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