Modulating Chemoselectivity in a Fe(II)/α-Ketoglutarate-Dependent Dioxygenase for the Oxidative Modification of a Nonproteinogenic Amino Acid

Abstract

Modification of aliphatic C–H bonds in a regio- and stereoselective manner can pose a formidable challenge in organic chemistry. In this context, the use of nonheme iron and α-ketoglutarate-dependent dioxygenases (αKGDs) represents an interesting tool for C–H activation as this enzyme family can catalyze a broad set of synthetically valuable reactions including hydroxylations, oxidations, and desaturations. The consensus reaction mechanism of αKGDs proceeds via the formation of a Fe(IV)-oxo complex capable of hydrogen atom transfer (HAT) from an sp3-hybridized substrate carbon center. The resulting substrate radical and Fe(III)–OH cofactor are considered to be the branch point toward the possible reaction outcomes which are determined by the enzyme’s active site architecture. To date, the modulation of the reaction fate in Fe(II)/α-ketoglutarate-dependent dioxygenases via enzyme engineering has been mainly elusive. In this study, we therefore set out to engineer the l-proline cis-4-hydroxylase SmP4H from Sinorhizobium meliloti for selective oxidative modifications of the nonproteinogenic amino acid l-homophenylalanine (l-hPhe) to produce pharmacological relevant small molecule intermediates. Using structure-guided directed evolution, we improved the total turnover number, the kcat, as well as the kcat/Km of the hydroxylation reaction yielding the desired γ-hydroxylation product by approximately 10-fold, >100-fold, and >300-fold, respectively. Notably, the exchange of only one amino acid in the active site (W40Y) allowed us to reprogram the natural hydroxylase to predominantly act as a desaturase, presumably through tyrosine’s capability to serve as a catalytic entity in the reaction mechanism. An investigation of the substrate scope revealed additional acceptance of the noncanonical amino acids l-homotyrosine and (S)-α-amino-3,4-dichlorobenzenebutanoic acid by SmP4H variants.