John H. Enemark
B.A. 1962, St. Olaf College
A.M. 1964, Harvard University
Ph.D. 1966, Harvard University

office CSML 320
phone (520) 621-2245
fax (520) 626-8065

Bioinorganic Chemistry; Molybdenum-Containing Enzymes; Electronic Structure and Spectroscopy; Pulsed EPR; Metal Nitrosyls

Molybdenum is an essential trace element for all forms of life and over 30 molybdoenzymes are known that catalyze oxidation-reduction reactions involved in the metabolism of carbon, nitrogen and sulfur. Of particular interest to our group is the enzyme sulfite oxidase (SO) that is required for normal neurological development in children. The determination of the crystal structure of chicken liver SO from protein prepared in our group has provided a basis for interpreting fatal point mutations in the highly homologous human enzyme. In addition, the novel structure of the molybdenum center of SO provides a target for the synthesis of new active site models as well as a framework for interpreting spectra from the enzyme. The large separation between the molybdenum center and the b-type heme center of SO raises fundamental questions about the process and pathways of intramolecular electron transfer in the protein. Our research involves an integrated program of chemical, biochemical and biophysical studies that include: synthesis of new compounds, X-ray structure determination, theoretical calculations, kinetics, and many types of spectroscopy (CW- and pulsed EPR, electronic, magnetic circular dichroism (MCD), resonance Raman, NMR, K- and L-edge X-ray absorption, photoelectron (PES)).

(top) Ribbon Diagram of Chicken Liver Sulfite Oxidase

(bottom) Mo Active Site of Chicken Liver Sulfite Oxidase

I. Models for Molybdenum-Containing Enzymes

The structure of SO has stimulated the synthesis of oxo-molybdenum compounds that contain three sulfur donor atoms. The first compounds exhibiting this previously unknown Mo coordination provide a rich framework for spectroscopic characterization of the MoOS3 unit. Emphasis is also being given to preparing compounds that contain both an oxo-molybdenum center and an iron porphyrin center and to investigating the weak coupling between the Mo(V) and low-spin Fe(III) centers by both NMR and EPR spectroscopy.

II. Multi-Frequency Pulsed EPR Spectroscopy

The paramagnetic Mo(V) state of SO provides a highly sensitive probe of the active site. In collaboration with the EPR Facility of the University of Arizona we are applying multi-frequency pulsed EPR (ESEEM and ENDOR) to SO to investigate the detailed chemistry of the transient Mo(V) species formed during catalysis. In addition, we are using pulsed electron-electron double resonance (ELDOR) to directly measure the MoFe distance in solution to better understand the wide range in the rates of intramolecular electron transfer as a function of pH and anion concentration. Studies of mutant forms of SO should provide additional important insight concerning the critical requirements of the Mo center for effective catalysis.

III. Metal-Sulfur Covalency

Metal-sulfur bonds play an important role in many biological and industrial catalysts. We are investigating the covalency of Mo-S bonds by EPR spectroscopy, gas phase photoelectron spectroscopy, and X-ray absorption spectroscopy. These studies suggest that under appropriate conditions, Mo-S bonds function as an "electronic buffer" to oxidation state changes at the metal center.

IV. Kinetics of Intramolecular Electron Transfer

Intramolecular electron transfer between the molybdenum and heme domains of SO is a key feature of the proposed catalytic mechanism. The wide variation in the rates of intramolecular electron transfer with pH and anion concentration is currently under study and should provide fundamental insight concerning electron transfer processes in molybdoenzymes.


  • Westcott, B. L.; Gruhn, N. E.; Enemark, J. H. "Evaluation of Molybdenum-Sulfur Interactions in Molybdoenzyme Model Complexes by Gas-Phase Photoelectron Spectroscopy. The "Electronic Buffer" Effect." J. Am. Chem. Soc. 1998, 120, 3382-3386.

  • Enemark, J. H. "EPR and MCD Studies of Oxomolybdenum Centers in Sulfite Oxidase and Related Model Compounds." In Spectroscopic Methods in Bioinorganic Chemistry, ACS Symposium Series; Vol. 692; Solomon, E. I.; Hodgson, K. O., Eds.; American Chemical Society: Washington, DC, 1998, pp 360-371.

  • Rawn, C. J.; Birnie, I., D. P.; Bruck, M. A.; Enemark, J. H.; Roth, R. S. "Structural Investigation of Ba6-3xLn8+2xTi18O54 (x = 0.27, Ln = Sm) by Single Crystal X-ray Diffraction in Space Group Pnma (No. 62)." J. Mater. Res. 1998, 13, 187-196.

