Methionine ligand lability in bacterial monoheme cytochromes c: an electrochemical study

How to Turn an Electron Transfer Protein into a Redox Enzyme for Biosensing

Cytochrome c is a small globular protein whose main physiological role is to shuttle electrons within the mitochondrial electron transport chain. This protein has been widely investigated, especially as a paradigmatic system for understanding the fundamental aspects of biological electron transfer and protein folding. Nevertheless, cytochrome c can also be endowed with a non-native catalytic activity and be immobilized on an electrode surface for the development of third generation biosensors. Here, an overview is offered of the most significant examples of such a functional transformation, carried out by either point mutation(s) or controlled unfolding. The latter can be induced chemically or upon protein immobilization on hydrophobic self-assembled monolayers. We critically discuss the potential held by these systems as core constituents of amperometric biosensors, along with the issues that need to be addressed to optimize their applicability and response.

Alanine to serine substitutions drive thermal adaptation in a psychrophilic diatom cytochrome c6

In this study, we investigate the thermodynamic mechanisms by which electron transfer proteins adapt to environmental temperature by directly comparing the redox properties and folding stability of a psychrophilic cytochrome c and a mesophilic homolog. Our model system consists of two cytochrome c₆ proteins from diatoms: one adapted specifically to polar environments, the other adapted generally to surface ocean environments. Direct electrochemistry shows that the midpoint potential for the mesophilic homolog is slightly higher at all temperatures measured. Cytochrome c₆ from the psychrophilic diatom unfolds with a melting temperature 10.4 °C lower than the homologous mesophilic cytochrome c₆. Changes in free energy upon unfolding are identical, within error, for the psychrophilic and mesophilic protein; however, the chemical unfolding transition of the psychrophilic cytochrome c₆ is more cooperative than for the mesophilic cytochrome c₆. Substituting alanine residues found in the mesophile with serine found in corresponding positions of the psychrophile demonstrates that burial of the polar serine both decreases the thermal stability and decreases the midpoint potential. The mutagenesis data, combined with differences in the m-value of chemical denaturation, suggest that differences in solvent accessibility of the hydrophobic core underlie the adaptation of cytochrome c₆ to differing environmental temperature.

Determining Redox Potentials of the Iron-Sulfur Clusters of the AdoMet Radical Enzyme Superfamily

While protein film electrochemistry (PFE) has proven to be an effective tool in the interrogation of redox cofactors and assessing the electrocatalytic activity of many different enzymes, recently it has been proven to be useful for the study of the redox potentials of the cofactors of AdoMet radical enzymes (AREs). In this chapter, we review the challenges and opportunities of examining the redox cofactors of AREs in a high level of detail, particularly for the deconvolution of redox potentials of multiple cofactors. We comment on how to best assess the electroactive nature of any given ARE, and we see that when applied well, PFE allows for not only determining redox potentials, but also determining proton-coupling and ligand-binding phenomena in the ARE superfamily.

Functionally Distinct Bacterial Cytochrome c Peroxidases Proceed through a Common (Electro)catalytic Intermediate

The diheme cytochrome c peroxidase from Shewanella oneidensis (So CcP) requires a single electron reduction to convert the oxidized, as-isolated enzyme to an active conformation. We employ protein film voltammetry to investigate the mechanism of hydrogen peroxide turnover by So CcP. When the enzyme is poised in the active state by incubation with sodium l-ascorbate, the graphite electrode specifically captures a highly active state that turns over peroxide in a high potential regime. This is the first example of an on-pathway catalytic intermediate observed for a bacterial diheme cytochrome c peroxidase that requires reductive activation, consistent with the observed voltammetric response from the diheme cytochrome c peroxidase from Nitrosomonas europaea (Ne), which is constitutively active and does not require the same one electron activation. Mutational analysis at the active site of So CcP confirms that the rate-limiting step involves a proton-coupled single electron reduction of a high valent iron species centered on the low-potential heme, consistent with the same mutation in Ne CcP. The pH dependence of catalysis for wild-type So CcP suggests that reduction shifts the pK(a)'s of at least two amino acids. Mutation of His81 in "loop 1", a surface exposed loop thought to shift conformation during the reductive activation process, eliminated one of the pH dependent features, confirming that the loop 1 shifts, changing the environment of His81 during the rate-limiting step. The observed catalytic intermediate has the same electron stoichiometry and similar pH dependence to that previously reported for Ne CcP, which is constitutively active and therefore hypothesized to follow a different catalytic mechanism. The prominent similarities between the rate-limiting steps of differing mechanistic classes of bCcPs suggest unexpected similarities in the intermediates formed.

Methionine ligand lability of homologous monoheme cytochromes c

Direct electrochemical analysis of adsorbed bacterial monoheme cytochromes c has revealed a phenomenological loss of the axial methionine when examined using pyrolytic "edge-plane" graphite (EPG) electrodes. While prior findings have reported that the Met-loss state may be quantitatively understood using the cytochrome c from Hydrogenobacter thermophilus as a model system, here we demonstrate that the formation of the Met-loss state upon EPG electrodes can be observed for a range of cytochrome orthologs. Through an electrochemical comparison of the wild-type proteins from organisms of varying growth temperature optima, we establish that Met-ligand losses at graphite surfaces have similar energetics to the "foldons" for known protein folding pathways. Furthermore, a downward shift in reduction potential to approximately -100 mV vs standard hydrogen electrode was observed, similar to that of the alkaline transition found in mitochondrial cytochromes c. Pourbaix diagrams for the Met-loss forms of each cytochrome, considered here in comparison to mutants where the Met-ligand has been substituted to His or Ala, suggest that the nature of the Met-loss state is distinct from either a His-/aquo- or a bis-His-ligated heme center, yet more closely matches the pKa values found for bis-His-ligated hemes., We find the propensity for adoption of the Met-loss state in bacterial monoheme cytochromes c scales with their overall thermal stability, though not with the specific stability of the Fe-Met bond.

Axial iron coordination and spin state change in a heme c upon electrostatic protein-SAM interaction

A bacterial di-heme cytochrome c binds electrostatically to a gold electrode surface coated with a negatively charged COOH-terminated SAM adopting a sort of 'perpendicular' orientation. Cyclic voltammetry, Resonance Raman and SERRS spectroscopies indicate that the high-potential C-terminal heme center proximal to the SAM's surface undergoes an adsorption-induced swapping of one axial His ligand with a water molecule, which is probably lost in the reduced form, and a low- to high-spin transition. This coordination change for a bis-His ligated heme center upon an electrostatically-driven molecular recognition is as yet unprecedented, as well as the resulting increase in reduction potential. We discuss it in comparison with the known methionine ligand lability in monoheme cytochromes c occurring upon interaction with charged molecular patches. One possible implication of this finding in biological ET is that mobile redox partners do not behave as rigid and invariant bodies, but in the ET complex are subjected to molecular changes and structural fluctuations that affect in a complex way the thermodynamics and the kinetics of the process.