Reaction constant contains one of the reactants

I need to provide the reaction rate functions of a system of reactions consisting of 5 reactions. One of them has the form: $$r\_1 = \frac{k\_a}{K\_D + \frac{K\_D}{K\_I}X\_4}\cdot X\_2\cdot X\_4.$$ I do not know how to handle the term of the reaction constant of this reaction rate function given that one of the reactants, $X\_4,$ is inside the constant. What I ultimately need, is to write a differential equation concerning the reaction dynamics. How can I handle this problem ?


