[ Eq(Derivative(x1, t), (1*k1*k4*x3/(k2 + x3)*x4/(k3 + x4) + 1*k1*k5*x5 + 1*k1*k6 + (-1)*k1*k7*x1)/k1),
  Eq(Derivative(x2, t), ((-1)*k1*k11*x2 + 1*k1*k18*x6/(k19 + x6)*x5/(k20 + x5)*x3/(k21 + x3))/k1),
  Eq(Derivative(x4, t), ((-1)*k1*k12*x4 + 1*k1*k14*x2)/k1),
  Eq(Derivative(x5, t), ((-1)*k1*k10*x5 + 1*k1*k15*x7)/k1),
  Eq(Derivative(x6, t), ((-1)*k1*k8*x6 + 1*k1*k13*x1)/k1),
  Eq(Derivative(x7, t), ((-1)*k1*k9*x7 + 1*k1*k16*x6*k17/(k17 + x3))/k1) ]