Biocatalysis continues to emerge as a powerful technique for the efficient synthesis of optically pure pharmaceuticals that are difficult to access via conventional chemistry. The power of biocatalysis can be enhanced if two or more reactions can be achieved by a single whole cell biocatalyst containing a pathway designed de-novo to facilitate a required synthetic sequence. The enzymes transketolase (TK) and transaminase (TAm) respectively catalyze asymmetric carbon--carbon bond formation and amine group addition to suitable substrate molecules. The ability of a transaminase to accept the product of the transketolase reaction can allow the two catalysts to be employed in series to create chiral amino-alcohols from achiral substrates. As proof of principle, the beta-alanine: pyruvate aminotransferase (beta-A:P TAm) from Pseudomonas aeruginosa has been cloned, to create plasmid pQR428, for overexpression in E.coli strain BL21gold(DE3). Production of the beta-A:P TAm alongside the native transketolase (overexpressed from plasmid pQR411), in a single E.coli host, has created a novel biocatalyst capable of the synthesis of chiral amino alcohols via a synthetic two-step pathway. The feasibility of using the biocatalyst has been demonstrated by the formation of a single diastereoisomer of 2-amino-1,3,4-butanetriol (ABT) product, in up to 21% mol/mol yield, by the beta-A:P TAm, via transamination of L-erythrulose synthesized by TK, from achiral substrates glycolaldehyde (GA) and beta-hydroxypyruvate (beta-HPA). ABT synthesis was achieved in a one-pot process, using either whole cells of the dual plasmid strain or cell lysate, while the dual alcohol-amine functionality of ABT makes it an excellent synthon for many pharmaceutical syntheses.
A microplate-based HPLC assay for transketolase is described for rapidly determining substrate and product concentration suitable for optimisation of biocatalytic process conditions and screening directed evolution libraries. Transketolase catalyses the enantioselective carbon-carbon bond formation of chiral keto-diol products. The assay was used to determine dissociation constants for the two cofactors required by transketolase with 5-11% error. The preparation of samples by microplate-based fermentation, cell lysis, addition of cofactor, addition of substrates was also evaluated and optimised for increased transketolase activity. The whole process enables 3-fold improved enzyme variants to be identified from a single measurement.
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