Novel transaminases from thermophiles: from discovery to application

Transaminases (TAs) are promising biocatalysts for chiral amine synthesis; however, only few thermophilic TAs have been described to date. In this work, a genome mining approach was taken to seek novel TAs from nine thermophilic microorganisms. TA sequences were identified from their respective genome sequences and their Pfam were predicted confirming that TAs class I-II are the most abundant (50%), followed by class III (26%), V (16%), IV (8%) and VI (1%). The percentage of open reading frames (ORFs) that are TAs ranges from 0.689% in Thermococcus litoralis to 0.424% in Sulfolobus solfataricus. A total of 94 putative TAs were successfully cloned and expressed into E. coli, showing mostly good expression levels when using a chemical chaperone media containing d-sorbitol. Kinetic and end-point colorimetric assays with different amino donors-acceptors confirmed TAs activity allowing for initial exploration of the substrate scope. Stereoselective and non-stereoselective serine-TAs were selected for the synthesis of hydroxypyruvate (HPA). Low HPA reaction yields were observed with four non-stereoselective serine-TAs, whilst two stereoselective serine-TAs showed significantly higher yields. Coupling serine-TA reactions to a transketolase to yield l-erythrulose (Ery) substantially increased serine conversion into HPA. Combining both stereoselective serine-TAs and transketolase using the inexpensive racemic D/L-serine led to high Ery yield (82%). Thermal characterization of stereoselective serine-TAs confirmed they have excellent thermostability up to 60°C and high optimum temperatures.