[ Eq(Derivative(x1, t), k3/(k11 + x2) - k8*x1),
  Eq(Derivative(x2, t), k1*((k4 + k5*x1)*x2*x3/(k12 + x1**2) - k9*x2)),
  Eq(Derivative(x3, t), k1*k2*(k6*x1 - (k10*x3 + k7*x1*x3/(k13 + x1)))) ]