The effect of surface charge on the reduction potential and heme propionate ionization of a monoheme cytochrome: spectroscopic and potentiometric studies of trifluoroacetylated cytochrome c551 from Pseudomonas aeruginosa

Effect of Bioconjugation on the Reduction Potential of Heme Proteins

The modification of protein surfaces employing cationic and anionic species enables the assembly of these biomaterials into highly sophisticated hierarchical structures. Such modifications can allow bioconjugates to retain or amplify their functionalities under conditions in which their native structure would be severely compromised. In this work, we assess the effect of this type of bioconjugation on the redox properties of two model heme proteins, that is, cytochrome c (CytC) and myoglobin (Mb). In particular, the work focuses on the sequential modification by 3-dimethylamino propylamine (DMAPA) and 4-nonylphenyl 3-sulfopropyl ether (S1) anionic surfactant. Bioconjugation with DMAPA and S1 are the initial steps in the generation of pure liquid proteins, which remain active in the absence of water and up to temperatures above 150 °C. Thin-layer spectroelectrochemistry reveals that DMAPA cationization leads to a distribution of bioconjugate structures featuring reduction potentials shifted up to 380 mV more negative than the native proteins. Analysis based on circular dichroism, MALDI-TOF mass spectrometry, and zeta potential measurements suggest that the shift in the reduction potentials are not linked to protein denaturation, but to changes in the spin state of the heme. These alterations of the spin states originate from subtle structural changes induced by DMAPA attachment. Interestingly, electrostatic coupling of anionic surfactant S1 shifts the reduction potential closer to that of the native protein, demonstrating that the modifications of the heme electronic configuration are linked to surface charges.

Molecular statistics of cytochrome c: structural plasticity and molecular environment

Nuclear magnetic resonance experiments performed on yeast mitochondrial cytochrome c (Cytc), a paradigmatic electron transfer protein, reveal that the two oxidation states have similar structures, but different mobility: despite the few structural differences compared with the reduced form, the oxidized form displays a larger unfolding propensity. Molecular dynamics simulations performed on both NMR reduced and NMR oxidized forms show that the reduced form has a larger solvent-accessible surface area (SASA). Starting from this observation, a molecular statistical approach was then applied in order to correlate the molecular surface to molecular mobility. Simulations started from biased initial conditions corresponding to different molecular sizes were combined with the maximal constrained entropy method. The NMR structure of oxidized Cytc is more suited to expose a smaller SASA than the NMR structure of the reduced form, but the accessible conformational landscape at 300 K around the NMR oxidized structure is flatter than for the NMR reduced structure. Protein configurations of smaller SASA and size display larger plasticity when they resemble the NMR oxidized structure, whereas they are more rigid when they resemble the NMR reduced structure. Implications of the results for the protein properties during its functional process are discussed.