fumarate(out) + succinate(in) => fumarate(in) + succinate(out)
fumarate(out) + succinate(in) => fumarate(in) + succinate(out)
fumarate(out) + succinate(in) => fumarate(in) + succinate(out)
fumarate(out) + succinate(in) => fumarate(in) + succinate(out)