of G protein-coupled receptors (GPCRs) and ligand-gated ion channels found in the central and peripheral nervous systems. GPCRs have a characteristic feature of activating different signalling pathways upon ligand binding and these ligands display several efficacy levels to differentially activate the receptor. GPCRs are primary drug targets due to their central role in several signal transduction pathways. Drug design for GPCRs is also most challenging due to their inherent promiscuity in ligand recognition, which gives rise to several side effects of existing drugs. Here, we have performed the ligand interaction study using the two prominent states of GPCR, namely the active and inactive state of the 5-HT_2A receptor. Active state of 5-HT_2A receptor model enhances the understanding of conformational difference which influences the ligand-binding site. A 5-HT_2A receptor active state model was constructed by homology modelling using active state β₂-adrenergic receptor (β₂-AR). In addition, virtual screening and docking studies with both active and inactive state models reveal potential small molecule hits which could be considered as agonist-like and antagonist-like molecules. The results from the all-atom molecular dynamics simulations further confirmed that agonists and antagonists interact in different modes with the receptor.     

New syntheses of deuterated protoporphyrin-IX derivatives for heme protein nmr studies

An efficient total synthesis of 1,5-di(trideuteromethyl)protoporphyrin-IX (3) dimethyl ester from monopyrrole precursors is described, the synthesis proceeding through crystalline tripyrrene and a,c-biladiene salt intermediates. The 2- and 4-vinyl groups in (3) are formed from the corresponding (2-chloroethyl) substituents by way of base-promoted dehydrochlorination. In protio solvents, this synthetic step is shown to exchange out preferentially deuterons in the 1-methyl group, and this observation is exploited in an efficient synthesis of the 1,3-di(trideuteromethyl)protoporphyrin-IX (22) dimethyl ester from 2,4-diacetyldeuteroporphyrin-IX (20) dimethyl ester (which is in turn accessible from commercially available protoporphyrin-IX (5)). Thus, basic exchange in deuterated solvent of (20) gives the deuterated analog, which after reduction and dehydration gives the 1,3-di(trideuteromethyl)protoporphyrin-IX analog (22), in which the vinyl H₂ and propionic CH₂·CO functions have also become deuterated.     

P NMR and mass spectrometry of atropinesterase and some serine proteases phosphorylated with a transition-state analogue

The serine residue in the active center of atropinesterase (AtrE), alpha-chymotrypsin (Chymo), and subtilisin A (Sub) and in alpha-chymotrypsinogen (Chymogen) was labeled with a diisopropylphosphoryl (DP) group. The labeled proteins were studied in buffered aqueous solution under various native and denaturing conditions with 31P NMR before and after being subjected to "ageing", a process leading to conversion of the DP group into a monoisopropylphosphoryl (MP) group. Besides, the model compounds Gly-Ser(DP), Gly-Glu-Ser(DP)-Gly-OEt, and diisopropyl hydrogen phosphate were investigated under similar conditions and in other solvents with different hydrogen-bonding capacity. Mass spectrometry was used to analyze products resulting from ageing in the presence of H2(18)O. The 31P chemical shift of the DP proteins increases according to a simple titration curve upon lowering the pH from 9.0 to 5.0. This is ascribed to protonation of a particular histidine residue in the active center that interacts with a nearby isopropoxy group by hydrogen bonding with the ester oxygen. In DP-AtrE, hydrogen bonding at the phosphoryl oxygen dominates the interaction between substituent and protein; in the other DP proteins, nonbonding interactions become more dominant in the order Chymogen less than Chymo less than Sub. DP-AtrE, DP-Chymo, and DP-Sub age according to first-order kinetics. The pH dependence of the reaction rate constant ka indicates that ageing is catalyzed by the protonated histidine, which is responsible for the increase in chemical shift. The direct interaction between the phosphoryl group and the histidine is lost upon ageing whereas there is an increase in the nonbonding interaction of the remaining isopropyl group with the protein in the order Chymo less than Sub less than AtrE. The maximum value of ka when the histidine is fully protonated (kam) increases in the same order. Ageing of the DP enzymes occurs exclusively by C-O fission, yielding 2-propanol and propene. Since the amount of 2-propanol decreased and that of propene increased in the order Chymo to Sub to AtrE, the increase in kam has been interpreted as a shift in character of ageing from mainly SN2 for Chymo to considerably SN1 for AtrE and Sub. This has been attributed to preferential stabilization of the SN1 transition state by an interplay of hydrogen-bonding and nonbonding interactions between the phosphoryl group and the protein.(ABSTRACT TRUNCATED AT 400 WORDS)     

Structure-Biological Activity Relationships in Alfalfa Antimycotic Saponins: The Relative Activity of Medicagenic Acid and Synthetic Derivatives Thereof Against Plant Pathogenic Fungi1

The antimycotic activity of medicagenic acid and of some synthetic derivatives thereof was tested against plant pathogenic fungi. In general they all possess antimycotic activity. Furthermore, in the case of Sclerotium rolfsii, compounds where the hydroxyl functions of the aglycon remained unchanged (medicagenic acid and its dimethyl ester) or could be enzymically released (3-0-β-D-glucoside of medicagenic acid dimethyl ester) were significantly more active than compounds where these functions were modified by acetylation or methylation. Selective 2-0-methylation of medicagenic acid and comparison of the antimycotic activity of the resulting derivative against S. rolfsii to that of other derivatives suggests that a potential free hydroxyl at position 3 is essential to antimycotic activity.     

