Mechanism of the glutathione transferase-catalyzed conversion of antitumor 2-crotonyloxymethyl-2-cycloalkenones to GSH adducts

Human glutathione (GSH) transferase (hGSTP1-1) processes with similar kinetic efficiencies the antitumor agents 2-crotonyloxymethyl-2-cyclohexenone (COMC-6), 2-crotonyloxymethyl-2-cycloheptenone (COMC-7), and 2-crotonyloxymethyl-2-cyclopentenone (COMC-5) to 2-glutathionylmethyl-2-cyclohexenone, 2-glutathionylmethyl-3-glutathionyl-2-cycloheptenone, and 2-glutathionylmethyl-2-cyclopentenone, respectively. This process likely involves initial enzyme-catalyzed Michael addition of GSH to the COMC derivative to give a glutathionylated enol(ate), which undergoes nonstereospecific ketonization, either while bound to the active site or free in solution, to a glutathionylated exocyclic enone. Free in solution, GSH reacts at the exomethylene carbon of the exocyclic enone, displacing the first GSH to give the final product. This mechanism is supported by the observation of multiphasic kinetics in the presence of high concentrations of hGSTP1-1 and the ability to trap kinetically competent exocyclic enones in aqueous acid using COMC-6 and COMC-7 as substrates. That the exocyclic enone is formed by nonstereospecific ketonization of an enol(ate) species is indicated by the observation that COMC-6 (chirally labeled with deuterium at the exomethylene carbon) gives stereorandomly labeled exocyclic enone. The isozymes hGSTP1-1, hGSTA1-1, hGSTA4-4, and hGSTM2-2 catalyze the conversion of COMC-6 to final product with similar efficiencies (K(m) = 0.08-0.34 mM, k(cat) = 1.5-6.1 s(-)(1)); no activity was detected with the rat rGSTT2-2 isozyme. Molecular docking studies indicate that in hGSTP1-1, the hydroxyl group of Tyr108 might serve as a general acid catalyst during substrate turnover. The possible significance of these observations with respect to the metabolism of COMC derivatives in multidrug resistant tumors is discussed.     

The `cyclophene' [2.2.2](1,2,4)cyclophan‐9‐ene

In the title compound, C₁₈H₁₆, the [2.2]paracyclophane geometry is restrained to a considerable extent despite the introduction of the extra C=C bridge; typical paracyclophane features, such as the elongated C—C bridges, are still observed. However, the bridgehead atoms of the C=C bridge are forced into unusually close proximity [2.657 (3) Å], which in turn causes the rings to be rotated to an interplanar angle of 13.7 (2)°. The packing involves hexagonally close‐packed layers of molecules parallel to the xy plane, corresponding to the known `7,11' pattern of paracyclophanes, but without significant short intermolecular contacts.     

C—H...π interactions in five ethynyl‐substituted [2.2]paracyclophanes: further examples of the `7,11' packing pattern

The monosubstituted derivative 4‐ethynyl[2.2]paracyclophane, C₁₈H₁₆, (I), and the four disubstituted isomers, 4,12‐, (II), 4,13‐, (III), 4,15‐, (IV), and 4,16‐diethynyl[2.2]paracyclophane, (V), all C₂₀H₁₆, show the usual distortions of the [2.2]paracyclophane framework. The crystal packing is analyzed in terms of C—H...π interactions, some with H...π as short as 2.47 Å, in which the cyclophane rings and/or the triple‐bond systems may act as acceptors. For compounds (I) and (IV), the known `7,11'‐type cyclophane packing is observed, with a herring‐bone pattern of molecules in a layer structure.     

5‐Methyl‐N‐[2‐nitro‐4‐(trifluoromethyl)phenyl]‐1H‐pyrazole‐3‐amine: a chain of hydrogen‐bonded R22(6) and R22(16) rings

The molecular skeleton of the title compound, C₁₁H₉F₃N₄O₂, is almost planar and exhibits a polarized (charge‐separated) electronic structure in the nitroaniline portion. Molecules are linked by N—H...N and C—H...O hydrogen bonds to form a chain in which centrosymmetric R₂²(6) and R₂²(16) rings alternate.     

Coexpression of CPR from Various Origins Enhances Biotransformation Activity of Human CYPs in S. pombe

Cytochrome P450 enzymes (CYPs or P450s) are the most important enzymes involved in the phase I metabolism of drugs (and other xenobiotics) in humans, and the corresponding drug metabolites are needed as reference substances for their structural confirmation and for pharmacological or toxicological characterization. We have previously shown that biotechnological synthesis of such metabolites is feasible by whole-cell biotransformation with human CYPs recombinantly expressed in the fission yeast Schizosaccharomyces pombe. It was the aim of this study to compare the activity of seven human microsomal CYPs (CYP2C9, CYP2D6, CYP3A4, CYP3A5, CYP3A7, CYP17, and CYP21) upon coexpression with NADPH-cytochrome P450 oxidoreductases (CPRs) from various origins, namely, human CPR (hCPR) and its homologues from fission yeast (ccr1) and the bishop’s weed Ammi majus (AmCPR), respectively. For this purpose, 28 recombinant strains were needed, with five of them having been constructed previously and 23 strains being newly constructed. Bioconversion experiments showed that coexpression of a CPR does not only influence the reaction rate but, in some cases, also exerts an influence on the metabolite pattern. For CYP3A enzymes, coexpression of hCPR yielded the best results, while for another two, hCPR was equally helpful as ccr1 (both CYP17 and CYP21) or AmCPR (CYP17 only), respectively. Interestingly, CYP2D6 displayed its highest activity when coexpressed with ccr1 and CYP2C9 with AmCPR. These results corroborate the view of CPR as a well-suited bio-brick in synthetic biology for the construction of artificial enzyme complexes.     

Chemical Probes for the Functionalization of Polyketide Intermediates

A library of functionalized chemical probes capable of reacting with ketosynthase‐bound biosynthetic intermediates was prepared and utilized to explore in vivo polyketide diversification. Fermentation of ACP mutants of S. lasaliensis in the presence of the probes generated a range of unnatural polyketide derivatives, including novel putative lasalocid A derivatives characterized by variable aryl ketone moieties and linear polyketide chains (bearing alkyne/azide handles and fluorine) flanking the polyether scaffold. By providing direct information on microorganism tolerance and enzyme processing of unnatural malonyl‐ACP analogues, as well as on the amenability of unnatural polyketides to further structural modifications, the chemical probes constitute invaluable tools for the development of novel mutasynthesis and synthetic biology.