A quantum chemical study of the mechanism of manganese catalase

The catalytic mechanism of manganese catalase has been studied using the Becke's three parameter hybrid method with the Lee. Yang and Parr correlation functional. On the basis of available experimental information on the geometric and electronic structure of the active manganese dimer complex, different possibilities were investigated. The mechanism finally suggested consists of eight steps. In the first steps, the first hydrogen peroxide becomes bound and its O-O bond is activated. This occurs in a spin-forbidden process found to be common in many biological processes where the O-O bond is cleaved. and two general rules are formulated for the requirements for a low activation energy in this type of reaction. As the O-O bond is cleaved a hydroxyl radical is initially formed in the overall rate-limiting step of the catalytic cycle. This radical is then immediately and irreversibly quenched in a strongly exothermic step. In the subsequent steps, the second hydrogen peroxide becomes bound and its two OH bonds are broken, leading to the formation of an O-2 molecule. which is released. Parallels between the reversal of the present O-O cleavage mechanism in manganese catalase and the recently suggested O-O bond formation in photosystem II are drawn.