PqsBC, a Condensing Enzyme in the Biosynthesis of the Pseudomonas aeruginosa Quinolone Signal: CRYSTAL STRUCTURE, INHIBITION, AND REACTION MECHANISM

Pseudomonas aeruginosa produces a number of alkylquinolone-type secondary metabolites best known for their antimicrobial effects and involvement in cell-cell communication. In the alkylquinolone biosynthetic pathway, the β-ketoacyl-(acyl carrier protein) synthase III (FabH)-like enzyme PqsBC catalyzes the condensation of octanoyl-coenzyme A and 2-aminobenzoylacetate (2-ABA) to form the signal molecule 2-heptyl-4(1H)-quinolone. PqsBC, a potential drug target, is unique for its heterodimeric arrangement and an active site different from that of canonical FabH-like enzymes. Considering the sequence dissimilarity between the subunits, a key question was how the two subunits are organized with respect to the active site. In this study, the PqsBC structure was determined to a 2 Å resolution, revealing that PqsB and PqsC have a pseudo-2-fold symmetry that unexpectedly mimics the FabH homodimer. PqsC has an active site composed of Cys-129 and His-269, and the surrounding active site cleft is hydrophobic in character and approximately twice the volume of related FabH enzymes that may be a requirement to accommodate the aromatic substrate 2-ABA. From physiological and kinetic studies, we identified 2-aminoacetophenone as a pathway-inherent competitive inhibitor of PqsBC, whose fluorescence properties could be used for in vitro binding studies. In a time-resolved setup, we demonstrated that the catalytic histidine is not involved in acyl-enzyme formation, but contributes to an acylation-dependent increase in affinity for the second substrate 2-ABA. Introduction of Asn into the PqsC active site led to significant activity toward the desamino substrate analog benzoylacetate, suggesting that the substrate 2-ABA itself supplies the asparagine-equivalent amino function that assists in catalysis.

Homohalocyclization: Electrophilic Bromine-Induced Cyclizations of Cyclopropanes

An efficient method for the halocyclization of cyclopropanes has been developed. The cyclopropanes undergo a 1,3-addition reaction to form homohalocyclization products compared to conventional alkene halocyclizations. The reaction can be induced by various electrophilic halogenating agents including 1,3-dibromo-5,5-dimethylhydantoin and N-iodosuccinimide. In cyclopropane derivatives with a preexisting stereocenter, excellent induced diastereoselectivities can be observed.

Enantioselective haloetherifications catalyzed by 1,1'-bi-2-naphthol (BINOL) phosphates: from symmetrical alkenediols to simple alkenols

Asymmetric haloetherifications can be conducted using 1,1'-bi-2-naphthol (BINOL) phosphates as catalyst. In combination with simple N-haloamides such as N-iodopyrrolidinone or N-bromosuccinimide, good enantioselectivities can be achieved. However, depending on the substrate, the choice of BINOL phosphate is important, and different catalysts show remarkably different selectivities.

The influence of G-quadruplex structure on DNA-based asymmetric catalysis using the G-quadruplex-bound cationic porphyrin TMPyP4·Cu

The DNA G-quadruplex-binding porphyrin TMPyP4·Cu was found to form efficient hybrid catalysts for DNA-based asymmetric catalysis when bound to DNA G-quadruplexes. Nucleobase substitution experiments using human telomere-derived G-quadruplex sequences suggest binding of the porphyrin on the 3'-face.

Hydrolase-like properties of a cofactor-independent dioxygenase

Mechanistic promiscuity: The (2-alkyl)-3-hydroxy-4(1H)-quinolone-cleaving dioxygenase Hod has an α/β-hydrolase fold and a Ser/His/Asp triad in its active site. Isatoic anhydride, a suicide substrate of serine hydrolases, inactivates Hod by covalent modification of the active-site serine, thus indicating that the α/β-hydrolase fold can accommodate dioxygenase chemistry without completely abandoning hydrolase-like properties.

New catalytic approaches towards the enantioselective halogenation of alkenes

The addition of electrophilic reagents to the carbon-carbon double bond is one of the most fundamental reactions in organic chemistry. Halogen electrophiles constitute probably the most important class of electrophiles and have been widely used to induce electrophilic addition reactions to alkenes like halolactonizations or dihalogenations. Despite their long history and high importance, catalytic, asymmetric variants of these reactions have been underdeveloped until very recently. During the last two years this has changed and many novel approaches have been reported. This review aims to cover these new developments through discussing the common themes as well as the suggested mechanistic scenarios.

Enantioselective haloetherification by asymmetric opening of meso-halonium ions

A new approach to enantioselective haloetherification reactions via desymmetrization of in situ-generated meso-halonium ions is described. The combination of N-haloamides as a halogen source and sodium salts of chiral phosphoric acids as catalysts can be used for the cyclization of symmetrical ene-diol substrates, yielding the haloetherification products under practical conditions in enantioenriched form.

Sequential double α-arylation of N-allylureas by asymmetric deprotonation and N→C aryl migration

On lithiation with lithium amides, N-allyl-N'-aryl ureas undergo rearrangement with transfer of the aryl ring from N to the allylic α carbon. From the α-arylated products, a further aryl transfer under the influence of a chiral lithium amide allows the enantioselective construction of 1,1-diarylallylamine derivatives. Stereoselectivity in these reactions results from the enantioselective formation of a planar chiral allyllithium under kinetic control.

Crystal structure of the T(6-4)C lesion in complex with a (6-4) DNA photolyase and repair of UV-induced (6-4) and Dewar photolesions

UV-light irradiation induces the formation of highly mutagenic lesions in DNA, such as cis-syn cyclobutane pyrimidine dimers (CPD photoproducts), pyrimidine(6-4)pyrimidone photoproducts ((6-4) photoproducts) and their Dewar valence isomers ((Dew) photoproducts). Here we describe the synthesis of defined DNA strands containing these lesions by direct irradiation. We show that all lesions are efficiently repaired except for the T(Dew)T lesion, which cannot be cleaved by the repair enzyme under our conditions. A crystal structure of a T(6-4)C lesion containing DNA duplex in complex with the (6-4) photolyase from Drosophila melanogaster provides insight into the molecular recognition event of a cytosine derived photolesion for the first time. In light of the previously postulated repair mechanism, which involves rearrangement of the (6-4) lesions into strained four-membered ring repair intermediates, it is surprising that the not rearranged T(6-4)C lesion is observed in the active site. The structure, therefore, provides additional support for the newly postulated repair mechanism that avoids this rearrangement step and argues for a direct electron injection into the lesion as the first step of the repair reaction performed by (6-4) DNA photolyases.

Alpha-pyridylation of chiral amines via urea coupling, lithiation and rearrangement

2-, 3- and 4-Bromopyridine, along with other brominated electron-deficient arenes, undergo palladium-catalyzed coupling with ureas of structure RNMeCONHMe. Where R is a chiral, alpha-substituted benzyl group, treatment with LDA leads to a benzylic organolithium which undergoes rearrangement with stereospecific and regiospecific transfer of the pyridyl group, generating a quaternary stereogenic center with high enantioselectivity. Alcoholysis of the urea under neutral conditions reveals the pyridyl amine.