Profiling of Diverse Pyridoxal-5'-Phosphate Dependent Enzymes Reveals Promiscuous Aldolase Activity with (2-Azaaryl)methanamines.

The elaboration of amine substrates through C-C bond-forming reactions is important in the synthesis of bioactive small molecules. Pyridoxal-5'-phosphate (PLP)-dependent enzymes have emerged as valuable biocatalysts for this class of reactions, due to their high stereoselectivity and ability to forge new C-C bonds on unprotected α-amino acid substrates. However, the use of abiological primary amines as pronucleophiles with enzymes such as threonine aldolase has been unexplored, moderating the utility of a biocatalytic approach in the synthesis of diverse 1,2-amino alcohols. In this report, we disclose the discovery and engineering of a PLP-dependent aldolase that accepts (2-azaaryl)methanamines in an aldol-type transformation. The 1,2-amino alcohol products generated, which contain representative heteroaromatic pharmacophores, are delivered with control over both the diastereoselectivity and enantioselectivity in the C-C bond-forming event. Protein engineering provided variants with improved binding affinity for the abiological substrate and decreased affinity for the native α-amino acid, overcoming inhibition of the abiotic reaction by components of lysate, a major challenge in reaction discovery with PLP-dependent enzymes such as threonine aldolases. This work represents the first known example of C-C bond formation on nonamino acid substrates with threonine aldolase and provides a platform for further development of complexity-building biocatalytic reactions with abiotic amine substrates.