CRYSTAL-STRUCTURES OF CYANIDE-BOUND AND TRIIODIDE-BOUND FORMS OF ARTHROMYCES-RAMOSUS PEROXIDASE AT DIFFERENT PH VALUES - PERTURBATIONS OF ACTIVE-SITE RESIDUES AND THEIR IMPLICATION IN ENZYME CATALYSIS

The structures of the cyanide and triiodide complexes of Arthromyces r amosus peroxidase (ARP) at different pH values were investigated by x- ray crystallography in order to examine the behavior of the invariant residues of arginine (Arg-52) and distal histidine (His-56) during the enzyme reaction as well as to provide the structural basis of the act ive site of peroxidase. The models of the cyanide complexes at pH 7.5, 5.0, and 4.0, respectively, were refined to the R-factors of 17.8, 17 .8, and 18.5% using 7.0-1.6-Angstrom resolution data, and those of the triiodide complexes at pH 6.5 and 5.0 refined to 16.9 and 16.8% using 7.0-1.9-Angstrom resolution data. The structures of the cyanide compl exes at pH 7.5, 5.0, and 4.0 are identical within experimental error. Cyanide ion bound to the heme in the bent conformation rather than in the tilt conformation. Upon cyanide ion binding, the N-epsilon atom of His-56 moved toward the ion by rotation of the imidazole ring around the C-beta-C-gamma bond, but there was little conformational change in the remaining residues. The distance between the N-epsilon atom of Hi s-56 and the nitrogen atom of the cyanide suggests the presence of a h ydrogen bond between them in the pH range investigated. In the triiodi de complexes, one of the two triiodides bound to ARP was located at th e distal side of the heme. When triiodide bound to ARP, unlike the rea rrangement of the distal arginine of cytochrome c peroxidase that occu rs on formation of the fluoride complex or compound I, the side chain of Arg-52 moved little. The conformation of the side chain of His-56, however, changed markedly, Conformational flexibility of His-56 appear s to be a requisite for proton translocation from one oxygen atom to t he other of HOO- by acid-base catalysis to produce compound I. The iro n atom in each cyanide complex (low-spin ferric) is located in the hem e plane, whereas in each triiodide complex (high-spin ferric) the iron atom is displaced from the plane about 0.2 Angstrom toward the proxim al side.