Answer
Every oxidation reaction needs a reduction reaction to occur at the same time.
We can represent these 2 reactions by:
$FAD + C_4H_6O_4 --\gt FADH_2 + C_4H_4O_4$
Work Step by Step
Since every reduction reaction needs electrons to occur, there must be an electron donator.
When a oxidation reaction occurs, electrons are released.
Therefore, these 2 types of reactions must occur together, so the electrons are donated and received.
Reduction: $FAD + 2H --\gt FADH_2$
Oxidation: $C_4H_6O_4 --\gt C_4H_4O_4 + 2H$
Summing these 2 equations:
$FAD + 2H + C_4H_6O_4 --\gt FADH_2 + C_4H_4O_4 + 2H$
Since the hydrogen appears on both sides, we can remove it.
$FAD + C_4H_6O_4 --\gt FADH_2 + C_4H_4O_4$
