Electron transfer pathways, through space

FIGURE 15. Electron transfer pathways in FCB2. The best path for electron transfer from flavin to heme based on GREENPATH calculations. Dashed lines represent paths along hydrogen bonds and dotted lines represent a through-space jump. Path 1 involves a water molecule while path 2 does not. 

FIGURE 17. Electron transfer pathways in FCSD. Through-space jumps are indicated hy dotted lines and paths along hydrogen bonds are indicated by dashed lines. Four paths (ln4) with decreasing electronic coupling are indicated. Fp and Cy indicate residues in the flavo-protein and the cytochrome subunits, respectively. 

FIGURE 18. Electron transfer pathways from the 2Fe-2S center to the flavin ring in fumarate reductase. Dotted lines represent through-space jumps. The four best paths (ln4) are indicated, all of which involve an initial transfer of an electron from the 2Fe-2S center to the SG atom of Cys57 which forms a ligand to an iron atom. 

The proposed electron transfer pathway from the proximal to the distal FeS cluster is shown in Figure 10. It involves a series of hydrogen bonds over the 16 distance between these two 4Fe clusters. The thin dotted lines indicate the most likely pathway whereas, the thick lines include two ithrough-space jumpsi, which are considered to incur a penalty in terms of electron transfer rate. The pathway is predominantly through the FeoS clusters, the cysteine ligands, and two histidines. The final site at which electrons are transferred to the electron donor or acceptor appears to be histidine 185, the ligand to the distal cluster. 

Kinetics of electron tranter. The central problem in investigations of the kinetics of electron transfer is to establish the mechanism and pathways of electron transfer. Is the through space mechanism relevant to this process, or have specific electron conduction pathways (through bond or superexchange) been designed into electron transfer proteins Once this question is resolved, then a quantitative understanding of electron transfer rates may be feasible. 

In PCMH, two equally efficient pathways for electron transfer from the flavin N5 to the heme iron were identified in GREENPATH calculations. One path follows the tyrosyl covalent link to FAD at the C8-methyl position, whereby electrons can travel from C8M via the Tyr 384 phenolic ether bond, tunnel through the tyrosine ring atoms and make a through-space jump across the subunit interface to the carbonyl oxygen of Ala 49 in the cytochrome (Figure 16). From there the electrons follow backbone... 

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