Functions of electrostatic potentials and conserved distal and proximal His-Asp H-bonding networks in haem peroxidases.

Introduction Haem-containing peroxidases from plants, fungi and bacteria are evolutionarily related [ 11. They are built from two structural domains enveloping ferric protoporphyrin IX and contain 11 conserved helices. The two domains, it appears, originated from an early gene duplication event [2]. The general picture of these peroxidases has emerged largely from the crystal structure of mitochondria1 yeast cytochrome c peroxidase (CCP) [3] and the alignment of a large number of amino acid sequences. The latter also showed that three distantly related classes or evolutionary branches of peroxidases exist: intracellular peroxidases of prokaryotic origin (class I), fungal (class 11) and plant (class 111) peroxidases targeted for the secretory pathway via the endoplasmic reticulum [ 11. Figure 1 shows examples of two distinct peroxidases from each class. Each pair shows very different reactivity, despite the fact that they are 40-45% identical in amino acid sequence. In the present paper we discuss the structural basis for these distinct differences. We show that the electrostatic potential imposed by the apo-protein on the haem active site can be very different for such a pair of similar peroxidases.