Chemfig: multiple arrows from one molecule

Question

I would like to have several arrows going from one molecule (chemfig). I managed left and right, but now I also want one going 90° down from the molecule in the middle. This is what I tried (ignore the spacing problems on the right side):

\documentclass[12pt,a4paper,twocolumn]{article}

\usepackage{chemfig}

\usepackage{tikz}

\begin{document}

\schemestart
excretion
\arrow{<-}[90]
APAP-sulf \arrow{<-[SULT][sulfation]}[0,1.3]
    \chemname{\chemfig{HO-[:30]*6(-=-(-NH-[:-30](=[:-90]O)-[:30])=-=)}}{Acetaminophen}
    \arrow{->[UGT][glucorination]}[0,1.3]APAP-gluc
    \arrow {->}[-90] excretion
    \arrow{->}[-90] NAPQI       
\schemestop
\end{document}

Answer 1

Here is a proposal. Please make sure that you have updated your TeX distribution when you run it because it relies on some recent updates of the tikzmark library. You can remember

1. elements of a chemfig with the @{<name>}{<element>} syntax and
2. other things with the \tikzmarknode{<name>}{<object>} syntax.

And then you can use an overlay TikZ picture to draw arrows and whatever. I used the calc library to determine a possibly suitable starting point of the arrow. If you drop blue in the \draw command, the arrow will be black.

\documentclass[12pt,a4paper,twocolumn]{article}

\usepackage{chemfig}

\usepackage{tikz}
\usetikzlibrary{tikzmark,calc}

\begin{document}

\schemestart
excretion
\arrow{<-}[90]
APAP-sulf \arrow{<-[SULT][sulfation]}[0,1.3]
\chemname{\chemfig{@{HO}{HO}-[:30]*6(-=-(-NH-[:-30](=[:-90]@{O}{O})-[:30])=-=)}}{Acetaminophen}
    \arrow{->[UGT][glucorination]}[0,1.3]APAP-gluc
    \arrow {->}[-90] excretion
    \arrow{->}[-90] \tikzmarknode{NAPQI}{NAPQI}
\schemestop
\begin{tikzpicture}[overlay,remember picture]
\draw[blue,-latex] ($(O)!0.1!(HO)-(0,0.5)$) -- (NAPQI.north west);
\end{tikzpicture}
\end{document}

Here is a second proposal with a vertical arrow. If you want to restore the arrow from excretion, just remove opacity=0.

\documentclass[12pt,a4paper,twocolumn]{article}

\usepackage{chemfig}

\usepackage{tikz}
\usetikzlibrary{tikzmark}

\begin{document}

\schemestart
excretion
\arrow{<-}[90]
APAP-sulf \arrow{<-[SULT][sulfation]}[0,1.3]
\chemname{\chemfig{@{HO}{HO}-[:30]*6(-=-(-NH-[:-30](=[:-90]@{O}{O})-[:30])=-=)}}{
\tikzmarknode{A}{Acetaminophen}}
\arrow{->[UGT][glucorination]}[0,1.3]APAP-gluc
\arrow {->}[-90] excretion
\arrow{->}[-165,3,opacity=0] \tikzmarknode{NAPQI}{NAPQI}
\schemestop
\begin{tikzpicture}[overlay,remember picture]
\draw[-latex,shorten >=4pt,shorten <=2pt] (A) -- (NAPQI);
\end{tikzpicture}
\end{document}

And here is a version that does not rely on tikzmark.

\documentclass[12pt,a4paper,twocolumn]{article}

\usepackage{chemfig}

\begin{document}

\schemestart
excretion
\arrow{<-}[90]
APAP-sulf \arrow{<-[SULT][sulfation]}[0,1.3]
\chemname{\chemfig{@{HO}{HO}-[:30]*6(-=-(-NH-[:-30](=[:-90]@{O}{O})-[:30])=-=)}}{
Acetaminophen}
\arrow{->[UGT][glucorination]}[0,1.3]APAP-gluc
\arrow {->}[-90] excretion
\arrow{->}[-165,3,opacity=0] NAPQI
\arrow{<-}[90]
\schemestop
\end{document}