Spectroscopic Analysis of Tautomeric Forms of 5-Phenyltetrazole

Experimental and theoretical directions of electronic transition moments and the UV spectra of anionic and neutral 5-phenyltetrazole (5-PhTH) have been determined. Based on the correlation between the theoretical and experimental data it has been found that the 2 H-tautomeric form is predominant in polyvinyl alcohol (PVA) film.

Crystal structure of YecO from Haemophilus influenzae (HI0319) reveals a methyltransferase fold and a bound S-adenosylhomocysteine

The crystal structure of YecO from Haemophilus influenzae (HI0319), a protein annotated in the sequence databases as hypothetical, and that has not been assigned a function, has been determined at 2.2-A resolution. The structure reveals a fold typical of S-adenosyl-L-methionine-dependent (AdoMet) methyltransferase enzymes. Moreover, a processed cofactor, S-adenosyl-L-homocysteine (AdoHcy), is bound to the enzyme, further confirming the biochemical function of HI0319 and its sequence family members. An active site arginine, shielded from bulk solvent, interacts with an anion, possibly a chloride ion, which in turn interacts with the sulfur atom of AdoHcy. The AdoHcy and nearby protein residues delineate a small solvent-excluded substrate binding cavity of 162 A(3) in volume. The environment surrounding the cavity indicates that the substrate molecule contains a hydrophobic moiety and an anionic group. Many of the residues that define the cavity are invariant in the HI0319 sequence family but are not conserved in other methyltransferases. Therefore, the substrate specificity of YecO enzymes is unique and differs from the substrate specificity of all other methyltransferases sequenced to date. Examination of the Enzyme Commission list of methyltransferases prompted a manual inspection of 10 possible substrates using computer graphics and suggested that the ortho-substituted benzoic acids fit best in the active site.

Exploration of the conformational space of oxytocin and arginine-vasopressin using the electrostatically driven Monte Carlo and molecular dynamics methods

Conformational analysis of the neurohypophyseal hormones oxytocin (OT) and arginine-vasopressin (AVP) has been carried out using two different computational approaches and three force fields, namely by the Electrostatically Driven Monte Carlo (EDMC) method, with the Empirical Conformational Energy Program for Peptides (ECEPP/3) force field or with the ECEPP/3 force field plus a hydration-shell model, and by simulated-annealing molecular dynamics with the Consistent Valence Force Field (CVFF). The low-energy conformations obtained for both hormones were classified using the minimal-tree clustering algorithm and characterized according to the locations of beta-turns in the cyclic moieties. Calculations with the CVFF force field located conformations with a beta-turn at residues 3 and 4 as the lowest energy ones both for OT and for AVP. In the ECEPP/3 force field the lowest energy conformation of OT contained a beta-turn at residues 2 and 3, conformations with this location of the turn being higher in energy for AVP. The latter difference can be attributed to the difference in the size of the side chain in position 3 of the sequences: the bulkier phenylalanine residue of AVP in combination with the bulky Tyr2 residue hinders the formation of a turn at residues 2 and 3. Conformations of OT and AVP with a turn at residues 3,4 were in the best agreement with the x-ray structures of deaminooxytocin and pressinoic acid (the cyclic moiety of vasopressin), respectively, and with the nmr-derived distance constraints. Generally, the low-energy conformations obtained with the hydration-shell model were in a better agreement with the experimental data than the conformations calculated in vacuo. It was found, however, that the obtained low-energy conformations do not satisfy all of the nmr-derived distance constraints and the nuclear Overhauser effect pattern observed in nmr studies can be fully explained only by assuming a dynamic equilibrium between conformations with beta-turns at residues 2,3, 3,4, and 4,5. The low-energy structures of OT with a beta-turn at residues 2,3 have the disulfide ring conformations close to the model proposed recently for a potent bicyclic antagonist of OT [M. D. Shenderovich et al. (1994) Polish Journal of Chemistry, Vol. 25, pp. 921-927], although the native hormone differs from the bicyclic analogue by the conformation of the C-terminal tripeptide. This finding confirms the hypothesis of different receptor-bound conformations of agonists and antagonists of OT.

Molecular modeling of singlet-oxygen binding to anthraquinones in relation to the peroxidating activity of antitumor anthraquinone drugs

Anthraquinone derivatives are important anti-cancer drugs possessing, however, undesirable peroxidating and, in consequence, cardiotoxic properties. This results from the mediation by these compounds of the one-electron reduction processes of the oxygen molecule, which produces the highly toxic superoxide anion radical and other active oxygen species. This article summarizes the results of our studies on the molecular aspects of the mechanism of anthraquinone-mediated peroxidation which were carried out using enzymatic-assay, electrochemical, and quantum-mechanical methods.

