Sven Bordewick

Edible mushrooms are widely appreciated for their appealing flavours, low caloric values and high content of presumably health-protecting metabolites. Their long history of safe use together with the looming worldwide food crisis have revived the idea of generating meat analogues and protein isolates by the controlled fermentation of mycelia of these edible fungi as a dietary option. The occurrence of proteins, polysaccharides, smaller metabolites, metal ions and toxins in mycelia and fruiting bodies is compared among the three most popular species, Agaricus bisporus (button mushroom), Pleurotus ostreatus (oyster mushroom), Lentinus edodes (shiitake) and some closely related species. Large effects of substrate chemistry, strain, developmental stage and ecological interactions result in a wide variation of the concentrations of some metabolites in both mycelial cells and fruiting bodies. This is obviously a result of the high adaptation abilities required to survive in natural habitats. Fungal bioprocesses are decoupled from agricultural production and can be operated anytime, anywhere, and on any scale according to demand. It is concluded that fungal biomass, if produced under food-grade conditions and on an industrial scale, could provide a safe and nutritious meat substitute and protein isolates with a high biological value for future vegan foods.

The biocatalytic system comprised of RizA and acetate kinase (AckA) combines the specific synthesis of bioactive arginyl dipeptides with efficient ATP regeneration. Immobilization of this coupled enzyme system was performed and characterized in terms of activity, specificity and reusability of the immobilisates. Co-immobilization of RizA and AckA into a single immobilisate conferred no disadvantage in comparison to immobilization of only RizA, and a small addition of AckA (20:1) was sufficient for ATP regeneration. New variants of RizA were constructed by combining mutations to yield variants with increased biocatalytic activity and specificity. A selection of RizA variants were co-immobilized with AckA and used for the production of the salt-taste enhancers Arg-Ser and Arg-Ala and the antihypertensive Arg-Phe. The best variants yielded final dipeptide concentrations of 11.3 mM Arg-Ser (T81F_A158S) and 11.8 mM Arg-Phe (K83F_S156A), the latter of which represents a five-fold increase in comparison to the wild-type enzyme. T81F_A158S retained more than 50% activity for over 96 h and K83F_S156A for over 72 h. This study provides the first example of the successful co-immobilization of an l-amino acid ligase with an ATP-regenerating enzyme and paves the way towards a bioprocess for the production of bioactive dipeptides.

The l-amino acid ligase RizA from B. subtilis selectively synthesizes dipeptides containing an N-terminal arginine. Many arginyl dipeptides have salt-taste enhancing properties while Arg-Phe has been found to have an antihypertensive effect. A total of 21 RizA variants were created by site-directed mutagenesis of eight amino acids in the substrate binding pocket. The variants were recombinantly produced in E. coli and purified by affinity chromatography. Biocatalytic reactions were set up with arginine and four amino acids differing in size and polarity (aspartic acid, serine, alanine, and phenylalanine) and were analyzed by RP-HPLC with fluorescence detection. Variant T81F significantly improved the yield in comparison to wild type RizA for aspartic acid (7 to 17%), serine (33 to 47%) and alanine (12 to 17%). S84F increased product yield similarly for aspartic acid (7 to 17%) and serine (33 to 42%). D376E increased the yield with alanine (12 to 19%) and phenylalanine (11 to 26%). The largest change was observed for S156A, which showed a yield for Arg-Phe of 40% corresponding to a 270% increase in product concentration. This study expands the knowledge about positions governing the substrate specificity of RizA and may help to inform future protein engineering endeavors.

Arginyl dipeptides like Arg-Ser, Arg-Ala, and Arg-Gly are salt-taste enhancers and can potentially be used to reduce the salt content of food. The l-amino acid ligase RizA from B. subtilis selectively synthesizes arginyl dipeptides. However, industrial application is prevented by the high cost of the cofactor adenosine triphosphate (ATP). Thus, a coupled reaction system was created consisting of RizA and acetate kinase (AckA) from E. coli providing ATP regeneration from acetyl phosphate. Both enzymes were recombinantly produced in E. coli and purified by affinity chromatography. Biocatalytic reactions were varied and analyzed by RP-HPLC with fluorescence detection. Under optimal conditions the system produced up to 5.9 g/L Arg-Ser corresponding to an ATP efficiency of 23 g Arg-Ser per gram ATP. Using similar conditions with alanine or glycine as second amino acid, 2.6 g/L Arg-Ala or 2.4 g/L Arg Gly were produced. The RizA/AckA system selectively produced substantial amounts of arginyl dipeptides while minimizing the usage of the expensive ATP.