THERMODYNAMIC AND KINETIC-STUDIES OF CYTOCHROME-C FROM DIFFERENT SPECIES

The direct heterogeneous electron transfer reactions of cytochrome c from different vertebrate species are compared in this report. A kinetic study revealed a biphasic temperature dependence at neutral pH for the formal heterogeneous electron transfer rate constants of these cytochromes. The properties of the electrode-solution interface are believed to cause this biphasic kinetic effect. This effect does not correlate with the conformational transitions evident in both formal potential and circular dichroism (CD) measurements as a function of temperature. The conformational transitions seen in the formal potential and CD measurements arise from the alkaline isomerization and heme crevice changes. These are subtle effects compared with the structural changes associated with unfolding of the protein that are evident in differential scanning calorimetric experiments. The conformational stabilities of cytochrome c between species relate to the differences in their amino acid sequences. The amino acid interactions which stabilize the structure of these different cytochrome c molecules have been evaluated and will be discussed.