[ Eq(Derivative(x1, t), ((-1)*k12*k1*(1 - x2)*x1*x3 + 1*k12*k2*k1**k11*x2*x3)/k12),
  Eq(Derivative(x2, t), (1*k12*k1/k1**k11*(1 - x2)*x1 + (-1)*k12*k2*x2)/k12),
  Eq(Derivative(x3, t), (-1)*k12*k8*x2/(x2 + k9)*x4/k12),
  Eq(Derivative(x4, t), ((-1)*k12*k8*x2/(x2 + k9)*x4 + 1*k12*k5*(k6 - x4)*(x3 - x4) + (-1)*k12*k7*x4)/k12) ]