[ Eq(Derivative(x1, t), -k1*x1*x17),
  Eq(Derivative(x2, t), k1*x1*x17),
  Eq(Derivative(x3, t), 4*(x2 - x3)/k12),
  Eq(Derivative(x4, t), 4*(x3 - x4)/k12),
  Eq(Derivative(x5, t), 4*(x4 - x5)/k12),
  Eq(Derivative(x6, t), 4*(x5 - x6)/k12),
  Eq(Derivative(x7, t), k3*x2*x18/(1 + k11*x16) - k5*x7),
  Eq(Derivative(x8, t), k5*x7 - k4*x8),
  Eq(Derivative(x9, t), 4*(x8 - x9)/k13),
  Eq(Derivative(x10, t), 4*(x9 - x10)/k13),
  Eq(Derivative(x11, t), 4*(x10 - x11)/k13),
  Eq(Derivative(x12, t), 4*(x11 - x12)/k13),
  Eq(Derivative(x13, t), 4*(x20 - x13)/k14),
  Eq(Derivative(x14, t), 4*(x13 - x14)/k14),
  Eq(Derivative(x15, t), 4*(x14 - x15)/k14),
  Eq(Derivative(x16, t), 4*(x15 - x16)/k14),
  Eq(Derivative(x17, t), -k1*x1*x17),
  Eq(Derivative(x18, t), k2*x6 + k9*x19 - k8*x18 - k3*x2*x18/(1 + k11*x16)),
  Eq(Derivative(x19, t), k8*x18 - k9*x19 + k4*x8),
  Eq(Derivative(x20, t), k10 + k6*x12 - k7*x20) ]