The smallest microperoxidase-5 (MP-5) was obtained from proteolytic digestion of cytochrome c. The heme c group of MP-5 links the pentapeptide Cys14-Ala-Gln-Cys-His18 through two thioether bonds. MP-5 exists in the ferric resting state with His18 as the proximal ligand and with water in the sixth coordination site. From the optical spectral studies on the aggregation of MP-5 and Nα-acetyl-microperoxidase-5 (Ac-MP-5), we observed that MP-5 and Ac-MP-5 exhibit different aggregation states. At concentrations greater than 10 μM, MP-5 exists as a mixture of monomers and dimers. The intermolecular coordination of MP-5 is abolished by acetylation, and Ac-MP-5 clearly exists as a high-spin, six-coordinate ferriheme. However, the porphyrin-porphyrin interaction between Ac-MP-5 cannot be eliminated. The substrate, o-methoxyphenol, was oxidized by hydrogen peroxide using Ac-MP-5 and Ac-MP-8 as catalysts. Comparisons of the peroxidase activities of Ac-MP- 5, MP-8, and Ac-MP-8 suggest that microperoxidases (MPs) retain the same peroxidase activity as long as His-18 coordinates to ferriheme. Circular dichroism (CD) analyses of Ac-MP-5 and Ac-MP-8 show β-turns in the peptide region, indicating rigidity. Based on CD analyses of MPs, we have built a molecular model for Ac-MP-5 containing a β-turn in the peptide region.
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