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Heme attachment motif mobility tunes cytochrome c redox potential.

ABSTRACT: Hydrogen exchange (HX) rates and midpoint potentials (Em) of variants of cytochrome c from Pseudomonas aeruginosa (Pa cyt c551) and Hydrogenobacter thermophilus (Ht cyt c552) have been characterized in an effort to develop an understanding of the impact of properties of the Cys-X-X-Cys-His pentapeptide c-heme attachment (CXXCH) motif on heme redox potential. Despite structural conservation of the CXXCH motif, Ht cyt c552 exhibits a low level of protection from HX for amide protons within this motif relative to Pa cyt c551. Site-directed mutants have been prepared to determine the structural basis for and functional implications of these variations on HX behavior. The double mutant Ht-M13V/K22M displays suppressed HX within the CXXCH motif as well as a decreased Em (by 81 mV), whereas the corresponding double mutant of Pa cyt c551 (V13M/M22K) exhibits enhanced HX within the CXXCH pentapeptide and a modest increase in Em (by 30 mV). The changes in Em correlate with changes in axial His chemical shifts in the ferric proteins reflecting the extent of histidinate character. Thus, the mobility of the CXXCH pentapeptide is found to impact the His-Fe(III) interaction and therefore the heme redox potential.

SUBMITTER: Michel LV

PROVIDER: S-EPMC2606054 | biostudies-literature | 2007 Oct

REPOSITORIES: biostudies-literature

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Heme attachment motif mobility tunes cytochrome c redox potential.

Michel Lea V LV Ye Tao T Bowman Sarah E J SE Levin Benjamin D BD Hahn Megan A MA Russell Brandy S BS Elliott Sean J SJ Bren Kara L KL

Biochemistry 20070928 42

Hydrogen exchange (HX) rates and midpoint potentials (Em) of variants of cytochrome c from Pseudomonas aeruginosa (Pa cyt c551) and Hydrogenobacter thermophilus (Ht cyt c552) have been characterized in an effort to develop an understanding of the impact of properties of the Cys-X-X-Cys-His pentapeptide c-heme attachment (CXXCH) motif on heme redox potential. Despite structural conservation of the CXXCH motif, Ht cyt c552 exhibits a low level of protection from HX for amide protons within this mo ...[more]

PMID: 17900177

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Project description:Cytochrome-c-peroxidase (CCP) contains a five-coordinate heme active site. The reduction potential for the ferric to ferrous couple in CCP is anomalously low and pH dependent (Eo = ~-180 mV vs. S.H.E. at pH 7). The contribution of the protein environment to the tuning of the redox potential of this couple is evaluated using site directed mutants of several amino acid residues in the environment of the heme. These include proximal pocket mutation to residues Asp-235, Trp-191, Phe-202 and His-175, distal pocket mutation to residues Trp-51, His-52, and Arg-48; and a heme edge mutation to Ala-147. Where unknown, the structural changes resulting from the amino acid substitution have been studied by X-ray crystallography. In most cases, ostensibly polar or charged residues are replaced by large hydrophobic groups or alternatively by Ala or Gly. These latter have been shown to generate large, solvent filled cavities. Reduction potentials are measured as a function of pH by spectroelectrochemistry. Starting with the X-ray derived structures of CCP and the mutants, or with predicted structures generated by Molecular Dynamics (MD), predictions of redox potential changes are modeled using the Protein Dipoles Langevin Dipoles (PDLD) method. These calculations serve to model an electrostatic assessment of the redox potential change with simplified assumptions about heme iron chemistry, with the balance of the experimentally observed shifts in redox potential being thence attributed to changes in the ligand set and heme coordination chemistry, and/or other changes in the structure not directly evident in the X-ray structures (e.g. ionization states, specific roles played by solvent species, or conformationally flexible portions of the protein). Agreement between theory and experiment is good for all mutant proteins with the exception of the mutation Arg 48 to Ala, and His 52 to Ala. In the former case, the influence of phosphate buffer is adduced to account for the discrepancy, and measurements made in a bis-tris propane/2-(N-morpholino)ethanesulfonic acid buffer system agree well with theory. For the latter case, an unknown structural element relevant to His-52, and/or solvent influence in the mutant akin to anion binding in the distal pocket (though lacking proof that it is) manifests in this mutant. The use of exogenous (sixth) ligands in dissecting the contributions to control of redox potential are also explored as a pathway for model building.

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Heme attachment motif mobility tunes cytochrome c redox potential.

Publications

Heme attachment motif mobility tunes cytochrome c redox potential.

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