High side chain promiscuity of the terminal enzyme in the homologation pathway for l-phenylalanine and l-tyrosine.

Natural product (NPs) and their derivatives are a major source of small-molecule drugs, and the building blocks of these NPs are often amino acids. These include both proteinogenic and nonproteinogenic amino acids (NPAAs), the latter of which expand the structural diversity of NPs. Homologation, or the addition of a methylene group to the amino acid side chain, is one modification that generates NPAAs. If the natural homologation pathway can be characterized and engineered, it could be used to diversify NPs. In this study, we investigated the terminal enzyme of this pathway, HphB, to determine its substrate scope. HphB was tested with various substrates that differed in backbone and/or side chain structures relative to its natural substrate. The results showed that HphB exhibits high promiscuity toward substrates with different side chains while maintaining strict specificity for the substrate backbone. Comparative analysis with two homologous enzymes from primary metabolic pathways revealed that HphB displays markedly higher substrate promiscuity. Bioinformatics analysis and structural modeling suggest that this promiscuity arises from the absence of a "lid" over the active site, resulting in increased solvent exposure of the substrate side chain. This study highlights the unique substrate flexibility of HphB and is a step toward engineering the homologation pathway to generate amino acid derivatives.