The redox centres and their location

It is generally accepted that the four redox centres are located in subunits I and II (see Refs. 85, 92, 96, 99). Cu is almost certainly in subunit II which, near the carboxy terminus, has a segment that is homologous to the copper-binding site of copper proteins [92,122,136] (Fig. 3.4). Moreover, Chan et al. [137] have recently 

Winter et al. [140] suggested that haem a is bound to subunit II. If so, it may be uniquely located because it is a bisimidazole complex and the number of conserved histidines is limited. It would then be expected to be near the C domain, sandwiched in part between transmembranous helices [92] (Fig. 3.4). This would agree with the perpendicularity between the haem and membrane planes observed by polarised spectroscopy of orientated enzyme specimens [141-143]. 

However, at present it cannot be excluded that both haem groups may be linked to the same protein, viz. subunit I. This subunit contains 11 invariant histidines, of which four are predicted to be located in the aqueous domains, and seven in the membrane (Fig. 3.5). 

Of the nine other conserved histidines in subunit I, two may be potential ligands of haem a (in case this haem is not associated with subunit II). In such a case subunit I would be similar to cytochrome b of the 6c, complex, which binds two haems (see below). Note that the conserved membranous histidines of subunit I (Fig. 3.5) are all predicted to be located in the lower aspect of the membrane. 

Recently an enhancement of spin relaxation of ferrous haem Uj-NO by ferric haem a has been demonstrated [149,150]. From this, and the known location of haem a near the C side of the membrane [99], it can be concluded that both haems are near this side with an Fe-Fe distance of approx. 15 A (about 10 A when projected on the same normal to the membrane). This would identify the lower aspect of Fig. 3.5 as the cytoplasmic side. 

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