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