Hematin iron valence in catalase and peroxidase compound I: Relationship to free radical reaction mechanism

The material in this paper is centered on the structure of compound I (first reaction intermediate) in the case of catalase and a classical peroxidase (horseradish peroxide, HRP). The concept of a π-cation radical is accepted for HRP but is rejected in the case of catalase. A possible mechanism for catalatic action previously proposed assumes FeV for the hematin iron of catalase and hydride ion transfer in the reduction of FeV by the second molecule of H 2O 2, no free radical being involved. In the case of HRP however, FeIV is assumed for compound I. A hypothetical ·OH needed to balance the reaction for the formation of compound I is thought to interact with the π electron cloud of the hematin prosthetic group, forming the now generally accepted π cation radical and an OH − ion. Attempts to apply the π cation mechanism to catalatic action lead to contradictions and implausible chemical reactions.