Crystal structures of human calcineurin and the human FKBP12-FK506-calcineurin complex

Calcineurin (CaN) is a calcium- and calmodulin-dependent protein serine/threonine phosphate which is critical for several important cellular processes, including T-cell activation. CaN is the target of the immunosuppressive drugs cyclosporin A and FK506, which inhibit CaN after forming complexes with cytoplasmic binding proteins (cyclophilin and FKBP12, respectively). We report here the crystal structures of full-length human CaN at 2.1 A resolution and of the complex of human CaN with FKBP12-FK506 at 3.5 A resolution. In the native CaN structure, an auto-inhibitory element binds at the Zn/Fe-containing active site. The metal-site geometry and active-site water structure suggest a catalytic mechanism involving nucleophilic attack on the substrate phosphate by a metal-activated water molecule. In the FKBP12-FK506-CaN complex, the auto-inhibitory element is displaced from the active site. The site of binding of FKBP12-FK506 appears to be shared by other non-competitive inhibitors of calcineurin, including a natural anchoring protein.

A theoretical study of the mechanism of oxygen binding by model anthraquinones. Part II. Quantum-mechanical studies of the energetics of oxygen binding to model anthraquinones

Anthracycline derivatives, which constitute an important class of antitumor drugs, exhibit undesirable cardiotoxicity owing to their mediation in the process of oxygen reduction to the superoxide anion radical. Earlier work showed that this mediation could be facilitated by the formation of complexes with the 1 delta g oxygen molecule prior to reduction. In this paper, we investigate the energetics of the possible peroxides formed by a series of model anthraquinones: 1,4-dihydroxyl- (quinizarin), 1,8-dihydroxyl-, 1-hydroxy-8-methoxy-, 1,8-dimethoxy-, 1,4,5-trimethoxy- and 1,4-dihydroxy-5-methoxy-9,10-anthracenedione, as well as of daunorubicin and demethoxydaunorubicin, by semi-empirical quantum-mechanical MNDO and PM3 methods, and limited STO-3G ab initio calculations. It was found that the oxygen-binding site is determined by three factors: the high electron density and high HOMO coefficients on the carbon atoms to which oxygen binds, the minimum loss of conjugation within the anthraquinone moiety on oxygen binding and the minimum number of bonds to other heavy atoms of the oxygen-binding carbons (the steric effect). For different molecules, the energy of the most stable oxygen complex is the greatest for compounds with the lowest ionization potential. On the basis of this and our earlier studies, it was concluded that the anthracycline derivatives with reduced ability to bind oxygen and, therefore, reduced cardiotoxicity, should possess a high symmetry of II-electron density distribution, a high ionization potential and have all of the oxygen-binding sites condensed to other rings or substituted by bulky groups.

A theoretical study of the mechanism of oxygen binding by model anthraquinones. I: Quantum mechanical evaluation of the oxygen-binding sites of 1,4-hydroquinone

The undesirable cardiotoxicity of some important classes of antitumor drugs, such as anthracycline derivatives, is caused by their mediation of the one-electron reduction processes of the oxygen molecule which produces the highly toxic superoxide anion radical. Recent studies enable the conclusion to be drawn that the first and rate-limiting stage of this process is the formation of complexes of the drug anthraquinone moiety with 1 delta g molecular oxygen. The complexes can easily undergo one-electron reduction, whose product dissociates into the unchanged drug molecule and the superoxide anion radical. The present study reports quantum mechanical calculations of the structure and the energies of the possible oxygen complexes of the most simplified model compound: 1,4-benzenediol (1,4-hydroquinone); 2,3-dihydro-2,3-epidioxy-, 2,5-dihydro-2,5-epidioxy- and 1,4-epidioxy-1,4-benzenediol (the 2,3-, 2,5- and 1,4-peroxide). Calculations were carried out with the use of ab initio (STO-3G, 4-31G, and 6-31G) and semiempirical MNDO methods with total geometry optimization. The optimized geometry parameters were found to be in a reasonable agreement with the available crystal data. During the oxygen complex formation with hydroquinone, charge transfer occurs from hydroquinone to the oxygen molecule. Supplementary MNDO calculations have shown that the stability of 2,3-peroxide is increased substantially upon the ionization of one of the hydroxyl groups of hydroquinone prior to oxygen binding, which increases the electron density of the benzene ring. These findings result in a prediction that the anthracycline derivatives with electron-withdrawing substituents in the II-electron moiety should exhibit diminished affinity towards oxygen and, consequently, diminished ability to peroxidation. It has also been found that the relative energies of different peroxides are well represented even in the STO-3G ab initio calculations which will enable the further extension of the study to the complete II-electron moiety of the actual anthracycline derivatives.

