\frac{dx_{1}}{dt} = \left(1 \cdot k_{20} \cdot 2 \cdot k_{2} \cdot x_{2} + -1 \cdot k_{20} \cdot \left(\left(k_{6} \cdot x_{3} + k_{4}\right) \cdot x_{1} + k_{1} \cdot x_{1}\right)\right) / k_{20}\\
\frac{dx_{2}}{dt} = \left(1 \cdot k_{20} \cdot k_{1} \cdot x_{1} + -1 \cdot k_{20} \cdot \left(k_{5} \cdot x_{2} + k_{2} \cdot x_{2} + k_{7} \cdot x_{2} \cdot x_{3} + k_{11} \cdot \left(-\exp\left(\left(-k_{12}\right) \cdot x_{4}\right)\right) \cdot x_{2}\right)\right) / k_{20}\\
\frac{dx_{3}}{dt} = \left(1 \cdot k_{20} \cdot \left(k_{10} + k_{16} \cdot x_{3} \cdot x_{1} + x_{2}^{k_{17}} / \left(k_{15} + x_{1} + x_{2}^{k_{17}}\right)\right) + -1 \cdot k_{20} \cdot \left(k_{8} \cdot x_{3} \cdot x_{1} + k_{9} \cdot x_{2} \cdot x_{3} + k_{3} \cdot x_{3} + k_{13} \cdot \left(1 - \exp\left(\left(-k_{14}\right) \cdot x_{4}\right)\right) \cdot x_{3}\right)\right) / k_{20}\\
\frac{dx_{4}}{dt} = -1 \cdot k_{20} \cdot k_{18} \cdot x_{4} / k_{20}