M. Musa

Recent advances in alcohol dehydrogenase-catalyzed asymmetric production of hydrophobic alcohols

The efficiency of biocatalytic redox reactions catalyzed by alcohol dehydrogenases (ADHs) have been the subject of considerable research recently. Two major challenges have restricted their application in asymmetric synthesis until now. First of all, most of the interesting substrates are either insoluble or sparingly soluble in aqueous media, the natural medium for enzymes. This drawback has been overcome by using non-aqueous media like organic solvents, ionic liquids, and supercritical carbon dioxide in mono- and biphasic reactions, and several ADHs show high activity at high concentrations of such reaction media. The second challenge is the strict substrate specificity for most ADHs. The continuous search for new ADHs, together with random and rational mutagenesis to widen substrate specificity, will help in attracting organic chemists to consider utilizing them in organic synthesis more often. The aim of this perspective is to highlight recent efforts to overcome the above-mentioned limitations.

A Single Point Mutation Reverses the Enantiopreference of Thermoanaerobacter ethanolicus Secondary Alcohol Dehydrogenase

Alcohol dehydrogenases (ADHs) are enzymes that catalyze the reversible reduction of carbonyl compounds to their corresponding alcohols. It is beyond doubt that they are important biocatalysts in asymmetric synthesis. Recent reports have shown that it is possible to use a number of ADHs for synthetic applications in nonaqueous media with high activities, which make them attractive choices to organic chemists. The stereopreferences of ADHs can be predicted by Prelog’s rule (Figure 1), which depends on the relative sizes of the two

Activity and selectivity of W110A secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus in organic solvents and ionic liquids: mono- and biphasic media

The asymmetric reduction of hydrophobic phenyl-ring-containing ketones and the enantiospecific kinetic resolution of the corresponding racemic alcohols catalyzed by Thermoanaerobacter ethanolicus W110A secondary alcohol dehydrogenase were performed in mono- and biphasic systems containing either organic solvents or ionic liquids. Both yield and enantioselectivity for these transformations can be controlled by changing the reaction medium. The enzyme showed high tolerance to both water-miscible and -immiscible solvents, which allows biotransformations to be conducted at high substrate concentrations.

Racemization of enantiopure secondary alcohols by Thermoanaerobacter ethanolicus secondary alcohol dehydrogenase.

Controlled racemization of enantiopure phenyl-ring-containing secondary alcohols is achieved in this study using W110A secondary alcohol dehydrogenase from Thermoanaerobacter ethanolicus (W110A TeSADH) and in the presence of the reduced and oxidized forms of its cofactor nicotinamide-adenine dinucleotide. Racemization of both enantiomers of alcohols accepted by W110A TeSADH, not only with low, but also with reasonably high, enantiomeric discrimination is achieved by this method. Furthermore, the high tolerance of TeSADH to organic solvents allows TeSADH-catalyzed racemization to be conducted in media containing up to 50% (v/v) of organic solvents.

Deracemization of Secondary Alcohols by using a Single Alcohol Dehydrogenase

We developed a single‐enzyme‐mediated two‐step approach for deracemization of secondary alcohols. A single mutant of Thermoanaerobacter ethanolicus secondary alcohol dehydrogenase enables the nonstereoselective oxidation of racemic alcohols to ketones, followed by a stereoselective reduction process. Varying the amounts of acetone and 2‐propanol cosubstrates controls the stereoselectivities of the consecutive oxidation and reduction reactions, respectively. We used one enzyme to accomplish the deracemization of secondary alcohols with up to >99 % ee and >99.5 % recovery in one pot and without the need to isolate the prochiral ketone intermediate.

