Reduction of aryl acids by white-rot fungi for the biocatalytic production of aryl aldehydes and alcohols

Mushroom-Mediated Reductive Bioconversion of Aldehyde-Rich Essential Oils for Aroma Alteration: A Rose-like Floral Bioflavor from Citronella Oil

The bioflavors are of high demand in food and beverage industries. The current study identified reductive processes mediated by mushroom species to alter the aroma of aldehyde-rich essential oils in the submerged culture. Neofomitella polyzonata, a polypore mushroom, reduced citronellal and citral in the citronella oil into corresponding alcohols that altered the oil aroma, creating a new bioflavor. The screening with 43 aldehydes showed its broad substrate scope within aromatic and linear aldehydes, yet influenced by the electronic and steric factors. Under an optimized condition, it efficiently converted up to 1.5 g/L citrusy and sharp citronella oil into a terpene alcohol-rich (citronellol and geraniol) floral, sweet, fresh, and rosy oily product within 12 h. The preparative-scale fermentation in the shake flask followed by distillation, an organic solvent-free downstream process, furnished the product in 87.2% w/w yield. Detailed sensory analyses and volatile chemo-profiling established the uniqueness in the product aroma and identified citronellol and geraniol as the key odorants. The chemometric analysis found best compositional similarity of this product with Damask or Turkish rose oils. The preference test for the water flavored with the fermented product (0.001-0.005% v/v) indicated its potential as a rosy bioflavor for the beverages.

Further Biochemical Profiling of Hypholoma fasciculare Metabolome Reveals Its Chemogenetic Diversity

Natural products with novel chemistry are urgently needed to battle the continued increase in microbial drug resistance. Mushroom-forming fungi are underutilized as a source of novel antibiotics in the literature due to their challenging culture preparation and genetic intractability. However, modern fungal molecular and synthetic biology tools have renewed interest in exploring mushroom fungi for novel therapeutic agents. The aims of this study were to investigate the secondary metabolites of nine basidiomycetes, screen their biological and chemical properties, and then investigate the genetic pathways associated with their production. Of the nine fungi selected, Hypholoma fasciculare was revealed to be a highly active antagonistic species, with antimicrobial activity against three different microorganisms: Bacillus subtilis, Escherichia coli, and Saccharomyces cerevisiae. Genomic comparisons and chromatographic studies were employed to characterize more than 15 biosynthetic gene clusters and resulted in the identification of 3,5-dichloromethoxy benzoic acid as a potential antibacterial compound. The biosynthetic gene cluster for this product is also predicted. This study reinforces the potential of mushroom-forming fungi as an underexplored reservoir of bioactive natural products. Access to genomic data, and chemical-based frameworks, will assist the development and application of novel molecules with applications in both the pharmaceutical and agrochemical industries.

Scope and limitations of biocatalytic carbonyl reduction with white-rot fungi

The reductive activity of various basidiomycetous fungi towards carbonyl compounds was screened on an analytical level. Some strains displayed high reductive activities toward aromatic carbonyls and aliphatic ketones. Utilizing growing whole-cell cultures of Dichomitus albidofuscus, the reactions were up-scaled to a preparative level in an aqueous system. The reactions showed excellent selectivities and gave the respective alcohols in high yields. Carboxylic acids were also reduced to aldehydes and alcohols under the same conditions. In particular, benzoic, vanillic, ferulic, and p-coumaric acid were reduced to benzyl alcohol, vanillin, dihydroconiferyl alcohol and 1-hydroxy-3-(4-hydroxyphenyl)propan, respectively.

Biocatalytic reduction of carboxylic acids

An increasing demand for non-petroleum-based products is envisaged in the near future. Carboxylic acids such as citric acid, succinic acid, fatty acids, and many others are available in abundance from renewable resources and they could serve as economic precursors for bio-based products such as polymers, aldehyde building blocks, and alcohols. However, we are confronted with the problem that carboxylic acid reduction requires a high level of energy for activation due to the carboxylate's thermodynamic stability. Catalytic processes are scarce and often their chemoselectivity is insufficient. This review points at bio-alternatives: currently known enzyme classes and organisms that catalyze the reduction of carboxylic acids are summarized. Two totally distinct biocatalyst lines have evolved to catalyze the same reaction: aldehyde oxidoreductases from anaerobic bacteria and archea, and carboxylate reductases from aerobic sources such as bacteria, fungi, and plants. The majority of these enzymes remain to be identified and isolated from their natural background in order to evaluate their potential as industrial biocatalysts.

