The structure of heme proteins Compounds I and II: some misconceptions

Many reactions catalyzed by heme proteins involve an oxidation of the heme to one or two equivalents above the ferric state. Such intermediates are often referred to as Compound II and Compound I, respectively. Several different notations are used in the literature to describe the chemical structures of these compounds, which has led to errors and misinterpretations. The main problems are: 1. For many biochemists the notations X - FeIV = O and X - Fe + = O are equivalent and are used interchangeably, whereas other biochemists interpret these notations to have quite different meanings. 2. It is inaccurate and misleading to illustrate the increased oxidation state of Compound I by just adding two positive charges to the structure. 3. The bond between the oxygen and iron is not a conventional double bond and illustrating it as such leads to misconceptions concerning its properties. 4. In several instances, including horseradish peroxidase Compound I, there is reason to doubt that the radical moiety of the porphyrin ring (or of the protein in other peroxidases) carries a positive charge. The purpose of this article is to promote the use of uniform, as well as chemically correct, formulae and equations in describing the structures and reactivities of Compounds I and II. To accomplish this, a new notation is proposed for the iron-oxygen bond in these compounds.