Electronic coupling protein efficiencies

Aromatic residues have been found in proteins at positions that probably enhance the electronic coupling in systems that have been selected by evolution for efficient ET. Examples are the tryptophan mediated reduction of quinone in the photosynthetic reaction center (31), the methylamine dehydrogenase (MADH) amicyanin system, where a Trp residue is placed at the interface between the two proteins (32), as well as the [cytochrome c peroxidase-cytochrome c] complex, where a Trp seems to have a similar function (33). 

Pseudomonas aeruginosa (P.a.) azurin has been ruthenated at His-83 (r 11.8 A) (54, 93, 113) the donor-acceptor separation is pictured in Fig. 19. Production of a5Ru(His-83) -Az(Cu ) was achieved by flash photolysis in the presence of [Ru(bpy)3]. The reduction of the protein was monitored at 625 nm, and the intramolecular Ru(II)-to-Cu(II) ET rate of 1.9 s was found to be independent of temperature. The Cu reorganization enthalpy was estimated to be <7 kcal mol (93, 113), a value confirming that blue copper is structured for efficient ET. Table XI compares ET rates for the blue copper proteins with those for heme proteins the blue copper rates are low in comparison with the heme protein rates over similar distances and driving forces. This effect could be a result of poor electronic coupling of asRu with the copper center, possibly owing to unfavorable ET pathways. 

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