dx1/dt = (-k5*x1) / (k6 + x1),
dx2/dt = (k5*k8*x1 + k5*x3*x1 + k5*x2*x1 - k6*x2*k7 - x2*k7*x1) / (k8*k6 + k8*x1 + k6*x3 + k6*x2 + x3*x1 + x2*x1),
dx3/dt = (-k8*k9*k10*x3 + k8*k9*x3^2 - k9*k10*x3^2 - k9*k10*x3*x2 + k9*x3^3 + k9*x3^2*x2 + k10*x2*k7) / (k8*k10 + k10*x3 + k10*x2),
dx4/dt = (k9*k10*x3 - k9*x3^2) / (k10),
y1 = x1,
y2 = x2
