Advances in equol production: Sustainable strategies for unlocking soy isoflavone benefits

Daidzein is an abundant polyphenol present in soy and other legumes. It accumulates mostly in seed and seed pod during the plant development in response to biotic and abiotic stress. Due to the high consumption of soy and soy derivative food, daidzein is one of the most abundant polyphenols in human and cattle diets. Similar to other phytochemicals, daidzein is not absorbed or metabolized by human cells, but transformed by enzymatic cascades of the gut microbiome. The product of daidzein "fermentation" is equol and related molecules. Equol [7-hydroxy-3-(4'-hydroxyphenyl)-chroman] is considered the most active metabolite of all soybean isoflavones, presenting a high level of antioxidant and estrogenic activity. This bioactive molecule presents estrogen-like activity by specifically binding to 5α-dihydrotestosterone and inhibiting its binding to the androgen receptor. Biotransformation of daidzein into equol requires enzymes that are not widespread along all microbiome taxa but restricted to a relatively small number of enzymes from the family Eggerthellaceae (phylum Actinomycetota, formerly Actinobacteria), strict anaerobes that inhabit the colon of humans and anoxic areas of the intestinal tract of other mammals. The isolation and characterization of equol-producing bacteria from the human gut resulted in the identification of bacterial species that, due to their metabolic capacity, were named Adlercreutzia equolifaciens, Slackia equolifaciens, Slackia isoflavoniconvertens, among others. Combining genomic and biochemical analysis, the puzzle of diadzein biotransformation was decoded. It includes an initial step catalyzed by β-glucosidases, enzymes that are commonly found in the intestinal tract and deglycosylate the natural form of daidzein in the plant, followed by the action of bacterial enzymes daidzein reductase, dihydrodaidzein reductase, tetrahydrodaidzein reductase, and dihydrodaidzein racemase. Undoubtedly, improving daidzein production from soy products or enhancing daidzein biotransformation by the human gut microbiome is of notorious biotechnological and biomedical interest. In addition, the chemistry catalyzed by the equol biosynthetic enzymes is of interest in the green chemistry field. However, the extreme oxygen sensitivity of the microbes and enzymes that participate in these pathways still impose a challenge for translating the knowledge obtained with recombinant enzymes and model organisms into the food industry. In this manuscript we review the process of equol biosynthesis and the effort in synthetic biology for production of equol into industrial or biomedical setups.