title: Electron tunneling in substrate-reduced trimethylamine dehydrogenase: kinetics of electron transfer and analysis of the tunneling pathway.
abstract: The reoxidation of substrate-reduced trimethylamine dehydrogenase by the artificial electron acceptor ferricenium hexafluorophosphate was studied by stopped-flow spectroscopy. The rate constants for the two sequential one-electron transfers from the reduced 4Fe-4S center to ferricenium ions were measured, the first (ka = 49 s-1) being about 7 times greater than the second (kb = 7.3 s-1) at 20 degrees C and neutral pH. The temperature dependence of the second electron transfer was studied over the range 10-40 degrees C, and the rate constant ranged from 5.7 to 19.2 s-1. Analysis of the temperature perturbation of kb by Marcus theory yielded values for the reorganizational energy of 1.95 eV and the electronic coupling matrix element of 0.26 cm-1. An electron tunneling pathway distance of 13 +/- 0.7 A was calculated which correlates with the shortest pathway measured from the 4Fe-4S center to the protein surface using the crystallographic coordinates of trimethylamine dehydrogenase. Tyr-442 is implicated in facilitating electron transfer from the enzyme to ferricenium ions. The data suggest a location for the docking site on the surface of trimethylamine dehydrogenase for the physiological electron acceptor (ETF).
authors.affiliation: Cambridge Centre for Molecular Recognition, Department of Biochemistry, University of Cambridge, UK.
authors.forename: E K
authors.initials: EK
authors.lastname: Wilson
authors.forename: F S
authors.initials: FS
authors.lastname: Mathews
authors.forename: L C
authors.initials: LC
authors.lastname: Packman
authors.forename: N S
authors.initials: NS
authors.lastname: Scrutton
mesh_terms: D004579:Electron Transport; D005296:Ferrous Compounds; D007700:Kinetics; D008772:Methylococcaceae / Q000201:enzymology; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D010089:Oxidoreductases, N-Demethylating / Q000737:chemistry* / Q000378:metabolism; D013053:Spectrophotometry; D013379:Substrate Specificity; D013816:Thermodynamics
chemical_list: D005296:Ferrous Compounds; C064804:ferrocenium; D010089:Oxidoreductases, N-Demethylating; C023186:trimethylamine dehydrogenase