Direct Spectroscopic Detection of a C-H-Cleaving High-Spin Fe(IV) Complex in a Prolyl-4-Hydroxylase

The Fe(ll)- and α-ketoglutarate (αKG)-dependent dioxygenases use mononuclear nonheme iron centers to effect hydroxylation of their substrates and decarboxylation of their cosubstrate, αKG, to CO₂ and succinate. Our recent dissection of the mechanism of taurine:aKG dioxygenase (TauD), a member of this enzyme family, revealed that two transient complexes accumulate during catalysis in the presence of saturating substrates. The first complex contains the long-postulated C-H-cleaving Fe(IV)-oxo intermediate, J, and the second is an enzyme-product(s) complex. Here, we demonstrate the accumulation of two transient complexes in the reaction of a prolyl-4-hydroxylase (P4H), a functional homologue of human αKG-dependent dioxygenases with essential roles in collagen biosynthesis and oxygen sensing. The kinetic and spectroscopic properties of these two P4H complexes suggest that they are homologues of the TauD intermediates. Most notably, the first exhibits optical absorption and M6ssbauer spectra similar to those of J and, like J, a large substrate deuterium kinetic isotope on its decay. The close correspondence of the accumulating states in the P4H and TauD reactions supports the hypothesis of a conserved mechanism for substrate hydroxylation by enzymes in this family.