1. Using stopped-flow technique we have investigated the electron transfer form cytochrome c to cytochrome aa3 and to the (porphyrin) cytochrome c-cytochromeaa3 complex.\ud \ud 2. In a low ionic strength medium, the pre-steady state reaction occurs in a biphasic way with rate constants of at least 2 · 108 M−1 · s−1 and about 107 M−1 · s−1 (I = 8.8 mM, pH 7.0, 10° C), respectively.\ud \ud 3. A comparison of the rate constants, determined in the presence of an excess of cytochrome c with those found in the presence of an excess of cytochrome aa3 reveals the existence of two slower reacting sites on the functional unit (2 hemes and 2 coppers) of cytochrome aa3. On basis of these results we discuss various models. If no site-site interactions are assumed (non-cooperative model) cytochrome aa3 has 2 high and 2 low affinity sites available for the reaction with ferrocytochrome c. If negative cooperativity occurs, cytochrome aa3 has 2 high affinity sites which change into 2 low affinity sites upon binding of one cytochrome c molecule. The latter model is favoured
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