Asymmetric Reduction of Substituted 2-Tetralones by Thermoanaerobacter pseudoethanolicus Secondary Alcohol Dehydrogenase

Ketones bearing two bulky substituents, named bulky–bulky ketones, were successfully reduced to their corresponding optically enriched alcohols by using various mutants of Thermoanaerobacter pseudoethanolicus secondary alcohol dehydrogenase (TeSADH). Substituted 2‐tetralones, in particular, were reduced to 2‐tetralols with high conversion and high enantioselectivity. The pharmacological importance of substituted 2‐tetralols as key drug‐building blocks makes our biocatalytic reduction method a highly essential tool. We showed that changing the position of the substituent on the aromatic ring of 2‐tetralones impacts their binding affinity and the reaction maximum catalytic rate. Docking studies with several TeSADH mutants explain how the position of the substituent on the tetralone influences the binding orientation of substituted 2‐tetralones and their reaction stereoselectivity.

Dual enzymatic dynamic kinetic resolution by Thermoanaerobacter ethanolicus secondary alcohol dehydrogenase and Candida antarctica lipase B

The immobilization of Thermoanaerobacter ethanolicus secondary alcohol dehydrogenase (TeSADH) using sol–gel method enables its use to racemize enantiopure alcohols in organic media. Here, we report the racemization of enantiopure phenyl-ring-containing secondary alcohols using xerogel-immobilized W110A TeSADH in hexane rather than the aqueous medium required by the enzyme. We further showed that this racemization approach in organic solvent was compatible with Candida antarctica lipase B (CALB)-catalyzed kinetic resolution. This compatibility, therefore, allowed a dual enzymatic dynamic kinetic resolution of racemic alcohols using CALB-catalyzed kinetic resolution and W110A TeSADH-catalyzed racemization of phenyl-ring-containing alcohols.

Isolation and characterization of naphthalene biodegrading Methylobacterium radiotolerans bacterium from the eastern coastline of the Kingdom of Saudi Arabia

Abstract Bioremediation is based on microorganisms able to use pollutants either as a source of carbon or in co-metabolism, and is a promising strategy in cleaning the environment. Using soil contaminated with petroleum products from an industrial area in Saudi Arabia (Jubail), and after enrichment with the polycyclic aromatic hydrocarbon (PAH) naphthalene, a Methylobacterium radiotolerans strain (N7A0) was isolated that can grow in the presence of naphthalene as the sole source of carbon. M. radiotolerans is known to be resistant to gamma radiation, and this is the first documented report of a strain of this bacterium using a PAH as the sole source of carbon. The commonly reported Pseudomonas aeruginosa (strain N7B1) that biodegrades naphthalene was also identified, and gas chromatography analyses have shown that the biodegradation of naphthalene by M. radiotolerans and P. aeruginosa did follow both the salicylate and phthalate pathways.

Effect of Gum Arabic on Stallion Sperm Survival During Cold Storage and Post Freezing

Abstract This study is aimed at investigating effects of supplementation of stallion’ semen extender with various concentrations of Gum Arabic (GA) versus egg yolk (EY) on viscosity, sperm motility and survival during cooling and freezing. Physical sperm characteristics; i.e. curvilinear velocity (VCL), straight line velocity (VSL), average path velocity (VAP), linearity (LIN) and straightness index (STR) were evaluated. Based on the sperm velocity (velocity of the average path), individual spermatozoons were classified into two major groups; i.e., progressively motile (>45 μm/sec) and immotile (0-45 μm/sec) spermatozoa. Addition of 3, 9 or 15% of GA to HF-20 extender resulted in linear decreases in VCL, VSL and VAP and a decrease in the percentage of progressively motile spermatozoa. Dilution of horse semen samples with high viscosityextenders (i.e., high percentage of GA) decreased the VCL, VSL and VAP in fresh and chilled semen. Freezing semen in high viscosity-extenders reduced percentage of progressively motile spermatozoa compared with those of low viscosity-extenders. In refrigerated and frozen semen samples, the extender containing 15% GA had detrimental effects on the percentage of progressively motile sperm cells and velocity of progressive motile sperm. Moreover, cooling sperm in extenders containing 9 or 15% of GA for 72 hours resulted in complete motility cessation. In conclusion, GA could replace EY in stallion semen extenders at a level of 3% to maintain the physical and biological characteristics of cold and frozen semen.