The crystal structure of ascorbate and manganese peroxidases: the role of non-haem metal in the catalytic mechanism.

First, the enzyme reacts with one peroxide equivalent resulting in oxidation of the haem iron to the oxyferryl, Fe(IV-0), centre and an organic radical, R', where R' is either the porphyrin or an amino acid-centred radical. Next, Compound I oxidizes one substrate molecule to give a substrate radical and Compound 11. Finally, Compound I1 is reduced by a second substrate molecule to the resting, Fe(III), state. In the past few years there has been a rapid growth in crystallographic structural information on haem peroxidases. In addition to cytochrome c peroxidase (CCP), whose structure has been known for over 10 years, the structures of myeloperoxidase [ 1 3 , lignin peroxidase (Lip) [ 2 ] , Arthromyces ramosw peroxidase [ 3 ] and Coprinus cinereus peroxidase [ 4 ] are now known. In addition, we have just completed the elucidation of the manganese peroxidase (MnP) structure from the white rot lignin-degrading Basidomycetous fungus Phanerochaete chrysosporium [ 51, and the recombinant pea cytosolic ascorbate peroxidase (APX) structure [6]. Despite the low level of sequence identity amongst these peroxidases (often < 20%) the overall helical topology is conserved. The level of conservation can best be appreciated by comparing the peroxidase family of enzymes with another family of haem enzymes that are also dominated by a-helices, the cytochromes P-450. In a short review, Li and Poulos [7] found that the peroxidases exhibit greater topological similarity than do the P-450s. For example, a com-