Probing the Cytochrome c Peroxidase−Cytochrome c Electron Transfer Reaction Using Site Specific Cross-Linking

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Abstract
Engineered cysteine residues in yeast cytochrome c peroxidase (CCP) and yeast iso-1-cytochrome c have been used to generate site specifically cross-linked peroxidase−cytochrome c complexes for the purpose of probing interaction domains and the intramolecular electron transfer reaction. Complex 2 was designed earlier [Pappa, H. S., & Poulos, T. L. (1995) Biochemistry 34, 6573−6580] to mimic the known crystal structure of the peroxidase−cytochrome c noncovalent complex [Pelletier, H., & Kraut, J. (1992) Science 258, 1748−1755]. Complex 3 was designed such that cytochrome c is tethered to a region of the peroxidase near Asp148 which has been suggested to be a second site of interaction between the peroxidase and cytochrome c. Using stopped flow methods, the rate at which the ferrocytochrome c covalently attached to the peroxidase transfers an electron to peroxidase compound I is estimated to be ≈0.5−1 s-1 in complex 3 and ≈800 s-1 in complex 2. In both complexes the Trp191 radical and not the Fe4+O oxyferryl center of compound I is reduced. Conversion of Trp191 to Phe slows electron transfer about 103 in complex 2. Steady state kinetic measurements show that complex 3 behaves like the wild type enzyme when either horse heart or yeast ferrocytochrome c is used as an exogenous substrate, indicating that the region blocked in complex 3 is not a functionally important interaction site. In contrast, complex 2 is inactive toward horse heart ferrocytochrome c at all ionic strengths tested and yeast ferrocytochrome c at high ionic strengths. Only at low ionic strengths and low concentrations of yeast ferrocytochrome c does complex 2 give wild type enzyme activity. This observation indicates that in complex 2 the primary site of interaction of CCP with horse heart and yeast ferrocytochrome c at high ionic strengths is blocked. The relevance of these results to the pathway versus distance models of electron transfer and to the interaction domains between peroxidase and cytochrome c is discussed.