{ diff(x1, t) = (1*k3*k1/(k2 + x4)*x4 + (-1)*k8*x1 + (-1)*k12*x1 + (-1)*k13*x1*x2)/k17,
  diff(x2, t) = (1*k5*x1 + 1*k6*x3*x1 + (-1)*k9*x2)/k17,
  diff(x3, t) = (1*k7*x1 + (-1)*k10*x3)/k17,
  diff(x4, t) = ((-1)*k1/(k2 + x4)*x4 + 1*k4*x3 + (-1)*k11*x4)/k17,
  diff(x5, t) = 0 }