Pulsed EPR Spectroscopy of 33S-Labeled Molybdenum Cofactor in Catalytically Active Bioengineered Sulfite Oxidase

• Published in: Inorganic chemistry 2014-01, Vol.53 (2), p.961-971
• Main Authors: Klein, Eric L., Belaidi, Abdel Ali, Raitsimring, Arnold M., Davis, Amanda C., Krämer, Tobias, Astashkin, Andrei V., Neese, Frank, Schwarz, Günter, Enemark, John H.
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic4023954

Molybdenum enzymes contain at least one pyranopterin dithiolate (molybdopterin, MPT) moiety that coordinates Mo through two dithiolate (dithiolene) sulfur atoms. For sulfite oxidase (SO), hyperfine interactions ( hfi ) and nuclear quadrupole interactions ( nqi ) of magnetic nuclei ( I ≠ 0) near the Mo(V) (d 1 ) center have been measured using high-resolution pulsed electron paramagnetic resonance (EPR) methods and interpreted with the help of the density functional theory (DFT) calculations. These have provided important insights about the active site structure and the reaction mechanism of the enzyme. However, it has not been possible to use EPR to probe the dithiolene sulfurs directly since naturally abundant 32 S has no nuclear spin ( I = 0). Here we describe direct incorporation of 33 S ( I = 3/2), the only stable magnetic sulfur isotope, into MPT using controlled in vitro synthesis with purified proteins. The electron spin echo envelope modulation (ESEEM) spectra from 33 S-labeled MPT in this catalytically active SO variant are dominated by the ‘inter-doublet’ transition arising from the strong nuclear quadrupole interaction, as also occurs for the 33 S-labeled exchangeable equatorial sulfite ligand [Klein, E. L., et al ., Inorg. Chem. 2012 , 51 , 1408 – 1418]. The estimated experimental hfi and nqi parameters for 33 S ( a iso = 3 MHz and e 2 Qq / h = 25 MHz) are in good agreement with those predicted by DFT. In addition, the DFT calculations show that the two 33 S atoms are indistinguishable by EPR and reveal a strong intermixing between their out-of-plane p z orbitals and the d xy orbital of Mo(V).