  • McMaster, J.; Enemark, J. H. "The Active Sites of Mo- and W-Containing Enzymes." Curr. Opin. in Chem. Biol. 1998, 2, 201-207.

  • Enemark, J. H.; Westcott, B. L. "Transition Metal Nitrosyls." In Inorganic Electronic Structure and Spectroscopy; Volume II: Applications and Case Studies; Solomon, E. I.; Lever, A. B. P., Eds.: John Wiley and Sons, 1999, pp 403-450.

  • Basu, P.; Valek, M.; Enemark, J. H. "Binuclear Oxomolybdenum-Metalloporphyrin Complexes." In Inorganic Synthesis; Coucouvanis, D., Ed., 1999; Vol. 33.

  • Fischer, B.; Enemark, J. H.; Basu, P. "A Chemical Approach to Systematically Designate the Pyranopterin Centers of Molybdenum and Tungsten Enzymes and Synthetic Models." J. Inorg. Biochem. 1998, 72, 13-21.

  • Raitsimring, A. M.; Pacheco, A.; Enemark, J. H. "ESEEM Investigations of the High pH and Low pH Forms of Chicken Liver Sulfite Oxidase." J. Am. Chem. Soc. 1998, 120, 11263-11278.

  • Pacheco, A.; Hazzard, J. T.; Tollin, G.; Enemark, J. H. "The pH Dependence of Intramolecular Electron Transfer Rates in Sulfite Oxidase at High and Low Anion Concentrations." JBIC, J. Biol. Inorg. Chem. 1999, 4, 390-401.

  • Inscore, F. E.; McNaughton, R.; Westcott, B.; Helton, M. E.; Jones, R.; Dhawan, I. K.; Enemark, J. H.; Kirk, M. L. "Spectroscopic Evidence for a Unique Bonding Interaction in Oxo-Molybdenum Dithiolate Complexes: Implications for Electron Transfer Pathways in the Pyranopterin Dithiolate Centers of Enzymes." Inorg. Chem. 1999, 38, 1401-1410.

  • Mader, M. L.; Carducci, M. D.; Enemark, J. H. "Analogues of the Molybdenum Center of Sulfite Oxidase: Oxomolybdenum(V) Complexes with Three Thiolate Sulfur Donor Atoms." Inorg. Chem.2000, 39, 525-531.

  • Izumi, Y.; Rose, K.; Glaser, T.; McMaster, J.; Basu, P.; Enemark, J. H.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. "Ligand K-edge and Metal L-edge X-Ray Absorption Spectroscopy and Density Functional Calculations of Oxomolybdenum Complexes with Thiolate and Related Ligands: Implications for Sulfite Oxidase." J. Am. Chem. Soc. 1999, 121, 10035-10046.

  • Astashkin, A. V.; Mader, M. L.; Enemark, J. H.; Pacheco, A.; Raitsimring, A. M. "Direct Detection of the Proton-Containing Group Coordinated to Mo(V) in the High-pH Form of Chicken Liver Sulfite Oxidase by Refocused Primary ESEEM Spectroscopy: Structural and Mechanistic Implications." J. Am. Chem. Soc. 2000, 122, 5294-5302.

  • Helton, M. E.; Pacheco, A.; McMaster, J.; Enemark, J. H.; Kirk, M. L. "An MCD Spectroscopic Study of the Molybdenum Active Site in Sulfite Oxidase: Insight into the Role of Coordinated Cysteine." J. Inorg. Biochem. 2000, 80, 227-233.

  • Astashkin, A. V.; Cosper, M. M.; Raitsimring, A. M.; Enemark, J. H. "ESEEM and Mims ENDOR Spectroscopy of cis,trans-(L-N2S2)MoVO(SCH2Ph): Detection of the,Two Benzylthiolate Protons." Inorg. Chem. 2000, 39, 4989-4992.

  • McMaster, J.; Carducci, M. D.; Yang, Y.-S.; Solomon, E. I.; Enemark, J. H. "Electronic Spectral Studies of Molybdenyl Complexes: II. MCD Spectroscopy of [MoOS4]- Centers." Inorg. Chem. 2001, 40, 687-702.

  • Enemark, J. H.; Cosper, M. M. "Molybdenum Enzymes and Sulfur Metabolism." In Metal Ions in Biological Systems; Sigel, A.; Sigel, H., Eds.; Marcel Dekker, Inc.: New York, 2001, in press.

date: Wed, 14 Aug 2002 19:17:06 GMT
time: Wed, 14 Aug 2002 19:17:06 GMT