Probing the role of a conserved salt bridge in the intramolecular electron transfer kinetics of human sulfite oxidase

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• 作者：Kayunta Johnson-Winters (1)
  Amanda C. Davis (2)
  Anna R. Arnold (2)
  Robert E. Berry (2)
  Gordon Tollin (2)
  John H. Enemark (2)
  
• 关键词：Cofactor ; Cytochrome ; Electrochemistry ; Electron transfer ; Enzyme kinetics
• 刊名：Journal of Biological Inorganic Chemistry
• 出版年：2013
• 出版时间：August 2013
• 年：2013
• 卷：18
• 期：6
• 页码：645-653
• 全文大小：504KB
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• 作者单位：Kayunta Johnson-Winters (1)
  Amanda C. Davis (2)
  Anna R. Arnold (2)
  Robert E. Berry (2)
  Gordon Tollin (2)
  John H. Enemark (2)
  
  1. Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, TX, 76019, USA
  2. Department of Chemistry and Biochemistry, The University of Arizona, Tucson, AZ, 85721, USA
  

文摘

Sulfite oxidase (SO) is a vital metabolic enzyme that catalyzes the oxidation of toxic sulfite to sulfate. The proposed mechanism of this molybdenum cofactor dependent enzyme involves two one-electron intramolecular electron transfer (IET) steps from the molybdenum center to the iron of the b 5-type heme and two one-electron intermolecular electron transfer steps from the heme to cytochrome c. This work focuses on how the electrostatic interaction between two conserved amino acid residues, R472 and D342, in human SO (hSO) affects catalysis. The hSO variants R472M, R472Q, R472K, R472D, and D342K were created to probe the effect of the position of the salt bridge charges, along with the interaction between these two residues. With the exception of R472K, these variants all showed a significant decrease in their IET rate constants, k et, relative to wild-type hSO, indicating that the salt bridge between residues 472 and 342 is important for rapid IET. Surprisingly, however, except for R472K and R472D, all of the variants show k cat values higher than their corresponding k et values. The turnover number for R472D is about the same as k et, which suggests that the change in this variant is rate-limiting in catalysis. Direct spectroelectrochemical determination of the Fe(III/II) reduction potentials of the heme and calculation of the Mo(VI/V) potentials revealed that all of the variants affected the redox potentials of both metal centers, probably due to changes in their environments. Thus, the position of the positive charge of R472 and that of the negative charge of D342 are both important in hSO, and changing either the position or the nature of these charges perturbs IET and catalysis.