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Figure 7.
FIGURE 7. The reaction mechanism for fumarate reduction by
flavocytochrome c[3] showing the pathways taken by each of the
two protons required to complete the catalytic cycle. This is an
abbreviated version of the mechanism previously reported (12).
One proton is transferred directly (from solvent or at least by
a pathway not involving Arg-381, Glu-378, or Arg-402) to the FAD
in a reaction coupled to its two-electron reduction. The
substrate is twisted out of plane and polarized by interactions
with positively charged residues. This facilitates hydride
transfer from flavin N5 to the fumarate C2. Arg-402 acts as the
active site acid donating a proton to fumarate C3. Arg-402 is
reprotonated via the proton pathway involving Glu-378 and
Arg-381.
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