Physicochemical properties of two atypical cytochromes c, Crithidia cytochrome c-557 and Euglena cytochrome c-558.

Cytochrome c-557 from Crithidia oncopelti and cytochrome c-558 from Euglena gracilis are mitochondrial cytochromes c that have an atypical haem-binding site. It was of interest to know whether the loss of one thioether bond affected the physicochemical properties of these cytochromes. The thermodynamic parameters of the redox potential were measured. The reaction with imidazole, the kinetics and thermodynamics of the alkaline isomerization and the effect of heating on the visible spectrum are described for the ferricytochromes. The kinetics of the loss of cyanide, the spectral changes occurring on reduction with dithionite at alkaline pH values and the reactivity with CO are described for the ferrocytochromes. In many respects the cytochromes of the two protozoans are very similar to the cytochromes of horse and yeast. The ferricytochromes do, however, undergo a reversible transition to high-spin species on heating, which may be due to the more flexible attachment of the prosthetic group. Similarly the alkaline isomers of cytochromes c-557 and c-558 give rise to high-spin proteins above pH 11. The alkaline isomerization of cytochrome c-558, involves a pKobs. of 10 and kinetics which do not obey the model of Davis et al. [(1974) J. Biol. Chem. 249, 2624-2632] for horse cytochrome c. It is proposed that a model involving two ionizations, followed by a conformation change, may fit the data. Both cytochromes c-557 and c-558 combine slowly with CO at neutral pH values.