The enantioselectivity of quinohaemoprotein alcohol dehydrogenases: mechanistic and structural aspects

Derivatives of natural organocatalytic cofactors and artificial organocatalytic cofactors as catalysts in enzymes

Catalytically active non-metal cofactors in enzymes carry out a variety of different reactions. The efforts to develop derivatives of natural occurring cofactors such as flavins or pyridoxal phosphate and the advances to design new, non-natural cofactors are reviewed here. We report the status quo for enzymes harboring organocatalysts as derivatives of natural cofactors or as artificial ones and their application in the asymmetric synthesis of various compounds.

PQQ-dependent Dehydrogenase Enables One-pot Bi-enzymatic Enantio-convergent Biocatalytic Amination of Racemic sec-Allylic Alcohols

The asymmetric amination of secondary racemic allylic alcohols bears several challenges like the reactivity of the bi-functional substrate/product as well as of the α,β-unsaturated ketone intermediate in an oxidation-reductive amination sequence. Heading for a biocatalytic amination cascade with a minimal number of enzymes, an oxidation step was implemented relying on a single PQQ-dependent dehydrogenase with low enantioselectivity. This enzyme allowed the oxidation of both enantiomers at the expense of iron(III) as oxidant. The stereoselective amination of the α,β-unsaturated ketone intermediate was achieved with transaminases using 1-phenylethylamine as formal reducing agent as well as nitrogen source. Choosing an appropriate transaminase, either the (R)- or (S)-enantiomer was obtained in optically pure form (>98 % ee). The enantio-convergent amination of the racemic allylic alcohols to one single allylic amine enantiomer was achieved in one pot in a sequential cascade.

Impact of the lanthanide contraction on the activity of a lanthanide-dependent methanol dehydrogenase - a kinetic and DFT study

Interest in the bioinorganic chemistry of lanthanides is growing rapidly as more and more lanthanide-dependent bacteria are being discovered. Especially the earlier lanthanides have been shown to be preferentially utilized by bacteria that need these Lewis acids as cofactors in their alcohol dehydrogenase enzymes. Here, we investigate the impact of the lanthanide ions lanthanum(iii) to lutetium(iii) (excluding Pm) on the catalytic parameters (vmax, KM, kcat/KM) of a methanol dehydrogenase (MDH) isolated from Methylacidiphilum fumariolicum SolV. Kinetic experiments and DFT calculations were used to discuss why only the earlier lanthanides (La-Gd) promote high MDH activity. Impact of Lewis acidity, coordination number preferences, stability constants and other properties that are a direct result of the lanthanide contraction are discussed in light of the two proposed mechanisms for MDH.

Similar but Not the Same: First Kinetic and Structural Analyses of a Methanol Dehydrogenase Containing a Europium Ion in the Active Site

Since the discovery of the biological relevance of rare earth elements (REEs) for numerous different bacteria, questions concerning the advantages of REEs in the active sites of methanol dehydrogenases (MDHs) over calcium(II) and of why bacteria prefer light REEs have been a subject of debate. Here we report the cultivation and purification of the strictly REE-dependent methanotrophic bacterium Methylacidiphilum fumariolicum SolV with europium(III), as well as structural and kinetic analyses of the first methanol dehydrogenase incorporating Eu in the active site. Crystal structure determination of the Eu-MDH demonstrated that overall no major structural changes were induced by conversion to this REE. Circular dichroism (CD) measurements were used to determine optimal conditions for kinetic assays, whereas inductively coupled plasma mass spectrometry (ICP-MS) showed 70 % incorporation of Eu in the enzyme. Our studies explain why bacterial growth of SolV in the presence of Eu3+ is significantly slower than in the presence of La3+ /Ce3+ /Pr3+ : Eu-MDH possesses a decreased catalytic efficiency. Although REEs have similar properties, the differences in ionic radii and coordination numbers across the series significantly impact MDH efficiency.

Asymmetric oxidation by Gluconobacter oxydans

Asymmetric oxidation is of great value and a major interest in both research and application. This review focuses on asymmetric oxidation of organic compounds by Gluconobacter oxydans. The microbe can be used for bioproduction of several kinds of important chiral compounds, such as vitamin C, 6-(2-hydroxyethyl)amino-6-deoxy-alpha-L-sorbofuranose, (S)-2-methylbutanoic acid, (R)-2-hydroxy-propionic acid and 5-keto-D-gluconic acid. Characteristics of the bacteria and research progress on the enantioselective biotransformation process are introduced.

