DIRECT-DETECTION AND MEASUREMENT OF ELECTRON RELAYS IN A MULTICENTERED ENZYME - VOLTAMMETRY OF ELECTRODE-SURFACE FILMS OF ESCHERICHIA-COLI FUMARATE REDUCTASE, AN IRON-SULFUR FLAVOPROTEIN

Intramolecular electron relays operating in a multicentered enzyme are revealed by protein film voltammetry. The membrane-extrinsic catalyti c domain of E. coli fumarate reductase (FrdAB) adsorbs to electroactiv e monolayer coverage at a rotating pyrolytic graphite edge electrode, giving characteristic voltammetric signals that are resolved and assig ned to redox-active sites. At pH 7.3 (2 degrees C) signals attributabl e to Centers 1 ([2Fe-2S]) and 3 ([3Fe-4S]) and FAD are enveloped toget her around -50 mV, while Center 2 ([4Fe-4S]) appears as a weaker signa l at -305 mV. At pH 9.5, similar voltammetry is observed, the main dif ference being that the FAD component shifts to the negative edge of th e enveloper. The prominence of the two-electron FAD signal enables act ive-site redox transformations to be tracked and examined over a range of conditions. Scans at rates up to 20 V s(-1) in the absence of fuma rate shaw that electrons are relayed to the FAD, most obviously by Cen ters 1 and 3. Upon adding fumarate, the signals undergo transformation s ss specific centers engage in catalytic electron transport. A sigmoi dal wave originating in the FAD envelope region is joined by a second wave close to the potential of Center 2. This is particularly evident under conditions optimizing enzyme catalytic control (as opposed to ma ss-transport control), i.e. high fumarate levels, high rotation rate, and pH 9.0 at which the enzyme is less active than at pH 7.0. Intramol ecular electron transport is partitioned between different relay syste ms depending on catalytic demand and proficiency of the FAD as electro n acceptor. At high pH, the less favorable driving force for electron transfer from Centers 1 and 3 places a greater burden on Center 2. Cat alytic voltammograms show hysteresis in the presence of oxalacetate, a nd inhibitor binding preferentially to oxidized FAD. Reductive activat ion is slow but accelerates sharply below the potential of Center 2, s howing that this cluster is much more effective than the others in red ucing the inhibitor-bound active site. The results demonstrate how vol tammetry can be used to quantify intramolecular electron transfer amon g multiple sites in complex enzymes.