Characterization of electron tunneling and hole hopping reactions between different forms of maug and methylamine dehydrogenase within a natural protein complex

Respiration, photosynthesis, and metabolism require the transfer of electrons through and between proteins over relatively long distances. It is critical that this electron transfer (ET) occur with specificity to avoid cellular damage, and at a rate that is sufficient to support the biological activity. A multistep hole hopping mechanism could, in principle, enhance the efficiency of long-range ET through proteins as it does in organic semiconductors. To explore this possibility, two different ET reactions that occur over the same distance within the protein complex of the diheme enzyme MauG and different forms of methylamine dehydrogenase (MADH) were subjected to kinetic and thermodynamic analysis. An ET mechanism of single-step direct electron tunneling from diferrous MauG to the quinone form of MADH is consistent with the data. In contrast, the biosynthetic ET from preMADH, which contains incompletely synthesized tryptophan tryptophylquinone, to the bis-Fe(IV) form of MauG is best described by a two-step hole hopping mechanism. Experimentally determined ET distances matched the distances determined from the crystal structure that would be expected for single-step tunneling and multistep hopping. Experimentally determined relative values of electronic coupling (H_AB) for the two reactions correlated well with the relative H _AB values predicted from computational analysis of the structure. The rate of the hopping-mediated ET reaction is also 10-fold greater than that of the single-step tunneling reaction despite a smaller overall driving force for the hopping-mediated ET reaction. These data provide insight into how the intervening protein matrix and redox potentials of the electron donor and acceptor determine whether the ET reaction proceeds via single-step tunneling or multistep hopping.