Long-Range Electron Transfer Reactions between Hemes of MauG and Different Forms of Tryptophan Tryptophylquinone of Methylamine Dehydrogenase

摘要

The diheme enzymeMauG catalyzes the post-translational modification of a precursor protein of methylamine dehydrogenase (preMADH) to complete the biosynthesis of its protein-derived tryptophan tryptophylquinone (TTQ) cofactor. This six-electron oxidation of preMADH requires long-range electron transfer (ET) as the structure of the MauG-preMADH complex reveals that the shortest distance between the modified residues of preMADH and the nearest heme of MauG is 14.0A ° [Jensen, L. M. R., Sanishvili, R., Davidson, V. L., and Wilmot, C. M. (2010) Science 327, 1392-1394]. The kinetics of two ET reactions betweenMADH and MauG have been analyzed. Interprotein ET from quinolMADHto the high-valent bis- Fe(IV) form of MauG exhibits a Kd of 11.2 μM and a rate constant of 20 s-1. ET from diferrous MauG to oxidized TTQ ofMADH exhibits a Kd of 10.1 μMand a rate constant of 0.07 s-1. These similar Kd values are much greater than that for the MauG-preMADHcomplex, indicating that the extent of TTQ maturity rather than its redox state influences complex formation. The difference in rate constants is consistent with a larger driving force for the faster reaction. Analysis of the structure of the MauG-preMADH complex in the context of ET theory and these results suggests that direct electron tunneling between the residues that form TTQ and the five-coordinate oxygen-binding heme is not possible, and that ET requires electron hopping via the six-coordinate heme.