The reactions of the fungal enzymes Arthromyces ramosus peroxidase (ARP) and Phanerochaete chrysosporium lignin peroxidase (LiP) with hydrogen peroxide (H 2 O 2 ) have been studied. Both enzymes exhibited catalase activity with hyperbolic H 2 O 2 concentration dependence (K m Ϸ 8 -10 mM, k cat Ϸ 1-3 s ؊1 ). The catalase and peroxidase activities of LiP were inhibited within 10 min and those of ARP in 1 h. The inactivation constants were calculated using two independent methods; LiP, k i Ϸ 1 The heme peroxidases have been classified into two distinct groups, termed the animal (found only in animals) and plant (found in plants, fungi, and prokaryotes) superfamilies (1). The plant peroxidases, which share similar overall protein folds and specific features, such as catalytically essential histidine and arginine residues in their active sites, have been subdivided into three classes on the basis of sequence comparison (2, 3). In class I are intracellular enzymes including yeast cytochrome c peroxidase, ascorbate peroxidase (APX) from plants, and bacterial geneduplicated catalase-peroxidases (4). Class III contains the secretory plant peroxidases such as those from horseradish (HRP), barley, or soybean. These peroxidases seem to be biosynthetic enzymes involved in processes such as plant cell wall formation and lignification. Class II consists of the secretory fungal peroxidases such as lignin peroxidase (LiP) from Phanerochaete chrysosporium, manganese peroxidase from the same source, and Coprinus cinereus peroxidase or Arthromyces ramosus peroxidase (ARP), which have been shown to be essentially identical in both sequence and properties (5). The main role of class II peroxidases appears to be the degradation of lignin in wood.All peroxidases studied so far share much the same catalytic cycle that proceeds in three distinct and essentially irreversible steps (6) and is often referred to as the "peroxidase ping-pong." The resting ferric enzyme reacts with H 2 O 2 in a two-electron process to generate the intermediate known as compound I. Compound I is discharged in two sequential single-electron reactions with reducing substrate yielding radical products, which are often highly reactive, and water. The first reduction step results in the formation of another enzyme intermediate, compound II. In the final step compound II is reduced back to ferric peroxidase.The peroxidase ping-pong provides an adequate description of the peroxidase reaction; however, continuing work has revealed some limitations of the basic model. The compound I reduction steps have been shown to consist of reversible substrate binding followed by substrate oxidation (7). The formation and nature of compound I have been studied intensively. Both a neutral peroxidase-peroxide complex and a charged complex (known as compound 0) have been observed (8), and variations have been identified in the electronic structures of the compound Is of different peroxidases (9 -14). Furthermore, certain peroxidases have been found to utilize H 2 O 2 to reduce compoun...

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