Coordination dynamics of iron center enables the C-H bond activation: QM/MM insight into the catalysis of hydroxyglutarate synthase (HglS)

Hydroxyglutarate synthase (HglS) from Pseudomonas putida is an iron (II) dependent non-heme oxygenase, which is responsible for the biodegradation of 2-oxoadipate (2OA) to D-2-hydroxyglutarate (D-2HG) in plant lysine catabolism. Here detailed mechanisms for the chemical steps involved in decarboxylation and hydroxylation in the biodegradation of 2OA have been explored by extensive MD simulations and QM/MM calculations. Our study revealed the existence of two coordination modes of the Fe(IV)-oxo species during the reactions of HglS, and the "proximal" one is responsible for the C-H bond activation. The reaction initiates with the attack of Fe(III)superoxo species on 2OA, resulting in the FeII-O-O-2OA species. After the release of CO2 and the subsequent O-O cleavage, the reactions lead to the "distal" Fe(IV)-oxo species. This species can undergo a coordination switch to evolve into the "proximal" conformation of Fe(IV)-oxo, which is the rate-limiting step with an energy barrier of 20.6 kcal/mol. Moreover, residues in the second coordination sphere, including Arg74, Gln266, Val402, and Ser403, not only strengthen the substrate binding but also facilitate the ferryl-flip, leading to the "proximal" Fe(IV)-oxo conformation that is ideal for the C-H bond activation. These findings may expand our understanding of the coordination dynamics of the iron center in HglS enzyme, which has general implications for other non-heme oxygenases.