Structure of Paracoccus pantotrophus Cytochrome cd

The structure of the oxidised enzyme from P. pantotrophus shows that within one monomer the edge to edge inter-heme distance is circa 12 (the FeoFe distance is 20). This distance suggests that the rate of electron transfer between the hemes should be on the microsecond timescale, unless electron transport is obligatorily associated with other slower events, for example rearrangement of chemical bonds. 

Such an involvement of amino acid side chain ligand switching within each catalytic cycle was a novel proposal and as such needs to be scrutinised by a variety of experimental procedures as well as analysis in the context of information known for cytochrome cd nitrite reductase from another source (see below). However, it is interesting to note that something similar has recently been proposed for the protocatechuate 3,4- 

All these observations prompt the question of what values are obtained for the redox potentials of the c and d hemes under equilibrium conditions. This issue is currently under study and all that can be said here is that the enzyme does not give a straightforward redox titration. In particular, and in contrast to the observations made under pulse radiolysis conditions, the c and d hemes titrate together, suggesting a cooperativity of behaviour between them (A. KoppenhTer and K. Turner et al. unpublished data). It remains for future work to elucidate the molecular basis of this effect. 

It is clear that a good deal of information needs to be obtained in order to interpret the structural data in particular, the question has to be faced as to whether the oxidised crystal structure is that of a resting state or whether the heme iron ligand switching occurs on each catalytic cycle. We do, however, know that the oxidised protein in solution has the same His/His coordination of the c-type heme as in the oxidised state of the crystalline enzyme. This was determined from MCD spectroscopy (Cheesman et al., 1997). Other solution spectroscocpic measurements have shown that the ligation of the d heme is very likely the same as in the crystalline state (Cheesman et al., 1997). These studies also showed that the d heme iron appeared to be in an unusual room temperature high/low spin equilibrium. 

In concluding this section we note that cytochrome cd nitrite reductase also has an oxidase activity (F,Fp et al., 1995). This four electron reaction, which contrasts with the one electron reduction of nitrite to nitric oxide, is outside the scope of this article. 

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