Probing the iron-substrate orientation for taurine/α-ketoglutarate dioxygenase using deuterium electron spin echo envelope modulation spectroscopy

The structural relationship between substrate taurine and the non-heme Fe(II) center of taurine/alpha-ketoglutarate (alpha KG) dioxygenase (TauD) was measured using electron spin echo envelope modulation (ESEEM) spectroscopy. Studies were conducted on TauD samples treated with NO, cosubstrate alpha KG, and either protonated or specifically deuterated taurine. Stimulated echo ESEEM data were divided to eliminate interference from H-1 and N-14 modulations and accentuate modulations from H-2. For taurine that was deuterated at the C-1 position (adjacent to the sulfonate group), H-2 ESEEM spectra show features that arise from dipole-dipole and deuterium nuclear quadrupole interactions from a single deuteron. Parallel measurements taken for taurine deuterated at both C-1 and C-2 show an additional ESEEM feature at the deuterium Larmor frequency. Analysis of these data at field positions ranging from g = 4 to g = 2 have allowed us to define the orientation of substrate taurine with respect to the magnetic axes of the Fe(II)-NO, S = (3)/(2), paramagnetic center. These results are discussed in terms of previous X-ray crystallographic studies and the proposed catalytic mechanism for this family of enzymes.