| Budget Amount *help |
¥15,600,000 (Direct Cost: ¥15,600,000)
Fiscal Year 2006: ¥4,800,000 (Direct Cost: ¥4,800,000)
Fiscal Year 2005: ¥5,200,000 (Direct Cost: ¥5,200,000)
Fiscal Year 2004: ¥5,600,000 (Direct Cost: ¥5,600,000)
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| Research Abstract |
Regioselective reactions catalyzed by enzymes are very important biologically and chemically. Therefore, molecular mechanisms of regioselective reactions have been studied for various enzymes with various spectroscopic methods. We have studied the molecular mechanism of heme oxygenase (HO), which catalyzes regioselective degradation of heme (iron protoporphyrinIX) to α-biliverdin, CO, and free iron ion. Interestingly, HO regiospecifically oxidize the α-meso position of the heme to form α-biliverdin isomer while non-enzymatic heme degradation forms all four possible α, β, γ, δ-biliverdin isomers at nearly identical yield. We have found two essential amino acid residues, Asp-140 and Arg-183 in rat-HO, for the a-regioselective heme degradation. We showed that the Asp-140 and Arg-183 residues control oxygen activation process and the substate (heme) binding process via hydrogen-bonding interactions, respectively. With combination of the Asp-140 and Arg-183 residues, the a-meso position of the heme is placed at the nearest position from the activated oxygen species, resulting in the a-regioselective reaction. This mechanism let us imagine that we can switch the regioselectivity of HO if we can control an orientation of the heme in HO so that the other meso position is placed at the nearest position from the activated oxygen species. To realize this idea, on the basis of the crystal structure of the wild type HO, we designed a mutant, in which the other meso position is placed at the nearest position from the activated oxygen species. In this project, we succeeded in conversion of α-selective HO to δ-selective HO with the present strategy. Furthermore, we also succeeded in controlling heme orientation in HO with mutation of amino acid residues, which results in β-selective and δ-selective HO.
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