= Sequential two-site enzyme reaction network
An <enzyme> may bind one molecule of a <substrate> to form $C_1$, then a second to form $C_2$, with product-release reactions $C_1\to E+P$ and $C_2\to C_1+P$. The <law of mass action> gives conservation laws $e+c_1+c_2=e_0$ and $s+c_1+2c_2+p=s_0$. Scaling substrate by $s_0$, complexes by $e_0$ and time by $(k_1e_0)^{-1}$ exposes the small ratio $\epsilon=e_0/s_0$: the substrate evolves on the slow scale while the complex equations have the form $\epsilon v_i'=g_i$. This is a starting point for <quasi-steady-state approximation> when $\epsilon\ll1$.
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