\frac{dx_{1}}{dt} = \left(1 \cdot k_{28} \cdot \left(1 + k_{17} \cdot k_{25} / \left(k_{25} + k_{18}\right)\right) \cdot k_{2} \cdot k_{4} / \left(k_{4} + x_{4}\right) + -1 \cdot k_{28} \cdot k_{1} \cdot x_{1}\right) / k_{28}\\
\frac{dx_{2}}{dt} = \left(1 \cdot k_{28} \cdot \left(k_{20} + \left(1 - k_{20}\right) \cdot k_{18} / \left(k_{25} + k_{18}\right)\right) \cdot k_{2} \cdot k_{6} / \left(k_{6} + x_{3}\right) + -1 \cdot k_{28} \cdot k_{1} \cdot x_{2}\right) / k_{28}\\
\frac{dx_{3}}{dt} = \left(1 \cdot k_{28} \cdot k_{9} \cdot \left(1 + k_{21} \cdot k_{25} / \left(k_{25} + k_{22}\right)\right) + 1 \cdot k_{28} \cdot k_{8} \cdot x_{1} \cdot \left(k_{10} + \left(1 - k_{10}\right) \cdot k_{11} / \left(x_{4} + k_{11}\right)\right) \cdot \left(1 + k_{23} \cdot k_{25} / \left(k_{25} + k_{24}\right)\right) + -1 \cdot k_{28} \cdot k_{7} \cdot x_{3}\right) / k_{28}\\
\frac{dx_{4}}{dt} = \left(1 \cdot k_{28} \cdot k_{14} \cdot \left(k_{15} + \left(1 - k_{15}\right) \cdot k_{16} / \left(x_{3} + k_{16}\right)\right) + 1 \cdot k_{28} \cdot k_{13} \cdot x_{2} + -1 \cdot k_{28} \cdot k_{12} \cdot x_{4}\right) / k_{28}