Kinetic and structural studies of N-acetyl-microperoxidase-5 and -microperoxidase-8

The smallest microperoxidase-5 (MP-5) was obtained from proteolytic digesti on of cytochrome c. The heme c group of MP-5 links the pentapeptide Cys(14) -Ala-Gln-Cys-His(18) through two thioether bonds. MP-5 exists in the ferric resting state with His(18) as the proximal ligand and with water in the si xth coordination site. From the optical spectral studies on the aggregation of MP-5 and N alpha-acetyl-microperoxidase-5 (Ac-MP-5), we observed that M P-5 and Ac-MP-5 exhibit different aggregation states. At concentrations gre ater than 10 mu M, MP-5 exists as a mixture of monomers and dimers. The int ermolecular coordination of MP-5 is abolished by acetylation, and Ac-MP-5 c learly exists as a high-spin, six-coordinate ferriheme. However, the porphy rin-porphyrin interaction between Ac-MP-5 cannot be eliminated. The substra te, 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 peroxid ase activity as long as His-18 coordinates to ferriheme. Circular dichroism (CD) analyses of Ac-MP-5 and Ac-MP-8 show beta-turns in the peptide region , indicating rigidity. Based on CD analyses of MPs, we have built a molecul ar model for Ac-MP-5 containing a beta-turn in the peptide region. (C) 1999 Elsevier Science Inc. All rights reserved.