{ diff(x1, t) = 0,
  diff(x2, t) = (1*k15*k7*(1 - k9*x2)*x2 + (-1)*k15*k8*x5*x2/(k10 + x2))/k15,
  diff(x3, t) = 0,
  diff(x4, t) = (1*k15*(k14 + k12*x5*x2/(k13 + x2)) + (-1)*k15*k11*x4)/k15,
  diff(x5, t) = (1*k15*(k6 + k3*x2 + k4*x5*x4/k5) + (-1)*k15*k2*x5)/k15 }