I have the following chemical reaction scheme with a shaded polygon (using link) as shown in MWE below:
MWE
\documentclass{article}
\usepackage{chemfig}
\begin{document}
%\schemedebug{true} % uncomment to see node names
\schemestart
P \arrow(P--Q){<=>[$\mathrm{k_{pq}}$]} Q
\arrow{<=>[$\mathrm{k_{qs}}$]}[30] S \arrow(S--T){<=>[][$\mathrm{k_{st}}$]}[-30] T
\arrow(@Q--R){<=>[][$\mathrm{k_{qr}}$]}[-30] R
\arrow(@R--@T){<=>[][$\mathrm{k_{rt}}$]}
\arrow(@T--U){<=>[][$\mathrm{k_{tu}}$]} U
\arrow(@U--V){->[$\mathrm{k_{uv}}$]} V
%
\chemmove{%
\draw [red,fill=red,fill opacity=0.5,thick] ([shift={(-5pt,1em)}]Q.north west) -- ([yshift=1em]S.north) --
([yshift=1em]T.north) -- ([shift={(5pt,1em)}]U.north east) --
([shift={(5pt,-1.5em)}]U.south east) --
([yshift=-1.5em]T.south) -- ([yshift=-2em]R.south) --
([shift={(-5pt,-2em)}]Q.south west) -- cycle;
}
\schemestop
\end{document}
However, the shaded polygon is overlaid on the reaction scheme as shown in the figure. How do I move back the shaded polygon to the back of the reaction scheme? I need the reaction scheme placed on the shaded polygon. Can this be done?