Spin-State Rationale for the Peroxo-Stabilizing Role of the Thiolate Ligand in Superoxide Reductase

摘要

Nature uses cysteine to modulate the electronic properties of the metal centers in metalloproteins.[1] Particularly interesting is the contrast in function between two mononuclear iron enzymes with similar square pyramidal {Fe~(II)(N)_4(Cys_(apical))} active sites: cytochrome P450[2] and superoxide reductase (SOR).[3], [4] The well-studied cytochrome P450 has an {Fe~(II)(porphyrin dianion)(Cys)} site that activates dioxygen to oxidize hydrocarbon substrates.[5] Dioxygen activation is proposed to occur via a low-spin FeIII-OOH intermediate that converts into a high-valent iron-oxo oxidizing species[6], [7] with the heterolytic OO bond-cleavage step assisted by a push of electron density from the trans thiolate ligand (Scheme 1 A).[8] In contrast, superoxide reductase (SOR) uses a similar {Fe~(II)(His)4(Cys)} site to convert superoxide into hydrogen peroxide as a means of protecting anaerobes and microaerophiles from the inimical effects of superoxide that may be formed from occasional intrusion of O2 into the cell.