Role of Conserved Tyrosine 343 in Intramolecular Electron Transfer in Human Sulfite Oxidase

2003 
Abstract Tyrosine 343 in human sulfite oxidase (SO) is conserved in all SOs sequenced to date. Intramolecular electron transfer (IET) rates between reduced heme (FeII) and oxidized molybdenum (MoVI) in the recombinant wild-type and Y343F human SO were measured for the first time by flash photolysis. The IET rate in wild-type human SO at pH 7.4 is about 37% of that in chicken SO with a similar decrease in k cat. Steady-state kinetic analysis of the Y343F mutant showed an increase inK m sulfite and a decrease ink cat resulting in a 23-fold attenuation in the specificity constantk cat/K m sulfiteat the optimum pH value of 8.25. This indicates that Tyr-343 is involved in the binding of the substrate and catalysis within the molybdenum active site. Furthermore, the IET rate constant in the mutant at pH 6.0 is only about one-tenth that of the wild-type enzyme, suggesting that the OH group of Tyr-343 is vital for efficient IET in SO. The pH dependences of IET rate constants in the wild-type and mutant SO are consistent with the previously proposed coupled electron-proton transfer mechanism.
    • Correction
    • Source
    • Cite
    • Save
    • Machine Reading By IdeaReader
    44
    References
    29
    Citations
    NaN
    KQI
    []