Semiempirical Molecular Orbital Calculation for the Redox Property of C-Terminal Active Site　Sequence of Human Thioredoxin Reductase

Semiempirical molecular orbital calculation (MOPAC) was used to estimate the enthalpy difference (ΔH) between the reduce and oxidized states of the C-terminal rebox center of human thioredoxin reductase. Heat of formation was computed by WinMOPAC 3.5Pro for the model peptides, N-Acetyl-Ser-Ile-Leu-Gly-X1-X2-Gly, whose-X1-X2-sequence was-Cys SeCys-(natural sequence), -SeCys-Cys-(reverse sequence), -Cys-Cys, and-SeCys-SeCys-. Calculation by Hamiltonian AM1 and PM3 agreed that the oxidized state with selenosulfide bonds and a diselenide bond were more favoralbe than their reduced states. Only the peptide that contained-Cys-Cys-sequence was shown to have lower enthalpy when the two Cys were in the reduced form. It has been reported that substitution of SeCys498 to Cys results in the mujtant TrxRs retaining only about 1% of the enzyme activity. The results of computational estimation supported the experimental hypothesis that the inactivation by SeCys498Cys mutation was due to the unfavorable formation of disulfide bond between Cys497-Cys498.