[ Eq(Derivative(x1, t), k1 - k2*x1*x2),
  Eq(Derivative(x2, t), 2*k2*x2*x1 - k3*x2/(k4 + x2) + k5*(x4 - x2)),
  Eq(Derivative(x3, t), k1 - k2*x3*x4),
  Eq(Derivative(x4, t), 2*k2*x3*x4 - k3*x4/(k4 + x4) - k5*(x4 - x2)) ]