Native lactoperoxidase, compound III, and the reduced forms (at pH 6 and 9) were studied using X-ray absorption spectroscopy (XAS). Native lactoperoxidase has four pyrrole nitrogen ligands at an average distance of 2.04 ± 0.01 Å, a proximal ligand at 1.91 ± 0.02 Å, and a sixth (distal) ligand at 2.16 ± 0.03 Å. Lactoperoxidase native enzyme has a first coordination shell structure that is similar to that of native lignin peroxidase [Sinclair, R., Yamazaki, I., Bumpus, J., Brock, B., Chang, C.-S., Albo, A., & Powers, L. (1992) Biochemistry 31, 4892‒4900] and different from that of horseradish peroxidase [Chance, B., Powers, L., Ching, Y., Poulos, T., Schonbaum, G., Yamazaki, I., & Paul, K. (1984) Arch. Biochem. Biophys. 235, 596‒611], Similarly, lactoperoxidase compound III resembles lignin peroxidase compound III. The five-coordinated ferrous form was stable at pH 9, but at pH 6 it was rapidly converted to the six-coordinated form with a distal ligand at 2.18 ± 0.03 Å. No evidence typical of changes in spin state was obtained at the different pH values.
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