{ diff(x1, t) = (1*(k10*x3 + k1*x1*x2/(k2 + x2)) + (-1)*k13*(k3*x1*x2/(k4 + x1) + 1/k5*x1))/k13,
  diff(x2, t) = (1*k13*k6*x2 + (-1)*k13*k7*x1*x2)/k13,
  diff(x3, t) = (1*k14*k8*x3*x4/(k9 + x4) + (-1)*k14*(1/k5*x3 + k10*x3))/k14,
  diff(x4, t) = (-1)*k14*(k11*x4*x3 + 1/k12*x4)/k14 }