@article{DongYangReschkeetal.2017,
  author    = {Dong, Chao and Yang, Jing and Reschke, Stefan and Leimk{\"u}hler, Silke and Kirk, Martin L.},
  title     = {Vibrational Probes of Molybdenum Cofactor-Protein Interactions in Xanthine Dehydrogenase},
  series = {Inorganic chemistry},
  volume    = {56},
  journal   = {Inorganic chemistry},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0020-1669},
  doi       = {10.1021/acs.inorgchem.7b00028},
  pages     = {6830 -- 6837},
  year      = {2017},
  abstract  = {The pyranopterin dithiolene (PDT) ligand is an integral component of the molybdenum cofactor (Moco) found in all molybdoenzymes with the sole exception of nitrogenase. However, the roles of the PDT in catalysis are still unknown. The PDT is believed to be bound to the proteins by an extensive hydrogen bonding network, and it has been suggested that these interactions may function to fine-tune Moco for electron- and atom-transfer reactivity in catalysis. Here, we use resonance Raman (rR) spectroscopy to probe Moco-protein interactions using heavy-atom congeners of lumazine, molecules that bind tightly to both wild-type xanthine dehydrogenase (wt-XDH) and its Q102G and Q197A variants following enzymatic hydroxylation to the corresponding violapterin product molecules. The resulting enzyme-product complexes possess intense near-IR absorption, allowing high-quality rR spectra to be collected on wt-XDH and the Q102G and Q197A variants. Small negative frequency shifts relative to wt-XDH are observed for the low-frequency Moco vibrations. These results are interpreted in the context of weak hydrogen-bonding and/or electrostatic interactions between Q102 and the -NH2 terminus of the PDT, and between Q197 and the terminal oxo of the Mo equivalent to O group. The Q102A, Q102G, Q197A, and Q197E variants do not appreciably affect the kinetic parameters k(red) and k(red)/K-D, indicating that a primary role for these glutamine residues is to stabilize and coordinate Moco in the active site of XO family enzymes but to not directly affect the catalytic throughput. Raman frequency shifts between wt-XDH and its Q102G variant suggest that the changes in the electron density at the Mo ion that accompany Mo oxidation during electron-transfer regeneration of the catalytically competent active site are manifest in distortions at the distant PDT amino terminus. This implies a primary role for the PDT as a conduit for facilitating enzymatic electron-transfer reactivity in xanthine oxidase family enzymes.},
  language  = {en}
}
@article{DongYangLehnkuehleretal.2014,
  author    = {Dong, Chao and Yang, Jing and Lehnkuehler, Silke and Kirk, Martin L.},
  title     = {Pyranopterin dithiolene distortions relevant to electron transfer in xanthine oxidase/dehydrogenase},
  series = {Inorganic chemistry},
  volume    = {53},
  journal   = {Inorganic chemistry},
  number    = {14},
  publisher = {American Chemical Society},
  address   = {Washington},
  issn      = {0020-1669},
  doi       = {10.1021/ic500873y},
  pages     = {7077 -- 7079},
  year      = {2014},
  abstract  = {The reducing substrates 4-thiolumazine and 2,4-dithiolumazine have been used to form Mo-IV-product complexes with xanthine oxidase (XO) and xanthine dehydrogenase. These Mo-IV-product complexes display an intense metal-to-ligand charge-transfer (MLCT) band in the near-infrared region of the spectrum. Optical pumping into this MLCT band yields resonance Raman spectra of the Mo site that are devoid of contributions from the highly absorbing FAD and 2Fe2S clusters in the protein. The resonance Raman spectra reveal in-plane bending modes of the bound product and low-frequency molybdenum dithiolene and pyranopterin dithiolene vibrational modes. This work provides keen insight into the role of the pyranopterin dithiolene in electron-transfer reactivity.},
  language  = {en}
}