No abstract

The reactivity of a series of para-substituted phenolic compounds in the peroxidation catalyzed by chloroperoxidase was investigated, and the results were interpreted on the basis of the binding characteristics of the substrates to the active site of the enzyme. Marked selectivity effects are observed. These operate through charge, preventing phenolic compounds carrying amino groups on the substituent chain to act as substrates for the enzyme, and through size, excluding potential substrates containing bulky substituents to the phenol nucleus. Also, chiral recognition is exhibited by chloroperoxidase in the oxidation of N-acetyltyrosine, where only the L isomer is oxidized. Kinetic measurements show that, in general, the efficiency of chloroperoxidase in the oxidation of phenols is lower than that of horseradish peroxidase. Paramagnetic NMR spectra and relaxation rate measurements of chloroperoxidase-phenol complexes are consistent with binding of the substrates close to the heme, in the distal pocket, with the phenol group pointing toward the iron atom. On the other hand, phenolic compounds which are not substrates for chloroperoxidase bind to the enzyme with a much different disposition, with the phenol group very distant from the iron and probably actually outside the active-site cavity.

No abstract

A series of 2-[[(4-aryl-1-piperazinyl)alkyl]thio]thieno[2,3-d]pyrimidin-4 (1H)-one and 3-substituted 2-[[(4-aryl-1-piperazinyl)alky]thio]thieno[2,3-d]pyrimidin-4 (3H)-one derivatives was prepared and evaluated for in vitro 5-HT1A receptor affinity by radioligand binding assays; the selectivity for 5-HT1A receptors rather than alpha 1-adrenoceptors was also examined (ratio of the IC50 alpha 1 to IC50 5-HT1A). The binding tests gave indications about the best features of the [(arylpiperazinyl)alkyl]thio moiety and of the substituents on the thiophene and pyrimidinone rings for efficacious and selective 5-HT1A ligands. The most effective derivative for displacing [3H]-8-OH-DPAT from rat hippocampal membranes was the 3-amino-2-[[3-[4-(2-methoxyphenyl)-1-piperazinyl] propyl]thio]-5,6-dimethylthieno[2,3-d]pyrimidin-4(3H)-one (70) (IC50 = 0.3 nM) with selectivity of 24 for the 5-HT1A over the alpha 1-adrenoceptor. Compound 73, where the 2-methoxyphenyl on the N4 piperazine ring was replaced with a pyrimidine group, showed the best selectivity, with a ratio of 74, while its affinity IC50 for 5-HT1A was 6.8 nM. These results, compared to those for compounds 46 (IC50 24 nM; selectivity 2) and 49 (IC50 226 nM; selectivity 5), N3 unsubstituted analogues of derivatives 70 and 73, show the importance of an amino group in position 3 of the thienopyrimidine system for the interaction with 5-HT1A receptor binding sites, although this fragment can affect the affinity and selectivity only if linked to the (arylpiperazinyl)alkyl moiety. The better selectivity of piperidine 74 (IC50 0.8; selectivity 45) compared to the analogous piperazine 70 is also noteworthy. Twenty of the 30 molecules used for determining the binding affinity to 5-HT1A and alpha 1-adrenergic receptors were selected for QSAR analysis using a series of molecular descriptors and calculated with the TSAR software.

No abstract

A detailed three-dimensional model of manganese peroxidase was constructed using lignine peroxidase as the structural scaffold. This is the only protein in the peroxidase family except for cytochrome c peroxidase for which a resolved crystal structure is available.The model was built using the following procedure: (a) structurally preserved regions were derived from similar regions in the sequence alignment of the two proteins; (b) non-similar regions were modelled by searching a set of resolved protein structures for fragments which fitted in geometrically and choosing the best fitting fragment. Side chains were constructed by calculating rotamer-rotamer interaction energies and minimizing intramolecular energy. Model refinement was performed by molecular mechanics calculation.The quality of the model was assessed on the basis of the propensity of the amino acids to be inserted into regular secondary-structure elements and to be exposed to solvent.All the lignine peroxidase regions not used for model construction because of the lack of similarity, except the helix fragment Leu261 -Phe269, correspond to external loops, suggesting reliable modelling.The manganese peroxidase model structure was analyzed in detail and several functionally relevant structural features were predicted, the most important being : (a) the very close structural similarity between lignine and manganese peroxidase active sites, suggesting a similar mode of hydrogen peroxide activation; (b) the substitution of polar residues for the hydrophobic amino acids exposed at the edge of the channel involved in substrate recognition in lignine peroxidase, suggesting that manganese peroxidase does not directly bind aromatic substrates; (c) the location of residues potentially able to bind Mn2+, spatially positioned on the side of the 3-CH3 heme edge.

A detailed three-dimensional model of manganese peroxidase was constructed using lignine peroxidase as the structural scaffold. This is the only protein in the peroxidase family except for cytochrome c peroxidase for which a resolved crystal structure is available.The model was built using the following procedure: (a) structurally preserved regions were derived from similar regions in the sequence alignment of the two proteins; (b) non-similar regions were modelled by searching a set of resolved protein structures for fragments which fitted in geometrically and choosing the best fitting fragment. Side chains were constructed by calculating rotamer-rotamer interaction energies and minimizing intramolecular energy. Model refinement was performed by molecular mechanics calculation.The quality of the model was assessed on the basis of the propensity of the amino acids to be inserted into regular secondary-structure elements and to be exposed to solvent.All the lignine peroxidase regions not used for model construction because of the lack of similarity, except the helix fragment Leu261 -Phe269, correspond to external loops, suggesting reliable modelling.The manganese peroxidase model structure was analyzed in detail and several functionally relevant structural features were predicted, the most important being : (a) the very close structural similarity between lignine and manganese peroxidase active sites, suggesting a similar mode of hydrogen peroxide activation; (b) the substitution of polar residues for the hydrophobic amino acids exposed at the edge of the channel involved in substrate recognition in lignine peroxidase, suggesting that manganese peroxidase does not directly bind aromatic substrates; (c) the location of residues potentially able to bind Mn2+, spatially positioned on the side of the 3-CH3 heme edge.

High Affinity and Selectivity of [[(Arylpiperazinyl)alkyl]thio]thieno [2,3-d]pyrimidinone Derivatives for the 5-HT 1A Receptor. Synthesis and Structure-Affinity Relationships.-Modifications performed on the lead thienopyrimidinone refer in particular to: 1) the variation of substituents of the thienopyrimidine nucleus and the substitution of the thienopyrimidine system; 2) the substitution of the amino group of the pyrimidine ring and 3) the substitution on the piperazine ring of the orthomethoxyphenyl nucleus. The affinity of the new pyrimidinones like (V), (IX), and (XV) for 5-HT 1A receptors and the selectivity versus α 1 A receptors are studied. The importance of the non-pharmacophoric thienopyrimidine portion for affinity and selectivity towards 5-HT 1A receptor is obvious. Compound (Va), structurally related to the lead compound, and the isosteric pyridopyrimidinone (IX) show the highest affinity and selectivity.