\frac{dx_{1}}{dt} = \left(-x_{1}\right) + k_{4} \cdot x_{2}^{2} / \left(k_{3}^{2} + x_{2}^{2}\right)\\
\frac{dx_{2}}{dt} = \left(-k_{5}\right) \cdot x_{2} + 1 / \left(1 + x_{1}^{2}\right) \cdot \left(k_{1} + k_{2} \cdot x_{2}^{2} / \left(1 + x_{2}^{2}\right)\right)