The reduced minus oxidized difference spectra of cytochromes a and a3

We have re-investigated the reduced minus oxidized difference spectra of the two heme centers of cytochrome oxidase, cytochromes a and a3. In contrast to data obtained in an earlier study (Vanneste, W.H. (1966) Biochemistry 5, 838-848), we find that the spectrum for cytochrome a3 agrees with that found with a 5-coordinate high-spin heme A model compound. Small but significant additional differences are noted for both heme centers.