Determination of the distance between the Mo(V) and Fe(III) heme centers of wild type human sulfite oxidase by pulsed EPR spectroscopy.
Intramolecular electron transfer (IET) between the molybdenum and heme centers of vertebrate sulfite oxidase (SO) is proposed to be a key step in the catalytic cycle of the enzyme. However, the X-ray crystallographic distance between these centers, R(MoFe) = 32.3 Å, appears to be too long for the rapid IET rates observed in liquid solution. The Mo and heme domains are linked by a flexible tether, and it has been proposed that dynamic interdomain motion brings the two metal centers closer together and thereby facilitates rapid IET. To date, there have been no direct distance measurements for SO in solution that would support or contradict this model. In this work, pulsed electron-electron double resonance (ELDOR) and relaxation induced dipolar modulation enhancement (RIDME) techniques were used to obtain information about R(MoFe) in the Mo(V)Fe(III) state of wild type recombinant human SO in frozen glassy solution. Surprisingly, the data obtained suggest a fixed structure with R(MoFe) = 32 Å, similar to that determined by X-ray crystallography for chicken SO, although the orientation of the R(MoFe) radius-vector with respect to the heme center was found to be somewhat different. The implications of these findings for the flexible tether model are discussed.
Department of Chemistry & Biochemistry, University of Arizona, Tucson, Arizona 85721, USA. andrei@u.arizona.edu
Andrei V
AV
Astashkin
Asha
A
Rajapakshe
Matthew J
MJ
Cornelison
Kayunta
K
Johnson-Winters
John H
JH
Enemark
D000818:Animals; D020134:Catalytic Domain; D002645:Chickens; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D004579:Electron Transport; D005290:Ferric Compounds / Q000737:chemistry*; D006418:Heme / Q000737:chemistry*; D006801:Humans; D008982:Molybdenum / Q000737:chemistry*; D050862:Oxidoreductases Acting on Sulfur Group Donors / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D013696:Temperature
D005290:Ferric Compounds; D011994:Recombinant Proteins; D006418:Heme; D008982:Molybdenum; D050862:Oxidoreductases Acting on Sulfur Group Donors; C503599:SUOX protein, human