Preparation, hydrolysis and intramolecular transesterification of 3'-deoxy-3'-thioinosine 3'-S-dimethylphosphorothiolate

The hydrolytic reactions of the dimethyl ester of 3'-deoxy-3'-thioinosine 3'-S-phosphorothiolate have been followed over a wide aciditiy range by HPLC. At pH > 3, only hydroxide ion catalyzed isomerization to the 2'-dimethylphosphate takes place, whereas under more acidic conditions hydrolysis to the 2'-monomethylphosphate and 3'-S-monomethylphosphorothiolate competes. The latter is the only product accumulating in very acidic solutions (1 M hydrochloric acid). Mechanisms of the reactions are discussed.     

Studies of the conformation of bilirubin and its dimethyl ester in dimethyl sulphoxide solutions by nuclear magnetic resonance.

The conformation of bilirubin and its dimethyl ester in dimethyl sulphoxide (DMSO) was investigated by n.m.r. spectroscopy. The chemical shifts of the pyrrole NH and Lactam protons of bilirubin and its dimethyl ester in DMSO indicate a strong interaction with the solvent. Inter-proton distances were calculated from nuclear Overhauser effects (NOE), selective and non-selective relaxation times (T1) and rotational correlation times taken from 13C relaxation times. The interproton distances indicate that the conformation of the skeleton of bilirubin and its dimethyl ester in DMSO is similar to that of bilirubin and mesobilirubin in the crystalline state and in chloroform solutions, except for a possible slight twist of the pyrrolenone rings about the methine bonds, which may be a consequence of solvation of the NH groups by DMSO. Unlike in chloroform solutions, no direct hydrogen-bonding occurs between the carboxylic acid and the lactam groups of bilirubin in DMSO, as shown by the absence of an NOE between these groups. The fast exchange of the pyrrole NH protons with 2H shows that no hydrogen-bonding occurs between these protons and the propionic residues, in line with their solvation by DMSO. From the above results, and from the slowness of the internal motion of the propionic residues of bilirubin and its dimethyl ester, it is concluded that these residues are tied to the skeleton via bound solvent molecules.     

Graft copolymerization of methyl methacrylate onto mercaptochitin and some properties of the resulting hybrid materials

The graft copolymerization of methyl methacrylate onto mercaptochitin and some properties of the resulting graft copolymers have been studied. Methyl methacrylate was efficiently graft copolymerized onto mercaptochitin in dimethyl sulfoxide, and the grafting percentage reached 1300% under appropriate conditions. Although the side-chain ester groups were resistant to aqueous alkali, hydrolysis could be achieved with a mixture of aqueous sodium hydroxide and dimethyl sulfoxide. Subsequent treatment with acetic anhydride in methanol transformed the sodium carboxylate groups into carboxyl groups. Although the graft copolymers exhibited an improved affinity for organic solvents, those having sodium carboxylate or carboxyl units were characterized by a much more enhanced solubility and were soluble in common solvents. The hygroscopic nature of chitin decreased with an increase in the grafting extent but increased significantly upon hydrolysis of the ester groups. The enzymatic degradability of the graft copolymers, as evaluated with lysozyme, was also dependent on the grafting extent and much higher than that of the original chitin. DSC measurements revealed the presence of a glass transition phenomenon, which could be ascribed to the poly(methyl methacrylate) side chain.     

Impact of histidine residues on the transmembrane helices of viroporins

Abstract

The role of histidine in channel-forming transmembrane (TM) helices was investigated by comparing the TM helices from Virus protein ‘u' (Vpu) and the M2 proton channel. Both proteins are members of the viroporin family of small membrane proteins that exhibit ion channel activity, and have a single TM helix that is capable of forming oligomers. The TM helices from both proteins have a conserved tryptophan towards the C-terminus. Previously, alanine 18 of Vpu was mutated to histidine in order to artificially introduce the same HXXXW motif that is central to the proton channel activity of M2. Interestingly, the mutated Vpu TM resulted in an increase in helix tilt angle of 11° in lipid bilayers compared to the wild-type Vpu TM. Here, we find the reverse, when histidine 37 of the HXXXW motif in M2 was mutated to alanine, it decreased the helix tilt by 10° from that of wild-type M2. The tilt change is independent of both the helix length and the presence of tryptophan. In addition, compared to wild-type M2, the H37A mutant displayed lowered sensitivity to proton concentration. We also found that the solvent accessibility of histidine-containing M2 is greater than without histidine. This suggests that the TM helix may increase the solvent exposure by changing its tilt angle in order to accommodate a polar/charged residue within the hydrophobic membrane region. The comparative results of M2, Vpu and their mutants demonstrated the significance of histidine in a transmembrane helix and the remarkable plasticity of the function and structure of ion channels stemming from changes at a single amino acid site.