[ Diff(x[1], t) = (1*k[1]*k[2]*x[2] + (-1)*k[3]*x[1]/(k[4] + x[1]))/k[1],
  Diff(x[2], t) = (1*k[5]*(1 - x[2])/(k[6] + 1 - x[2]) + (-1)*k[7]*x[3]*x[2]/(k[8] + x[2]))/k[1],
  Diff(x[3], t) = (1*x[1]*k[9]*(1 - x[3])/(k[10] + 1 - x[3]) + (-1)*k[11]*x[3]/(k[12] + x[3]))/k[1] ]