Functional domains in the membrane

At present much evidence is quoted in favour of the idea [19-22,25,26,38] that the proton circuitry between respiration and ATP synthesis is confined to the membrane proper or its interphases with the aqueous media transversal localisation). Special high conductance pathways of the protons have been postulated along the membrane, with resistive and capacitative barriers against delocalisation of translocated protons into the bulk media. Typical of this idea is the prediction that the functionally relevant pmf (across a restricted membrane domain) is higher than that between the bulk aqueous phases (cf., above). However, other sets of experiments based on inhibitor titrations [35-37,89,90] suggest lateral localisation of protonic circuits. This implies that a particular respiratory chain complex would be able to drive ATP synthesis only in a limited membrane domain containing one or very few ATP synthase complexes. These two modes of localisation are, of course, not mutually exclusive. Transversal localisation does not necessarily require lateral localisation, but the latter is difficult to envisage unless the former is also true. 

Mitochondrial cytochrome oxidase (EC 1.9.3.1) consists of three polypeptides synthesised in the mitochondrion, and several others synthesised in the cytoplasm (Table 3.3). Preparations of the mammalian enzyme have been shown to contain up to 13 different polypeptides, and there is uncertainty at present as to which are true constituents of the enzyme. The present opposing views are those of Capaldi et al. [85,96], who regard eight polypeptides as unique constituents, and Kadenbach et al. [93,94], who suggest that all 13 are true parts of the enzyme. 

All polypeptides involved in one way or another have been sequenced (Table 3.3) in some cases the sequence is known only from the corresponding nucleotide sequence of mtDNA. 

Cytochrome oxidase contains two haem groups and two protein-bound coppers per minimal functional unit, i.e., the monomer (Table 3.4). Apparently a single copy of most polypeptides is present in this monomer. The haems are chemically identical (haem A), but are bound quite differently to the protein, which gives them widely different functions and spectroscopic properties (Table 3.4). The same is true of the two coppers, Cu and Cug. The haems will be called a and a, respectively. As discussed below, it is not certain whether they have different apoprotein parts, i.e., whether they are formally different cytochromes . 

Polypeptides of cytochrome oxidase from beef heart 

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