A theoretical study of glucosamine synthase. II. Combined quantum and molecular mechanics simulation of sulfhydryl attack on the carboxyamide group

Continuing our theoretical studies of glucosamine synthase catalysis, we have carried out MNDO and ab initio calculations of the first stage of the reaction, which involves the attack of a cysteine thiol group from the enzyme active site on the side chain carboxyamide group of glutamine, producing ammonia and thioester. The reactants were modelled by methyl mercaptate and acetamide, respectively. For two considered mechanisms of the reaction the energy surfaces were evaluated. Mechanism I, proposed by Chmara et al. (1985) involves the nucleophilic attack of a deprotonated thiol group on the carbonyl carbon atom. Mechanism II, postulated in our previous work (Tempczyk et al. 1989), assumes the concerted binding of the mercaptate sulphur to the carbonyl carbon and the sulfhydryl hydrogen to the amide nitrogen with simultaneous breaking of the S-H bond. The energy surface of mechanism I shows no minimum on the approach of the mercaptide anion towards the carbonyl carbon, which is also consistent with ab initio calculations in a 4-31 G basis set. Therefore, mechanism I seems to be unlikely. The same analysis of mechanism II shows that it leads to the desired products: methyl thioacetate and ammonia. The presence of a sulfhydryl hydrogen causes apparent pyramidicity of the amido nitrogen and lengthening of the C-N bond in the transition state, making conditions for the release of the ammonia molecule. The MNDO calculated energy barrier of the reaction is 50.1 kcal/mol and the approximate 4-31 G ab initio barrier (at the MNDO geometries of the substrate complex and the transition state) is 63 kcal/mol.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular determinants of singlet oxygen binding by anthraquinones in relation to their redox cycling activity

A series of model anthraquinones with varying symmetry of pi-electron density distribution have been examined to verify our previous hypothesis concerning the essential role of quinone-singlet oxygen complex formation by asymmetric anthraquinones in their peroxidating properties. Comparison of the results of enzymatic studies using NADH dehydrogenase with those of cyclovoltammetric measurements fully confirmed the assumption that one-electron transfer mediation is facilitated by the preceding quinone-oxygen complex formation. To extend the scope of the molecular determinants of oxygen binding found in our previous studies, CNDO/2 and molecular electrostatic field (MEF) calculations have been performed. It has been concluded that the analysis of molecular electrostatic field as well as the dipole moment components has to be taken into account to judge whether a mutual orientation of the quinone and oxygen molecule can be reached which enables binding to occur. The second important factor is the appropriate symmetry of the quinone outer filled orbitals which assures that binding is not forbidden by the Woodward-Hoffman rules. These characteristics also explain the lack of oxygen binding by some asymmetric anthraquinones. The efficient electron transfer mediation be anthraquinones requires, beside the formation of the intermediate quinone-oxygen complex, effective catalysis of this process by oxidoreductase enzyme. The results obtained with model anthraquinones indicated that compounds with more than one phenolic group and an unsubstituted quinone carbonyl are good NADH dehydrogenase substrates. Imino derivatives and compounds with a reduced number or without free phenolic groups exhibit low affinity towards the enzyme.

A theoretical study of glucosamine synthase. Part I. Molecular mechanics calculations on substrate binding

Glucosamine synthase transfers the gamma-amino group of glutamine to fructose, producing 1-glucosamine which is the key constituent of bacterial and fungal cell walls. In this study, model calculations were performed on substrate binding to the enzyme active site. Two models of the active site of glucosamine synthase were proposed, which assume two different sequences of aminoacids, Cys-Gly-Ile and Cys-Ala-Cys, the first one being the N-terminal sequence of the Escherichia coli enzyme. Several initial geometries were assumed for these tripeptides, the energy was then optimized by means of molecular mechanics. It has been found that the structure which is both energy optimal and satisfies the assumed cysteine sulphur arrangement consists of combinations of C7eq and C7ax conformations of single residues. Molecular mechanics calculations were then performed on glutamine and D-fructose-6-phosphate, which are the substrates of the enzymatic catalysis, and on their complex with the enzyme glutamine-binding site. The spatial configuration of the compounds under study, which is optimal as far as the reaction path is concerned, also turned out to be an energy minimum.

Comparative conformational analysis of cholesterol and ergosterol by molecular mechanics

A comparative conformational analysis of cholesterol and ergosterol has been carried out using molecular mechanics methods. These studies are aimed at giving a better understanding of the molecular nature of the interaction of these sterols with polyene macrolide antibiotics. Structures of cholesterol and ergosterol determined by X-ray methods have been used as initial geometries of these molecules for force field calculations. The calculation of steric energy has also been made for conformations which do not appear in the crystal. The latter conformers have different conformations of the side chain as well as different conformations of rings A and D. The rotational barriers around bonds C17-C20 and C20-C22 have also been calculated. The results obtained on differences and similarities in the conformations of cholesterol and ergosterol allow us to postulate a mechanism for differential interaction with the antibiotics. The relatively rigid side chain of ergosterol (stretched molecule) in comparison with the flexible side chain of cholesterol (bent molecule), allows better intermolecular contact of the first sterol molecule with a polyene macrolide and in consequence facilitates complex formation involving Van der Waal's forces.

