Electroanalytical chemistry of myoglobin with modification of distal histidine by cyanated imidazole

Microemulsion-controlled reaction sites in biocatalytic films for electrochemical reduction of vicinal dibromides

We report herein the electrochemical dehalogenation of vicinal dibromides in microemulsions using cross-linked films of the redox protein myoglobin (Mb) and poly-l-lysine (PLL) covalently bonded to carbon electrodes. Catalytic reduction of the dibromides to olefins was more efficient in an SDS microemulsion than in a CTAB microemulsion. SDS shifts the Mb redox potential more negative, but a comparison to Mb-SDS films suggests that the activation free energy of the reduction is controlled by an inner-sphere mechanism. SDS also enters the positively charged Mb-PLL films and preconcentrates the dibromide reactants, enhancing catalytic efficiency in SDS microemulsions. Shifts in formal potential and Soret absorbance bands for Mb-PLL films suggested binding of trans-1,2-dibromocyclohexane in the iron heme distal pocket with little catalysis. Results are consistent with active catalytic reduction sites for reactant bound on the protein surface and less-reactive sites in the distal heme pocket. Preconcentration into catalytic PLL films using SDS incorporated from microemulsions may be a general way to improve catalytic efficiency for nonpolar reactants in microemulsions.

Direct electrochemistry of engineered cytochrome b562 molecules with a ligand binding pocket

The rapid and reversible electron transfer reaction of cytochrome b562 was observed at an In2O3 electrode. The estimated heterogeneous electron transfer rate constant (k0') was k0' > or = 5.0 x 10(-3) cm s(-1) at pH 6.5. When the methionine-7 (Met-7) residue, which coordinates to the heme iron as an axial ligand, of the wild-type cytochrome b562 was replaced by an Ala or Gly residue, a water molecule bound to the heme iron and the electron transfer rate constants decreased to 1.3 x 10(-3) and 1.8 x 10(-3) cm s(-1), respectively. This decrease in the electron transfer rate would be due to the larger reorganization energy for the structural change at the redox site. The midpoint potential of cytochrome b562 was shifted negatively by approximately 135 mV by replacing Met-7 with Ala or Gly. Similar dissociation kinetics of cyanide for the mutated molecules as compared to native myoglobin was obtained.

Functional Evaluation of Heme Vinyl Groups in Myoglobin with Symmetric Protoheme Isomers

We replaced protoheme-IX in native myoglobin with the symmetric protohemes-III and -XIII, in order to investigate the role of heme vinyl-globin contacts on Mb function. The UV-visible spectra and the resonance Raman spectra in the high-frequency region (containing oxidation, spin, and coordination state marker lines) of the two reconstituted Mbs were very similar. However, the signal intensity of the Soret band in the CD spectra and the resonance Raman lines for vinyl bending modes in the low-frequency region notably differed, thereby reflecting altered heme peripheral contacts. The redox potentials, formal heterogeneous electron-transfer rates, and thermal denaturation temperatures of the two reconstituted Mbs were also indistinguishable. In addition, the oxygen binding properties of the ferrous deoxy Mbs were comparable. These results demonstrate that altered heme vinyl-globin interactions only slightly affect the physical properties of Mb. It is therefore likely that the orientation of protoheme-IX about the alpha,gamma-axis in the heme pocket is not necessarily a crucial factor for oxygen binding to native Mb.