Chlorobenzoic acid degradation by Lentinus (Panus) tigrinus: in vivo and in vitro mechanistic study-evidence for P-450 involvement in the transformation

Aim of this work was to investigate the ability of Lentinus (Panus) tigrinus to degrade and detoxify a chlorobenzoate (CBA) mixture composed of mono-, di- and tri-chlorinated isomers. The degradation process was investigated as a function of both the growing medium (i.e. low N Kirk's and malt extract-glucose medium) and cultivation conditions (i.e. stationary and shaken cultures). The majority of CBAs were quantitatively degraded within the early 15 d from spiking with the notable exception of the double ortho-chlorinated compounds, 2,6-di-, 2,3,6-tri- and 2,4,6-tri-CBA. Analysis of the degradation intermediates indicated the occurrence of side chain reduction, hydroxylation and methylation reactions. Although CBAs stimulated laccase production, in vitro experiments with a purified L. tigrinus laccase isoenzyme demonstrated its inability to participate in the initial attack on CBAs even in the presence of redox mediators; similar results were found with a Mn-peroxidase isoenzyme. Conversely, prompt degradation was observed upon 1h incubation of CBAs with a purified microsomal fraction containing cytochrome P-450 monooxygenase. The nature of some reaction products (i.e. hydroxylated derivatives), the dependency of the reaction on NADPH and its susceptibility to either CO or piperonyl butoxide inhibition confirmed the involvement of L. tigrinus cytochrome P-450 in the early steps of CBA degradation.

De novo biosynthesis of vanillin in fission yeast (Schizosaccharomyces pombe) and baker's yeast (Saccharomyces cerevisiae)

Vanillin is one of the world's most important flavor compounds, with a global market of 180 million dollars. Natural vanillin is derived from the cured seed pods of the vanilla orchid (Vanilla planifolia), but most of the world's vanillin is synthesized from petrochemicals or wood pulp lignins. We have established a true de novo biosynthetic pathway for vanillin production from glucose in Schizosaccharomyces pombe, also known as fission yeast or African beer yeast, as well as in baker's yeast, Saccharomyces cerevisiae. Productivities were 65 and 45 mg/liter, after introduction of three and four heterologous genes, respectively. The engineered pathways involve incorporation of 3-dehydroshikimate dehydratase from the dung mold Podospora pauciseta, an aromatic carboxylic acid reductase (ACAR) from a bacterium of the Nocardia genus, and an O-methyltransferase from Homo sapiens. In S. cerevisiae, the ACAR enzyme required activation by phosphopantetheinylation, and this was achieved by coexpression of a Corynebacterium glutamicum phosphopantetheinyl transferase. Prevention of reduction of vanillin to vanillyl alcohol was achieved by knockout of the host alcohol dehydrogenase ADH6. In S. pombe, the biosynthesis was further improved by introduction of an Arabidopsis thaliana family 1 UDP-glycosyltransferase, converting vanillin into vanillin beta-D-glucoside, which is not toxic to the yeast cells and thus may be accumulated in larger amounts. These de novo pathways represent the first examples of one-cell microbial generation of these valuable compounds from glucose. S. pombe yeast has not previously been metabolically engineered to produce any valuable, industrially scalable, white biotech commodity.

New oxidase from Bjerkandera arthroconidial anamorph that oxidizes both phenolic and nonphenolic benzyl alcohols

A new flavooxidase is described from a Bjerkandera arthroconidial anamorph. Its physicochemical characteristics, a monomeric enzyme containing non-covalently bound flavin adenine dinucleotide (FAD), and several catalytic properties, such as oxidation of aromatic and polyunsaturated aliphatic primary alcohols, are similar to those of Pleurotus eryngii aryl-alcohol oxidase (AAO). However, it also efficiently oxidizes phenolic benzyl and cinnamyl alcohols that are typical substrates of vanillyl-alcohol oxidase (VAO), a flavooxidase from a different family, characterized by its multimeric nature and presence of covalently-bound FAD. The enzyme also differs from P. eryngii AAO by having extremely high efficiency oxidizing chlorinated benzyl alcohols (1000-1500 s(-1) mM(-1)), a feature related to the different alcohol metabolites secreted by the Pleurotus and Bjerkandera species including chloroaromatics, and higher activity on aromatic aldehydes. What is even more intriguing is the fact that, the new oxidase is optimally active at pH 6.0 on both p-anisyl and vanillyl alcohols, suggesting a mechanism for phenolic benzyl alcohol oxidation that is different from that described in VAO, which proceeds via the substrate phenolate anion formed at basic pH. Based on the above properties, and its ADP-binding motif, partially detected after N-terminus sequencing, the new enzyme is classified as a member of the GMC (glucose-methanol-choline oxidase) oxidoreductase family oxidizing both AAO and VAO substrates.

Determination of the toxicity of several aromatic carbonylic compounds and their reduced derivatives on Phanerochaete chrysosporium using a Pseudomonas putida test system

We tested four aromatic carbonylic compounds and their corresponding reduced derivatives, possible substrates, and products of a biotransformation for toxicity against the white-rot fungus Phanerochaete chrysosporium. The bacterium Pseudomonas putida, which has been proven to be a good test organism for investigating toxic effects, was used as a primary screen. For both P. chrysosporium and P. putida, all ketones showed a higher toxicity than their corresponding alcohol derivatives. Within one chemical group a direct correlation between the hydrophobicity (logP values) of the compounds and their toxicity could be observed. Furthermore, all tested compounds also caused an isomerization of cis to trans unsaturated fatty acids in P. putida, a mechanism of this bacterium to adapt its membrane to toxic environmental influences. Toxicity of aromatic carbonylic compounds in an established biotransformation system with P. chrysosporium can be estimated by calculating the corresponding logP values of the substrates and potential products. P. putida can be used to test the toxicity of aromatic ketones to the basic diomycete P. chrysosporium.