Long-distance cofactor interactions in terminal oxidases studied by second-derivative absorption spectroscopy.

The electronic transitions of the two heme groups of cytochrome c oxidase have been resolved by application of second-derivative and cryogenic absorption spectroscopy. Both methods reveal a splitting of the ferrocytochrome a Soret transition into two features at 443 and 450 nm. The relative intensity of the 450 nm feature appears to depend on the ligation state of cytochrome a3, the solution pH, and complex formation with cytochrome c. The structural origin and mechanistic significance of this second Soret transition of cytochrome a are discussed in terms of the electron transfer and proton translocation activities of the enzyme.