\documentclass{article}
\usepackage{xymtex}
\usepackage{chemist}
\pagestyle{empty}
\begin{document}

\newcommand{\aminoacid}[1]{\tetrahedral{0==C;1==\utrigonal{0==C;1==(yl);3D==O;2==O$^-$};2==$^+$H$_3$N;3==#1;4==H}}
\newcommand{\alanine}{\aminoacid{CH$_3$}}
\alanine

\end{document}