Characterization of Electron Tunneling and Hole Hopping Reactions between Different Forms of MauG and Methylamine Dehydrogenase within a Natural Protein Complex

Abstract

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 which is sufficient to support the biological activity. A multi-step 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 values of ET distance matched the distances determined from the crystal structure that would be expected for single-step tunneling and multi-step hopping, respectively. Experimentally-determined relative values of electronic coupling (HAB) for the two reactions correlated well with the relative HAB values predicted from computational analysis of the structure. The rate of the hopping-mediated ET reaction is also ten-fold greater than that of the single-step tunneling reaction despite having a smaller overall driving force for the 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 multi-step hopping.

Diradical intermediate within the context of tryptophan tryptophylquinone biosynthesis

“…The W199F MauG mutant can form the bis-Fe(IV) state on reaction with H 2 O 2 , but cannot catalyze TTQ biosynthesis, as hole hopping through Trp199 is required for preTTQ oxidation ( Fig. S1A) (13,14). In contrast to the native MauG-preMADH crystals, the W199F MauG-preMADH crystals showed no changes at the preTTQ site after 60 d ( Fig.…”

“…A hydrogen bond connection postulated to play a role in R2 is also absent in the MauG-preMADH complex. Rather, the electron transfer from preMADH to MauG has been shown to occur through hole-hopping (13,14).…”

Mutation of Trp93 of MauG to tyrosine causes loss of bound Ca2+ and alters the kinetic mechanism of tryptophan tryptophylquinone cofactor biosynthesis

“…Trp 199 of MauG resides at the site of interaction with preMADH and is positioned approximately halfway between βTrp 108 of preMADH and the nearest haem of MauG. Site-directed mutagenesis [17] and kinetic and thermodynamic analyses [21] showed that Trp 199 mediates a hole-hopping mechanism of ET that is required for MauG-dependent TTQ biosynthesis. Previous studies also demonstrated that a critical role of the bis-Fe IV intermediate of MauG was not only to provide a high-potential oxidant for modification of preMADH, but also to shorten the distance for the required long-range ET from the substrate to the nearest Fe IV haem [14,15].…”

Roles of multiple-proton transfer pathways and proton-coupled electron transfer in the reactivity of the bis -Fe IV state of MauG

“…The catalytic mechanism of MauG is unusual in that the preMADH substrate does not make direct contact with either heme but instead binds to the surface of MauG several angstroms away (5). Catalysis requires long-range ET to bis-Fe IV MauG from the residues on preMADH that are modified via a hole-hopping mechanism through Trp199 (13,14), which resides at the MauGpreMADH interface (Fig. 1).…”