\frac{dx_{1}}{dt} = \left(1 \cdot k_{15} \cdot k_{10} \cdot x_{3} \cdot \left(k_{11} - x_{1}\right) / \left(k_{12} + k_{11} - x_{1}\right) + -1 \cdot k_{15} \cdot k_{13} \cdot x_{1} / \left(k_{14} + x_{1}\right)\right) / k_{15}\\
\frac{dx_{2}}{dt} = \left(1 \cdot k_{15} \cdot \left(k_{1} + k_{2} \cdot k_{16}\right) + -1 \cdot k_{15} \cdot \left(k_{3} + k_{4} \cdot x_{1}\right) \cdot x_{2}\right) / k_{15}\\
\frac{dx_{3}}{dt} = \left(1 \cdot k_{15} \cdot k_{5} \cdot x_{2} \cdot \left(k_{6} - x_{3}\right) / \left(k_{7} + k_{6} - x_{3}\right) + -1 \cdot k_{15} \cdot k_{8} \cdot x_{3} / \left(k_{9} + x_{3}\right)\right) / k_{15}\\
\frac{dx_{4}}{dt} = 0