Theoretical studies of the mechanism of the action of the neurohypophyseal hormones. I. Molecular electrostatic potential (MEP) and molecular electrostatic field (MEF) maps of some vasopressin analogues

Continuing our theoretical studies of the oxytocin and vasopressin analogues, we have analysed the molecular electrostatic potential (MEP) and the norm of the molecular electrostatic field (MEF) of [1-beta-mercaptopropionic acid]-arginine-vasopressin ([ Mpa1]-AVP), [1-(beta-mercapto-beta,beta-cyclopentamethylene)propionic acid]-arginine-vasopressin ([Cpp']-AVP), and [1-thiosalicylic acid]-arginine-vasopressin ([Ths']-AVP) whose low-energy conformations were calculated in our previous work. These compounds are known from experiment to exhibit different biological activity. The scalar fields mentioned determine the energy of interaction with either charged (MEP) or polar (MEF) species, the energy being in the second case either optimal or Boltzmann-averaged over all the possible orientations of the dipole moment versus the electrostatic field. The electrostatic interactions slowly vanish with distance and can therefore be considered to be the factor determining the molecular shape at greater distances, which can help in both predicting the interactions with the receptor at the stage of remote recognition and in finding the preferred directions of solvation by a polar solvent. In the analysis of the fields three techniques have been used: (i) the construction of maps in certain planes; (ii) the construction of maps on spheres centered in the charge center of the molecule under study and of poles chosen according to the main axes of the quadrupole moment; and (iii) the construction of surfaces corresponding to a given value of potential. The results obtained show that the shapes of both MEP and MEF are similar in the case of [Mpa1]-AVP and [Cpp1]-AVP (biologically active), while some differences emerge when comparing these compounds with [Ths1]-AVP (inactive). It has also been found that both MEP and MEF depend even more strongly on conformation.

Molecular mechanics calculations on deaminooxytocin and on deamino-arginine-vasopressin and its analogues

The backbone conformations of the cyclic moieties of 1-[beta-mercaptopropionic acid]-oxytocin [( Mpa1]-OT), [1-beta-mercaptopropionic acid]-arginine-vasopressin [( Mpa1]-AVP), [1-(beta'-mercapto-beta,beta-cyclopentamethylene)propionic acid]-arginine-vasopressin [( Cpp1]-AVP), and [1-thiosalicylic acid]-arginine-vasopressin [( Ths1]-AVP) have been analyzed by means of molecular mechanics. In these calculations, the side chains were simulated by pseudoatoms. For the three last compounds, the calculations were also performed on the whole molecules, in order to shed light on the differences in their biological activity. Their starting conformations were obtained by attaching the acyclic tail and side chains to the lowest energy conformations of the cyclic parts. In the case of [Ths1]-AVP, however, other starting conformations were also examined, which were obtained by attaching the planar benzene ring to the lowest energy conformations of [Mpa1]-AVP. In the calculations, all the degrees of freedom were relaxed and Weiner's force field was used, the parameters required for the benzene parts of [Ths1]-AVP being determined from the experimental data available, as well as from the results of molecular dynamics calculations on the model compounds. The lowest energy conformations of [Mpa1]-AVP and [Cpp1]-AVP are similar, while [Ths1]-AVP differs from them near the disulphide region, due to the presence of a planar benzene ring. Interactions involving the charged guanidine group of arginine make, in each case, an important contribution to the conformational energy. A model description of the shapes of the oxytocin and vasopressin ring has been proposed, which is based on the cyclohexane geometry. This description is in good correlation with the energetics of the conformations corresponding to different shapes.

An alternative concept for the molecular nature of the peroxidating ability of anthracycline anti-tumor antibiotics and anthracenodiones

Quantum chemical calculations of model anthraquinone molecules using the CNDO/2 method have revealed that superoxide anion radical formation following the single electron transfer mediated by anthraquinone anti-tumor antibiotics may occur in aerobic conditions as a result of the direct addition of an electron to the anthraquinone-oxygen low energy charge transfer complex that is formed with singlet oxygen. Cyclovoltammetric measurements have been performed in order to provide experimental evidence supporting the hypothesis. The structural requirements for an anthraquinone molecule not exhibiting peroxidating ability by the above mechanism have been postulated. They include maximum symmetry of electron density distribution (symmetry of the molecule), a decrease of the electron density of the pi electron system and an increase in the rigidity of the molecule.