Susana Prazeres

The Structure of an Electron Transfer Complex Containing a Cytochrome c and a Peroxidase

Efficient biological electron transfer may require a fluid association of redox partners. Two noncrystallographic methods (a new molecular docking program and 1H NMR spectroscopy) have been used to study the electron transfer complex formed between the cytochrome c peroxidase (CCP) ofParacoccus denitrificans and cytochromes c. For the natural redox partner, cytochrome c 550, the results are consistent with a complex in which the heme of a single cytochrome lies above the exposed electron-transferring heme of the peroxidase. In contrast, two molecules of the nonphysiological but kinetically competent horse cytochrome bind between the two hemes of the peroxidase. These dramatically different patterns are consistent with a redox active surface on the peroxidase that may accommodate more than one cytochrome and allow lateral mobility.

Mössbauer Characterization of Paracoccus denitrificans Cytochrome c Peroxidase FURTHER EVIDENCE FOR REDOX AND CALCIUM BINDING-INDUCED HEME-HEME INTERACTION

Mössbauer and electron paramagnetic resonance (EPR) spectroscopies were used to characterize the diheme cytochrome c peroxidase from Paracoccus denitrificans (L.M.D. 52.44). The spectra of the oxidized enzyme show two distinct spectral components characteristic of low spin ferric hemes (S = 1/2), revealing different heme environments for the two heme groups. The Paracoccus peroxidase can be non-physiologically reduced by ascorbate. Mössbauer investigation of the ascorbate-reduced peroxidase shows that only one heme (the high potential heme) is reduced and that the reduced heme is diamagnetic (S = 0). The other heme (the low potential heme) remains oxidized, indicating that the enzyme is in a mixed valence, half-reduced state. The EPR spectrum of the half-reduced peroxidase, however, shows two low spin ferric species with gmax = 2.89 (species I) and gmax = 2.78 (species II). This EPR observation, together with the Mössbauer result, suggests that both species are arising from the low potential heme. More interestingly, the spectroscopic properties of these two species are distinct from that of the low potential heme in the oxidized enzyme, providing evidence for heme-heme interaction induced by the reduction of the high potential heme. Addition of calcium ions to the half-reduced enzyme converts species II to species I. Since calcium has been found to promote peroxidase activity, species I may represent the active form of the peroxidatic heme.

Redox and Spin-State Control of the Activity of a Diheme Cytochrome C Peroxidase — Spectroscopic studies

Hydrogen peroxide formed in cells, as the result of incomplete reduction of oxygen, can be removed essentially by two ways: by peroxidases in a process of reduction to water or by catalase in a dismutation reaction. The actions of these enzymes are essential to prevent the accumulation of hydrogen peroxide, diminishing the risk of peroxide-induced damage of cell constituents [1].