Martina Letizia Contente

Continuous flow biocatalysis: production and in-line purification of amines by immobilised transaminase from Halomonas elongata

The continuous flow synthesis of a series of amines was successfully achieved by exploiting the enhanced stability and broad substrate scope of an immobilised transaminase from Halomonas elongata (HEWT). A series of substrates were tested in flow reactors and transformed into the corresponding amines in good to excellent yields. The process was implemented with an integrated in-line purification step for the recovery of the pure amines.

Stereoselective reduction of aromatic ketones by a new ketoreductase from Pichia glucozyma

A new NADPH-dependent benzil reductase (KRED1-Pglu) was identified from the genome of the non-conventional yeast Pichia glucozyma CBS 5766 and overexpressed in E. coli. The new protein was characterised and reaction parameters were optimised for the enantioselective reduction of benzil to (S)-benzoin. A thorough study of the substrate range of KRED1-Pglu was conducted; in contrast to most other known ketoreductases, KRED1-Pglu prefers space-demanding substrates, which are often converted with high stereoselectivity. A molecular modelling study was carried out for understanding the structural determinants involved in the stereorecognition experimentally observed and unpredictable on the basis of steric properties of the substrates. As a result, a new useful catalyst was identified, enabling the enantioselective preparation of different aromatic alcohols and hydroxyketones.

Highly Efficient Oxidation of Amines to Aldehydes with Flow‐based Biocatalysis

A new mild and efficient process for the aqueous preparation of aldehydes, which are employed as flavour and fragrance components in food, beverage, cosmetics, as well as in pharmaceuticals, was developed using a continuous‐flow approach based on an immobilised pure transaminase‐packed bed reactor. HEWT, an ω‐transaminase from the haloadapted bacterium Halomonas elongata, has been selected for its excellent stability and substrate scope. Sixteen different amines were rapidly (3–15 min) oxidised to the corresponding aldehydes (90 to 99 %) with only 1 to 5 equivalents of sodium pyruvate. The process was fully automated, allowing for the in‐line recovery of the pure aldehydes (chemical purity >99 % and isolated yields above 80 %), without any further work‐up procedure.

Stereoelectronic effects in the reaction of aromatic substrates catalysed by Halomonas elongata transaminase and its mutants.

A transaminase from Halomonas elongata and four mutants generated by an in silico-based design were recombinantly produced in E. coli, purified and applied to the amination of mono-substituted aromatic carbonyl-derivatives. While benzaldehyde derivatives were excellent substrates, only NO2-acetophenones were transformed into the (S)-amine with a high enantioselectivity. The different behaviour of wild-type and mutated transaminases was assessed by in silico substrate binding mode studies.

Seawater-Based Biocatalytic Strategy: Stereoselective Reductions of Ketones with Marine Yeasts

The large consumption of freshwater in fermentations and bio‐transformations is a matter of concern for the sustainability of many bio‐processes. The use of seawater to perform bio‐processes is a sustainable alternative. In this work, we used marine yeasts from deep‐sub‐seafloor sediments grown in seawater as bio‐catalysts to perform the stereoselective reduction of different ketones, and the bio‐transformations were accomplished in seawater as well. Strains of Meyerozyma guilliermondii and Rhodotorula mucilaginosa were able to reduce different aromatic ketones with high molar conversions and moderate‐to‐high enantioselectivity with no significant differences between bio‐catalysis performed in seawater and freshwater. Finally, the selected marine yeasts were used for the reduction of key intermediates in seawater for the synthesis of molecules of pharmaceutical interest (desogestrel, norgestrel, gestodene, pramipexole).

Recombinant S. cerevisiae expressing Old Yellow Enzymes from non-conventional yeasts: an easy system for selective reduction of activated alkenes

BackgroundOld Yellow Enzymes (OYEs) are flavin-dependent enoate reductases (EC 1.6.99.1) that catalyze the stereoselective hydrogenation of electron-poor alkenes. Their ability to generate up to two stereocenters by the trans- hydrogenation of the C = C double bond is highly demanded in asymmetric synthesis. Isolated redox enzymes utilization require the addition of cofactors and systems for their regeneration. Microbial whole-cells may represent a valid alternative combining desired enzymatic activity and efficient cofactor regeneration. Considerable efforts were addressed at developing novel whole-cell OYE biocatalysts, based on recombinant Saccharomyces cerevisiae expressing OYE genes.ResultsRecombinant S. cerevisiae BY4741∆Oye2 strains, lacking endogenous OYE and expressing nine separate OYE genes from non-conventional yeasts, were used as whole-cell biocatalysts to reduce substrates with an electron-poor double bond activated by different electron-withdrawing groups. Ketoisophorone, α-methyl-trans- cinnamaldehyde, and trans- β-methyl-β-nitrostyrene were successfully reduced with high rates and selectivity. A series of four alkyl-substituted cyclohex-2-enones was tested to check the versatility and efficiency of the biocatalysts. Reduction of double bond occurred with high rates and enantioselectivity, except for 3,5,5-trimethyl-2-cyclohexenone. DFT (density functional theory) computational studies were performed to investigate whether the steric hindrance and/or the electronic properties of the substrates were crucial for reactivity. The three-dimensional structure of enoate reductases from Kluyveromyces lodderae and Candida castellii, predicted through comparative modeling, resulted similar to that of S. cerevisiae OYE2 and revealed the key role of Trp116 both in substrate specificity and stereocontrol. All the modeling studies indicate that steric hindrance was a major determinant in the enzyme reactivity.ConclusionsThe OYE biocatalysts, based on recombinant S. cerevisiae expressing OYE genes from non-conventional yeasts, were able to differently reduce the activated double bond of enones, enals and nitro-olefins, exhibiting a wide range of substrate specificity. Moreover whole-cells biocatalysts bypassed the necessity of the cofactor recycling and, tuning reaction parameters, allowed the synthetic exploitation of endogenous carbonyl reductases. Molecular modeling studies highlighted key structural features for further improvement of catalytic properties of OYE enzymes.

