Fine tuning of the redox function of Pseudomonas aeruginosa cytochrome c551 through structural properties of a polypeptide loop bearing an axial Met residue

Pseudomonas aeruginosa cytochrome c551 (PA) possesses a long polypeptide loop near its heme, and a unique hydrogen bond network among Ser52, axial Met61, and the heme 13-propionate side chain, i.e., Ser52 amide NH is hydrogen bonded to axial Met61 carbonyl CO, Met61 amide NH to Ser52 carbonyl CO, and Ser52 side chain OH to the heme 13-propionate side chain, contributes to stabilization of the structure of the loop [Y. Matsuura, T. Takano, R.E. Dickerson, J. Mol. Biol. 156 (1982) 389–409]. In this study, the structure and redox function of S52N and S52G mutants were characterized in order to elucidate the role of Ser52 in functional regulation of the protein. We found that the redox function of PA was hardly affected by an S52N mutation, but was slightly by an S52G one. The functional similarity between the wild-type protein and the S52N mutant demonstrated that Asn52 in the mutant plays a similar pivotal role in the formation of the unique hydrogen bond network that stabilizes the structure of the loop as Ser52 in the wild-type protein does. On the other hand, the functional alteration induced by the S52G mutation can be attributed to a structural change of the loop due to the lack of the hydrogen bond between the Gly52 and heme 13-propionate side chain in the mutant. Thus, this study demonstrated that the function of the protein can be tuned through the structural properties of the polypeptide loop near its heme. A unique heme-Ser52-Met61 hydrogen bond network in Pseudomonas aeruginosa cytochrome c551 contributes to stabilization of the structure of a loop near its heme. Characterization of S52N and S52G mutants revealed that the redox function of the protein can be finely tuned through the structure of the loop. [Display omitted] ► The redox function of P. aeruginosa cytochrome c. is finely tuned by the Ser52 residue. ► The structure of the loop near the heme is slightly altered by the S52G mutation. ► The structural change of the loop increases the polarity of the heme environment. ► As a result, the redox function of the protein is affected by the mutation. ► Thus, the protein function can be tuned through the structure of the loop.