Functional Characterization of Three Specific Acyl-Coenzyme A Synthetases Involved in Anaerobic Cholesterol Degradation in Sterolibacterium denitrificans Chol1S.

The denitrifying betaproteobacterium Sterolibacterium denitrificans Chol1S catabolizes steroids such as cholesterol via an oxygen-independent pathway. It involves enzyme reaction sequences described for aerobic cholesterol and bile acid degradation as well as enzymes uniquely found in anaerobic steroid-degrading bacteria. Recent studies provided evidence that in S. denitrificans, the cholest-4-en-3-one intermediate is oxygen-independently oxidized to Δ4-dafachronic acid (C26-oic acid), which is subsequently activated by a substrate-specific acyl-coenzyme A (acyl-CoA) synthetase (ACS). Further degradation was suggested to proceed via unconventional ß-oxidation, where aldolases, aldehyde dehydrogenases, and additional ACSs substitute for classical ß-hydroxyacyl-CoA dehydrogenases and thiolases. Here, we heterologously expressed three cholesterol-induced genes that putatively code for AMP-forming ACSs and characterized two of the products as specific 3ß-hydroxy-Δ5-cholenoyl-CoA (C24-oic acid)- and pregn-4-en-3-one-22-oyl-CoA (C22-oic acid)-forming ACSs, respectively. A third heterologously produced ATP-dependent ACS was inactive with C26-, C24-, or C22-oic-acids but activated 3aα-H-4α-(3'propanoate)-7aß-methylhexahydro-1,5-indanedione (HIP) to HIP-CoA, a rather late intermediate of aerobic cholesterol degradation that still contains the CD rings of the sterane skeleton. This work provides experimental evidence that anaerobic steroid degradation proceeds via numerous alternate CoA-ester-dependent or -independent enzymatic reaction sequences as a result of aldolytic side chain and hydrolytic sterane ring C-C bond cleavages. The aldolytic side chain degradation pathway comprising highly exergonic ACSs and aldehyde dehydrogenases is considered to be essential for driving the unfavorable oxygen-independent C26 hydroxylation forward.IMPORTANCE The biological degradation of ubiquitously abundant steroids is hampered by their low solubility and the presence of two quaternary carbon atoms. The degradation of cholesterol by aerobic Actinobacteria has been studied in detail for more than 30 years and involves a number of oxygenase-dependent reactions. In contrast, much less is known about the oxygen-independent degradation of steroids in denitrifying bacteria. In the cholesterol-degrading anaerobic model organism Sterolibacterium denitrificans Chol1S, initial evidence has been obtained that steroid degradation proceeds via numerous alternate coenzyme A (CoA)-ester-dependent/independent reaction sequences. Here, we describe the heterologous expression of three highly specific and characteristic acyl-CoA synthetases, two of which play key roles in the degradation of the side chain, whereas a third one is specifically involved in the B ring degradation. The results obtained shed light into oxygen-independent steroid degradation comprising more than 40 enzymatic reactions.

Structural characterization of O- and C-glycosylating variants of the landomycin glycosyltransferase LanGT2.

The structures of the O-glycosyltransferase LanGT2 and the engineered, C-C bond-forming variant LanGT2S8Ac show how the replacement of a single loop can change the functionality of the enzyme. Crystal structures of the enzymes in complex with a nonhydrolyzable nucleotide-sugar analogue revealed that there is a conformational transition to create the binding sites for the aglycon substrate. This induced-fit transition was explored by molecular docking experiments with various aglycon substrates.

Mechanistic insights into a supramolecular self-assembling catalyst system: evidence for hydrogen bonding during rhodium-catalyzed hydroformylation.

The structural integrity and flexibility provided by intermolecular hydrogen bonds leads to the outstanding properties of the 6-diphenylphosphinopyridin-(2H)-1-one ligand (see scheme) in the rhodium-catalyzed hydroformylation of terminal alkenes, as demonstrated by the combination of spectroscopic methods and DFT computations. Hydrogen bonds were also detected in a competent intermediate of the catalytic cycle.

