Comparative study on stabilization mechanism of monomeric cytochrome c₅ from deep-sea piezophilic Shewanella violacea

Monomeric cytochrome c₅ from deep-sea piezophilic Shewanella violacea (SVcytc₅) was stable against heat and denaturant compared with the homologous protein from shallow-sea piezo-sensitive Shewanella livingstonensis (SLcytc₅). Here, the SVcytc₅ crystal structure revealed that the Lys-50 side chain on the flexible loop formed a hydrogen bond with heme whereas that of corresponding hydrophobic Leu-50 could not form such a bond in SLcytc₅, which appeared to be one of possible factors responsible for the difference in stability between the two proteins. This structural insight was confirmed by a reciprocal mutagenesis study on the thermal stability of these two proteins. As SVcytc₅ was isolated from a deep-sea piezophilic bacterium, the present comparative study indicates that adaptation of monomeric SVcytc₅ to high pressure environments results in stabilization against heat.