How to include a chemical reaction in the equation numbering?

Question

I would like to have my chemical reaction as equation number 0.1 and the equation (K_D) as an equation with the second number 0.2.

  \documentclass[12pt]{article}
    \usepackage{amsmath}
    \usepackage{chemmacros}

\begin{document}

\begin{align*}
\ch{S + E <>[ $k_{\mathrm{SI}}$ ][ $k_{\mathrm{IS}}$ ] E.I <>[ $k_{\mathrm{PI}}$ ][ $k_{\mathrm{IP}}$ ] P + E}
\begin{align*}

\begin{equation} \label{}
 K_{D} = \frac{[A][B]^{N_{max}}}{[AB_{N_{max}]}} = \frac{k_{off}}{k_{on}}.
\end{equation}

\end{document}

With Alex's suggestion I got this:

This is the SOLUTION:

\begin{equation}
  \ch{ A + B -> C + D }
\end{equation}

Answer 1

You can use numberwithin{equation}{section}:

    \documentclass[12pt]{article}
    \usepackage{amsmath}
    \usepackage{chemmacros}
    \begin{document}

    \numberwithin{equation}{section}
    \begin{align}
        \ch{S + E <>[ $k_{\mathrm{SI}}$ ][ $k_{\mathrm{IS}}$ ] E.I <>[                 $k_{\mathrm{PI}}$ ][ $k_{\mathrm{IP}}$ ] P + E}
    \end{align}

    \begin{equation} \label{}
         K_{D} = \frac{[A][B]^{N_{max}}}{[AB_{N_{max}]}} = \frac{k_{off}}        {k_{on}}.
    \end{equation}

    \end{document}