Self-sustaining closed-loop multienzyme-mediated conversion of amines into alcohols in continuous reactions

The synthesis of alcohols from amine starting materials is an excellent yet challenging strategy for the preparation of pharmaceuticals and polymers. Here we developed a versatile, self-sustaining closed-loop multienzymatic platform for the biocatalytic synthesis of a large range of non-commercially available products in a continuous flow with excellent yields (80 to >99%), reaction times and optical purity of secondary alcohols (>99 enantiomeric excess). This process was also extended to the conversion of biogenic amines into high-value alcohols, such as the powerful antioxidant hydroxytyrosol, and the synthesis of enantiopure 2-arylpropanols via the dynamic kinetic resolution of commercially affordable racemic amines. The system exploits the in situ immobilization of transaminases and redox enzymes which were combined to cater for a fully automated, ultra-efficient synthetic platform with cofactor recycling, in-line recovery of benign by-products and recirculation of the aqueous media that contains the recycled cofactors in catalytic amounts, which increases the efficiency of the system by over 20-fold.Alcohols serve as versatile intermediates for the synthesis of pharmaceuticals and other valuable compounds. Here, Contente and Paradisi developed self-sustainable biocatalytic flow systems for the conversion of amines into various non-commercially-available and high-value alcohols.

Development of a high-yielding bioprocess for 11-α hydroxylation of canrenone under conditions of oxygen-enriched air supply

A high yielding bioprocess for 11-α hydroxylation of canrenone (1a) using Aspergillus ochraceus ATCC 18500 was developed. The optimization of the biotransformation involved both fermentation (for achieving highly active mycelium of A. ochraceus) and biotransformation with the aim to obtain 11-α hydroxylation with high selectivity and yield. A medium based on sucrose as C-source resulted particularly suitable for conversion of canrenone into the corresponding 11-hydroxy derivative, whereas the use of O2-enriched air and dimethyl sulfoxide (DMSO) as a co-solvent for increasing substrate solubility played a crucial role for obtaining high yields (>95%) of the desired product in high chemical purity starting from 30mM (10.2g/L) of substrate. The structure of the hydroxylated product was confirmed by a combination of two-dimensional NMR proton-proton correlation techniques.

Complementary microbial approaches for the preparation of optically pure aromatic molecules

Different strategies for stereoselective microbial preparation of various chiral aromatic compounds are described. Optically pure 2-methyl-3-phenyl-1-propanol, ethyl 2-methyl-3-phenylpropanoate, 2-methyl-3-phenylpropanal, 2-methyl-3-phenylpropionic acid and 2-methyl-3-phenylpropyl acetate have been prepared using different microbial biotransformations starting from different prochiral and/or racemic substrates. (S)-2-Methyl-3-phenyl-1-propanol and (S)-2-methyl-3-phenylpropanal were prepared by biotransformation of 2-methyl cinnamaldehyde using the recombinant strain Saccharomyces cerevisiae BY4741ΔOye2Ks carrying a heterologous OYE gene from Kazachstania spencerorum. (R)-2-Methyl-3-phenylpropionic acid was obtained by oxidation of racemic 2-methyl-3-phenyl-1-propanol with acetic acid bacteria. Kinetic resolution of racemic 2-methyl-3-phenylpropionic acid was carried out by direct esterification with ethanol using dry mycelia of Rhizopus oryzae CBS 112.07 in organic solvent, giving (R)-ethyl 2-methyl-3-phenylpropanoate as major enantiomer. Finally, (R,S)-2-methyl-3-phenylpropyl acetate was enantioselectively hydrolysed employing different bacteria and yeasts having cell-bound carboxylesterases with prevalent formation of (R)- or (S)-2-methyl-3-phenyl-1-propanol, depending on the strain employed.

A stereospecific carboxyl esterase from Bacillus coagulans hosting nonlipase activity within a lipase-like fold

Microbial carboxylesterases are important biocatalysts that selectively hydrolyze an extensive range of esters. Here, we report the biochemical and structural characterization of an atypical carboxylesterase from Bacillus coagulans (BCE), endowed with high enantioselectivity toward different 1,2‐O‐isopropylideneglycerol (IPG or solketal) esters. BCE efficiently catalyzes the production of enantiopure (S)‐IPG, a chiral building block for the synthesis of β‐blockers, glycerophospholipids, and prostaglandins; efficient hydrolysis was observed up to 65 °C. To gain insight into the mechanistic bases of such enantioselectivity, we solved the crystal structures of BCE in apo‐ and glycerol‐bound forms at resolutions of 1.9 and 1.8 Å, respectively. In silico docking studies on the BCE structure confirmed that IPG esters with small acyl chains (≤ C6) were easily accommodated in the active site pocket, indicating that small conformational changes are necessary to accept longer substrates. Furthermore, docking studies suggested that enantioselectivity may be due to an improved stabilization of the tetrahedral reaction intermediate for the S‐enantiomer. Contrary to the above functional data implying nonlipolytic functions, BCE displays a lipase‐like 3D structure that hosts a “lid” domain capping the main entrance to the active site. In lipases the lid mediates catalysis through interfacial activation, a process that we did not observe for BCE. Overall, we present the functional‐structural properties of an atypical carboxyl esterase that has nonlipase‐like functions, yet possesses a lipase‐like 3D fold. Our data provide original enzymatic information in view of BCE applications as an inexpensive, efficient biocatalyst for the production of enantiopure (S)‐IPG.