INTRAMOLECULAR ELECTRON-TRANSFER IN TRIMETHYLAMINE DEHYDROGENASE - A THERMODYNAMIC ANALYSIS

Within the enzyme trimethylamine dehydrogenase [TMADH], intramolecular electron transfer occurs between a fully reduced covalently bound 6-S -cysteinylflavin [FMN] cofactor, and an oxidized iron-sulfur [4Fe-4S]( 2+) center. When the enzyme is reduced by the substrate trimethylamine , the kinetics of this intramolecular electron transfer [ET] reaction are biphasic, suggesting that ET ocurs via two alternative processes [ Falzon, L., st Davidson, V. L. (1996) Biochemistry 35, 2445-2452]. The formation of the FMN semiquinone was monitored by stopped-flow spectr oscopy, and the two rate constants for the biphasic reaction were dete rmined at temperatures ranging from 12 to 37 degrees C. Analysis of th ese rate constants by ET theory yielded values of 2.2 eV for the reorg anizational energy [lambda] associated with each reaction and electron ic couplings [H-AB] of 5.9 and 47 cm(-1) for the slower and faster ET reactions, respectively. The analysis also predicted average theoretic al distances between the two redox centers of 12.3 Angstrom for the sl ower reaction and 8.1 Angstrom for the faster reaction. These predicte d distances correlate well with the known crystal structure of TMADH a nd the most efficient pathways for ET that were predicted from the kno wn structure using the Greenpath program. This analysis suggests that for each reaction the ET event is rate-limiting, but coupled to a high ly unfavorable non-ET process, and that binding of a second molecule o f substrate to reduced TMADH decreases the efficiency of the intramole cular ET.