Structure and Folding of Cytochromes c with Different Thermal Stability

Abstract

Cytochrome c is found in a variety of bacterial species, from mesophiles to hyperthermophiles. Cytochrome c functions in respiratory electron transport chain, and is one of the best-characterized ones not only in terms of molecular biology such as biogenesis and evolution, but also in biophysical terms such as protein structure, stability, and folding. Four homologous cytochromes c, mesophilic Pseudomonas aeruginosa cytochrome c551 (PA c551), moderately thermophilic Hydrogenophilus thermoluteolus cytochrome c552 (PH c552), thermophilic Hydrogenobacter thermophilus cytochrome c552 (HT c552), and hyperthermophilic Aquifex aeolicus cytochrome c555 (AA c555) have been subjected to mutagenesis and structural analyses and provided substantial clues to the mechanism underlying protein stability at the amino-acid level. HT c552 is stabilized through five specific amino acid residues compared with PH c552 and PA c551. AA c555 is further stabilized by an additional helix structure compared with the other three proteins. As to folding, the apo AA c555 polypeptide can intrinsically form a holo-like structure without heme binding, this being the first case for a cytochrome c polypeptide. Molecular mechanisms underlying protein stability and folding in these bacterial homologous cytochromes c with different thermal stability are understood.