Improved selectivity of microbial biosensor using membrane coating. Application to the analysis of ethanol during fermentation

A ferricyanide mediated microbial biosensor for ethanol detection was prepared by surface modification of a glassy carbon electrode. The selectivity of the whole Gluconobacter oxydans cell biosensor for ethanol determination was greatly enhanced by the size exclusion effect of a cellulose acetate (CA) membrane. The use of a CA membrane increased the ethanol to glucose sensitivity ratio by a factor of 58.2 and even the ethanol to glycerol sensitivity ratio by a factor of 7.5 compared with the use of a dialysis membrane. The biosensor provides rapid and sensitive detection of ethanol with a limit of detection of 0.85 microM (S/N=3). The selectivity of the biosensor toward alcohols was better compared to previously published enzyme biosensors based on alcohol oxidase or alcohol dehydrogenases. The biosensor was successfully used in an off-line monitoring of ethanol during batch fermentation by immobilized Saccharomyces cerevisiae cells with an initial glucose concentration of 200 g l(-1).

Deuterium isotope effect on enantioselectivity in the Comamonas testosteroni quinohemoprotein alcohol dehydrogenase-catalyzed kinetic resolution of rac-2,2-dimethyl-4-hydroxymethyl-1,3-dioxolane, solketal

Isotopic substitution provides an effective tool to probe the mechanism of enzyme-catalyzed reactions. To our knowledge, kinetic isotope effects on the enantioselectivity of enzymes have not been reported. We investigated the effect of deuterium substitution on the enantiomeric ratio, E, of PQQ-containing quinohemoprotein alcohol dehydrogenase, QH-ADH, from Comamonas testosteroni in the ferricyanide-coupled kinetic resolution of rac-2,2-dimethyl-4-hydroxymethyl-1,3-dioxolane, solketal. Under otherwise identical conditions, we measured E=30 for solketal and E=6 for rac-2,2-dimethyl-4-[1,1-2H]hydroxymethyl-1,3-[5,5,4-2H]dioxolane, d(5)-solketal. It is proposed that isotopic substitution affects the relative kinetic weights of the initial hydron/deuteron transfer from substrate to cofactor and the subsequent proton/deuteron shift in the cofactor-product complex. The latter step becomes more important in the deuterated complex to the extent that the enantiomer discrimination in the first step is partially overruled.

Monitoring of ethanol during fermentation using a microbial biosensor with enhanced selectivity

The present study is concerning the construction of ferricyanide-mediated Gluconobacter oxydans cell ethanol biosensor. The size exclusion effect of a cellulose acetate membrane was used for elimination of glucose interferences during ethanol assays in real samples. A typical response time of the biosensor was 13 s with a high sensitivity of 3.5 microA mM(-1). The microbial biosensor exhibits a very low detection limit of 0.85 microM and a wide linear range from 2 to 270 microM. The operational stability was excellent. During 8.5 h of repetitive ethanol assays, no decrease in the sensor sensitivity was observed. The biosensor was successfully used in the off-line monitoring of ethanol fermentation with a good agreement with HPLC measurements (R(2)=0.998).

Direct hydride transfer in the reaction mechanism of quinoprotein alcohol dehydrogenases: a quantum mechanical investigation

Oxidation of alcohols by direct hydride transfer to the pyrroloquinoline quinone (PQQ) cofactor of quinoprotein alcohol dehydrogenases has been studied using ab initio quantum mechanical methods. Energies and geometries were calculated at the 6-31G(d,p) level of theory. Comparison of the results obtained for PQQ and several derivatives with available structural and spectroscopic data served to judge the feasibility of the calculations. The role of calcium in the enzymatic reaction mechanism has been investigated. Transition state searches have been conducted at the semiempirical and STO-3G(d) level of theory. It is concluded that hydride transfer from the Calpha-position of the substrate alcohol (or aldehyde) directly to the C(5) carbon of PQQ is energetically feasible. Copyright 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1732-1749, 2001