[ Eq(Derivative(x1, t), 1*k1*(k3*(1 - x1) - k2*x1)/k1),
  Eq(Derivative(x2, t), 1*k1*((1 - x2)*k4*x1 - k5*x2)/k1),
  Eq(Derivative(x3, t), 1*k1*(k6/(k6 + x4)*(1 - x3)*k7 - k8*x3)/k1),
  Eq(Derivative(x4, t), 1*k1*(k10/(k10 + x3)*k14 - (k13*x4*x1 + k11*x4 + k12*x6*x4/(k9 + x4)))/k1),
  Eq(Derivative(x5, t), 1*k1*(k16*x4*x1 - k15*x5)/k1),
  Eq(Derivative(x6, t), 1*k1*(k20*x2*x5 + k21*x6**5/(k17**5 + x6**5) - k19*x6/(k18 + x6))/k1) ]