Ligand self-assembling through complementary hydrogen-bonding in the coordination sphere of a transition metal center: the 6-diphenylphosphanylpyridin-2(1H)-one system.

Motivated by previous findings which had shown that transition metal catalysts based on the 6-diphenylphosphanylpyridone ligand (6-DPPon, 2) display properties as a self-assembling bidentate ligand-metal complex, we have performed a thorough study on the bonding situation of this ligand, alone and in the coordination sphere of a late transition metal. Thus, combining a number of spectroscopic methods (UV-vis, IR, NMR, X-ray), we gained insights into the unique structural characteristics of 2. These experimental studies were corroborated by DFT calculations, which were in all cases in good agreement with the experimental results. The free ligand 2 prefers to exist as the pyridone tautomer 2A and dimerizes to the pyridone-pyridone dimer 4A in solution as well as in the crystal state. The corresponding hydroxypyridine tautomer 2B is energetically slightly disfavored (ca. 0.9 kcal/mol within the up-conformer relevant for metal coordination); hence, hydrogen bond formation within the complex may easily compensate this small energy penalty. Coordination properties of 2 were studied in the coordination sphere of a platinum(II) center. As a model complex, [Cl(2)Pt(6-DPPon)(2)] (11) was prepared and investigated. All experimental and theoretical methods used prove the existence of a hydrogen-bonding interligand network in solution as well as in the crystal state of 11 between one 6-DPPon ligand existing as the pyridone tautomer 2A and the other ligand occupying the complementary hydroxypyridine form 2B. Dynamic proton NMR allowed to determine the barrier for interligand hydrogen bond breaking and, in combination with theory, enabled us to determine the enthalpic stabilization through hydrogen-bonding to contribute 14-15 kcal/mol.

Hydrogen-bond and solvent dynamics in transition metal complexes: a combined simulation and NMR-investigation.

Self-assembling ligands through complementary hydrogen-bonding in the coordination sphere of a transition metal provides catalysts with unique properties for carbon-carbon and carbon-heteroatom formation. Their most distinguishing chemical bonding pattern is a double-hydrogen-bonded motif, which determines much of the chemical functionality. Here, we discuss the possibility of double proton transfer (DPT) along this motif using computational and experimental methods. The infrared and NMR spectral signatures for the double-hydrogen-bonded motif are analyzed. Atomistic simulations and experiments suggest that the dynamics of the catalyst is surprisingly complex and displays at least three different dynamical regimes which can be distinguished with NMR spectroscopy and analyzed from computation. The two hydrogen bonds are kept intact and in rapid tautomeric exchange down to 125 K, which provides an estimate of 5 kcal/mol for the barrier for DPT. This is confirmed by the simulations which predict 5.8 kcal/mol for double proton transfer. A mechanistic interpretation is provided and the distribution of the solvent shell surrounding the catalyst is characterized from extensive simulations.

Self-assembly of bidentate ligands for combinatorial homogeneous catalysis: asymmetric rhodium-catalyzed hydrogenation.

The first chiral ligand library based on self-assembly through complementary hydrogen-bonding was realized. From a 10 x 4 ligand library, catalysts that show excellent activity and enantioselectivity for the asymmetric rhodium-catalyzed hydrogenation have been identified.

Hydrogen bonding as a construction element for bidentate donor ligands in homogeneous catalysis: regioselective hydroformylation of terminal alkenes.

A new concept for the construction of bidentate ligands for homogeneous metal complex catalysis is described. The concept relies on the self-assembly of monodentate ligands through hydrogen bonding. As a prototype of such systems, 6-diphenylphosphanyl-2-pyridone (6-DPPon) was shown to form a chelate in the coordination sphere of a transition metal center through unusual pyridone/hydroxypyridine hydrogen bonding (X-ray). This hydrogen bonding stays intact in a catalytic reaction as proven upon highly regioselective hydroformylation of terminal alkenes. Regioselectivities and reactivities observed rank the 6-DPPon/rhodium system among the most active and regioselective catalysts for n-selective hydroformylation of terminal alkenes.