Correlation between the optimal growth pressures of four Shewanella species and the stabilities of their cytochromes c 5.

Shewanella species live widely in deep-sea and shallow-water areas, and thus grow piezophilically and piezosensitively. Piezophilic and psychrophilic Shewanella benthica cytochrome c 5 (SB cytc 5) was the most stable against guanidine hydrochloride (GdnHCl) and thermal denaturation, followed by less piezophilic but still psychrophilic Shewanella violacea cytochrome c 5 (SV cytc 5). These two were followed, as to stability level, by piezosensitive and mesophilic Shewanella amazonensis cytochrome c 5 (SA cytc 5), and piezosensitive and psychrophilic Shewanella livingstonensis cytochrome c 5 (SL cytc 5). The midpoint GdnHCl concentrations of SB cytc 5, SV cytc 5, SL cytc 5, and SA cytc 5 correlated with the optimal growth pressures of the species, the correlation coefficient value being 0.93. A similar trend was observed for thermal denaturation. Therefore, the stability of each cytochrome c 5 is related directly to its host's optimal growth pressure. Phylogenetic analysis indicated that Lys-37, Ala-41, and Leu-50 conserved in piezosensitive SL cytc 5 and SA cytc 5 are ancestors of the corresponding residues in piezophilic SB cytc 5 and SV cytc 5, Gln, Thr, and Lys, respectively, which might have been introduced during evolution on adaption to environmental pressure. The monomeric Shewanella cytochromes c 5 are suitable tools for examining protein stability with regard to the optimal growth pressures of the source species.