I'm try to write a square-scheme reaction using MathJax. Until now I managed this one.
$$\begin{array}{c}
\ce{A} \quad \ce{<=>[$k_1$][$k_2$]} \quad \ce{B}\\[-1cm]
\!\scriptsize k_3 \Large\downharpoonleft\!\!\upharpoonright \scriptsize \!\!\! k_4 \qquad
\scriptsize k_5 \Large\downharpoonleft\!\!\upharpoonright \scriptsize \!\!\! k_6 \normalsize \\[-1cm]
\ce{C} \quad \ce{<=>[$k_7$][$k_8$]} \quad \ce{D}\\
\end{array}$$
$$\begin{array}{c} \ce{A} \quad \ce{<=>[$k_1$][$k_2$]} \quad \ce{B}\\[-1cm] \!\scriptsize k_3 \Large\downharpoonleft\!\!\upharpoonright \scriptsize \!\!\! k_4 \qquad \scriptsize k_5 \Large\downharpoonleft\!\!\upharpoonright \scriptsize \!\!\! k_6 \normalsize \\[-1cm] \ce{C} \quad \ce{<=>[$k_7$][$k_8$]} \quad \ce{D}\\ \end{array}$$
Is this result acceptable? Is there any other way to achieve this result?
&
separator. You do not need$$
if you use a\begin...\end
block (there's no harm if you do). $\endgroup$