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Papers In Press, published online ahead of print November 11, 2003
J. Biol. Chem, 10.1074/jbc.M301013200
Submitted on January 30, 2003
Revised on October 23, 2003
Accepted on November 10, 2003
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai, Miyagi 980-8577
Corresponding Author: shimizu{at}tagen.tohoku.ac.jp
The heme-regulated phosphodiesterase, Ec DOS, is a redox sensor that uses the heme in its PAS domain to regulate catalysis. The rate of O2 association (kon) with full-length Ec DOS is extremely slow at 0.0019 mM-1s-1, compared to >9.5 mM-1s-1 for 6-coordinated globin-type hemoproteins, as determined by the stopped-flow method. This rate is dramatically increased (up to 16-fold) in the isolated heme-bound PAS domain. Dissociation constants (Kd) calculated from the kinetic parameters are 340 mM and 20 mM for the full-length wild-type enzyme and its isolated PAS domain, respectively. Mutations at Met95 in the isolated PAS domain, which may be a heme axial ligand in the Fe(II) complex, lead to a further increase in the kon value by more than 30-fold, and consequently, a decrease in the Kd value to less than 1 mM. The kon value for CO binding to the full-length wild-type enzyme is also very low (0.00081 mM-1s-1). The kinetics of CO binding to the isolated PAS domain and its mutants are similar to those observed for O2. However, the Kd values for CO are considerably lower than those for O2.
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