Long-range electron transfer in recombinant peroxidases anisotropically orientated on gold electrodes

Long-range electron transfer (ET) in horseradish peroxidase (HRP) was studied with a wild-type recombinant form of HRP, rHRP, and recombinant forms containing histidine and cysteine tags at Gln1, Asn57, Asn189, or Ser309 amino acid residues of the protein. Chemisorption of the enzyme onto the Au electrodes through the tags introduced in different positions of the protein surface provided anisotropic orientations of the rHRPs on the Au surface, which allowed a restricted "rotation" of the rHRP molecules on the electrodes. Atomic force microscopy (AFM) studies revealed the monolayer coverage of the enzyme on gold surfaces and the specific orientations of different forms of rHRP, which may be characterized by different distances between the heme active site of rHRP and the gold electrode. The efficiency of long-range ET between the electrode and the heme of rHRP was estimated from direct non-catalytic electrochemistry of rHRPs differently orientated on Au and compared with the theoretically calculated values from the protein ET model (C. C. Page, C. C. Moser, X. Chen, P. L. Dutton, Nature, 1999, 402, 47-51), under the assumption that ET occurs within the protein structure between the heme and the tag-modified amino acid residue of the protein. Comparative analysis of the long-range ET through the rHRP showed that the highest ET rates, obtained for the rHRP forms containing the tags at C- or N-termini of the enzyme, did not correlate with the shortest ET distance, but were instead consistent with the directional ET along the most favourable ET pathway within the protein matrix.