Alanine to serine substitutions drive thermal adaptation in a psychrophilic diatom cytochrome c 6 .

In this study, we investigate the thermodynamic mechanisms by which electron transfer proteins adapt to environmental temperature by directly comparing the redox properties and folding stability of a psychrophilic cytochrome c and a mesophilic homolog. Our model system consists of two cytochrome c ₆ proteins from diatoms: one adapted specifically to polar environments, the other adapted generally to surface ocean environments. Direct electrochemistry shows that the midpoint potential for the mesophilic homolog is slightly higher at all temperatures measured. Cytochrome c ₆ from the psychrophilic diatom unfolds with a melting temperature 10.4 °C lower than the homologous mesophilic cytochrome c ₆ . Changes in free energy upon unfolding are identical, within error, for the psychrophilic and mesophilic protein; however, the chemical unfolding transition of the psychrophilic cytochrome c ₆ is more cooperative than for the mesophilic cytochrome c ₆ . Substituting alanine residues found in the mesophile with serine found in corresponding positions of the psychrophile demonstrates that burial of the polar serine both decreases the thermal stability and decreases the midpoint potential. The mutagenesis data, combined with differences in the m-value of chemical denaturation, suggest that differences in solvent accessibility of the hydrophobic core underlie the adaptation of cytochrome c ₆ to differing environmental temperature.