[ Eq(Derivative(x1, t), (-1)*k15*k1*x12*x1/(k2 + x1)/k15),
  Eq(Derivative(x2, t), (1*k15*k1*x12*x1/(k2 + x1) + (-1)*k15*k3*x2 + (-1)*k15*(k4*x6*x2*x4 - k5*x5))/k15),
  Eq(Derivative(x3, t), 1*k15*k3*x2/k15),
  Eq(Derivative(x4, t), (-1)*k15*(k4*x6*x2*x4 - k5*x5)/k15),
  Eq(Derivative(x5, t), 1*k15*(k4*x6*x2*x4 - k5*x5)/k15),
  Eq(Derivative(x6, t), ((-1)*k15*(k4*x6*x2*x4 - k5*x5) + 1*k15*k6*x7*x8/(k7 + x8))/k15),
  Eq(Derivative(x7, t), (1*k15*k8*x5*x9/(k9 + x9) + 1*k15*k10*x2*x9/(k11 + x9) + (-1)*k15*k12*x7 + (-1)*k15*k13*x7)/k15),
  Eq(Derivative(x8, t), (-1)*k15*k6*x7*x8/(k7 + x8)/k15),
  Eq(Derivative(x9, t), ((-1)*k15*k8*x5*x9/(k9 + x9) + (-1)*k15*k10*x2*x9/(k11 + x9))/k15),
  Eq(Derivative(x10, t), 1*k15*k12*x7/k15),
  Eq(Derivative(x11, t), 1*k15*k13*x7/k15),
  Eq(Derivative(x12, t), 0/k15) ]