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