Topography and image reconstruction

The topography of Complex IV with respect to the membrane has been studied both by the aid of several kinds of labelling and cross-linking techniques (see Refs. 85, 92, 95, 96, 99), and by image reconstruction of two-dimensional crystal structures [102-106]. The wealth of this information may be summarised as the model in Fig. 

which also includes information on the positions of the redox centres (see below). 

Cytochrome oxidase catalyses electron transfer from cytochrome c to dioxygen, reducing the latter to water without release of intermediates. The maximal electron transfer activity may reach 400 (moles of cytochrome c oxidised per second per mole of cytochrome aa ) in optimal conditions at pH 7 and 25°C, both in situ and in detergent solution, although much lower activities are often encountered in the latter case (see Ref. 99). In mitochondria, respiration with natural substrates proceeds at much lower rates (cf.. Section 2.3). The kinetic capacity of cytochrome oxidase greatly exceeds demands for reasons not understood at present. 

Cytochrome c delivers electrons to the oxidase after binding to a specific site in subunit II (ATj, approx. 10M) that is located on the C side of the membrane 

The positions of the redox centres with respect to the membrane are also shown (see the text). 

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