Recombinant Production of Arginyl Dipeptides by l-Amino Acid Ligase RizA Coupled with ATP Regeneration

Efficient synthesis of Ala-Tyr by L-amino acid ligase coupled with ATP regeneration system

The multi-enzyme coupling reaction system has become a promising biomanufacturing platform for biochemical production. Tyr is an essential amino acid, but the limited solubility restricts its use. Tyrosyl dipeptide has been paid more attention due to its higher solubility. In this study, an efficient enzymatic cascade of Ala-Tyr synthesis was developed by a L-amino acid ligase together with polyphosphate kinase (PPK). Two L-amino acid ligases from Bacillus subtilis and Bacillus pumilus were selected and applied for Ala-Tyr synthesis. The L-amino acid ligase from B. subtilis (Bs) was selected and coupled with the PPK from Sulfurovum lithotrophicum (PPKSL) for regenerating ATP to produce Ala-Tyr in one pot. In the optimization system, 40.1 mM Ala-Tyr was produced within 3 h due to efficient ATP regeneration with hexametaphosphate (PolyP(6)) as the phosphate donor. The molar yield was 0.89 mol/mol based on the substrates added, while the productivity of Ala-Tyr achieved 13.4 mM/h, which were the highest yield and productivity ever reported about Ala-Tyr synthesis with L-amino acid ligase.

Co-Immobilization of RizA Variants with Acetate Kinase for the Production of Bioactive Arginyl Dipeptides

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.

Mutagenesis of the l-Amino Acid Ligase RizA Increased 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.