Fig. 3.

Separated redox difference spectra of cytochrome a and Cu_A.

0.1 M sodium phosphate buffer pH 7.4 0.1% lauryl maltoside 22 °C. with 7.5 mM sodium formate present in the mixed valence sample (haem a Cu_A reduced/haem a₃ CuB oxidised). Formate complexes formed as described in Materials and methods.

A. Separated difference spectra +/− formate (haem a and Cu_A):

------ formate mixed valence minus oxidised formate (haem a Fe^2 + plus Cu_A⁺) (IV− VIII, Table 2);

formate mixed valence minus calculated Cu_A (haem a Fe^2 +); (i.e. haem a reduced minus oxidised).

formate mixed valence minus calculated haem a (Cu_A⁺) (i.e. Cu_A reduced minus oxidised).

B. Bacterial and mammalian Cu_A reduced minus oxidised difference spectra.

Cu_A difference spectrum obtained by reduction in the presence of formate (converted to extinction coefficients);

-------- Thermus thermophilus subunit II difference spectrum (digitised version from the cytochrome oxidase Web site http://www-bioc.rice.edu/~graham/CcO.Spectra.pc).

The troughs in the visible spectra are closely matched but in the NIR the bacterial subunit II spectrum is blue shifted by ~ 34 nm relative to the computed cytochrome c oxidase Cu_A . The 599 to 611 nm spike is due to the haem a spectral shift that occurs upon Cu_A reduction (see Discussion).