{ diff(x1, t) = (-1)*k6*k2*k1*x1*x3/x5/k6,
  diff(x2, t) = (1*k6*k2*k1*x1*x3/x5 + (-1)*k6*k4*x2)/k6,
  diff(x3, t) = (1*k6*k4*x2 + (-1)*k6*k3*x3)/k6,
  diff(x4, t) = 1*k6*k3*x3/k6,
  diff(x5, t) = 0/k6 }