<p>Two cytochromes <i>c</i><sub>5</sub> (SBcyt<i>c</i> and SVcyt<i>c</i>) have been derived from <i>Shewanella</i> living in the deep-sea, which is a high pressure environment, so it could be that these proteins are more stable at high pressure than at atmospheric pressure, 0.1 MPa. This study, however, revealed that SBcyt<i>c</i> and SVcyt<i>c</i> were more stable at 0.1 MPa than at higher pressure. In addition, at 0.1–150 MPa, the stability of SBcyt<i>c</i> and SVcyt<i>c</i> was higher than that of homologues from atmospheric-pressure <i>Shewanella</i>, which was due to hydrogen bond formation with the heme in the former two proteins. This study further revealed that cytochrome <i>c</i><sub>551</sub> (PMcyt<i>c</i>) of deep-sea <i>Pseudomonas</i> was more stable than a homologue of atmospheric-pressure <i>Pseudomonas aeruginosa</i>, and that specific hydrogen bond formation with the heme also occurred in the former. Although SBcyt<i>c</i> and SVcyt<i>c</i>, and PMcyt<i>c</i> are phylogenetically very distant, these deep-sea cytochromes <i>c</i> are commonly stabilized through hydrogen bond formation.</p> <p>Deep-sea cytochromes <i>c</i> are commonly stabilized through hydrogen bond formation.</p
