\frac{dx_{1}}{dt} = k_{1} / \left(1 + x_{2}\right) - k_{2} \cdot x_{1} \cdot x_{2}\\
\frac{dx_{2}}{dt} = k_{4} - x_{2} - k_{2} \cdot x_{1} \cdot x_{2} + k_{3} \cdot x_{3}\\
\frac{dx_{3}}{dt} = k_{2} \cdot x_{1} \cdot x_{2} - k_{3} \cdot x_{3}