High Thermal Stability and Unique Trimer Formation of Cytochromec′ from ThermophilicHydrogenophilus thermoluteolus

Sequence analysis indicated that thermophilic Hydrogenophilus thermoluteolus cytochrome c' (PHCP) and its mesophilic homolog, Allochromatium vinosum cytochrome c' (AVCP), closely resemble each other in a phylogenetic tree of the cytochrome c' family, with 55% sequence identity. The denaturation temperature of PHCP was 87 °C, 35 °C higher than that of AVCP. Furthermore, PHCP exhibited a larger enthalpy change value during its thermal denaturation than AVCP. While AVCP was dimeric, as observed previously, PHCP was trimeric, and this was the first observation as a cytochrome c'. Dissociation of trimeric PHCP and its protein denaturation reversibly occurred at the same time in a two-state transition manner. Therefore, PHCP is enthalpically more stable than AVCP, perhaps due to its unique trimeric form, in addition to the lower number of Gly residues in its putative α-helical regions.