Conformational transitions of a cytochrome c having a single thioether bridge

Solution measurements of the overall shape, secondary structure, and tryptophan fluorescence of Euglena gracilis ferricytochrome c558 at neutral pH and 25 degrees C indicate that the conformation of the native protein is very comparable to that of horse heart ferricytochrome c550. The increased susceptibility of the Euglena ferricytochrome c to denaturation by guanidine hydrochloride at neutral pH relative to the horse protein can be accounted for by the differences in their amino acid sequences. The kinetics of refolding of acid and guanidine denatured Euglena ferricytochrome c are similar to those observed with the horse protein with respect to the numbers of kinetic phases detected and to the fractional reaction associated with each phase. The time constants of the kinetic phases observed in the refolding of guanidine denatured Euglena ferricytochrome c are those predicted from the midpoint of the denaturation transition measured at equilibrium. Thus, the absence of one thioether bridge linking the heme with the polypeptide chain in Euglena cytochrome c does not appear to significantly perturb either the acquisition or the stability of the native conformation. The enhanced flexibility of the heme in denatured Euglena ferricytochrome c likely contributes to the extensive quenching of its tryptophan fluorescence and to its polymerization in acid media containing concentrated salt.