?
\documentclass{article}
\usepackage{chemfig}
%
\newcommand*\circleatom[1]{\tikz\node[circle,fill=green!30]{\printatom{#1}};}
\setatomsep{2em}
\setcompoundsep{7em}
%
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\begin{document}
\documentclass{article}
\usepackage{chemfig}
%
\newcommand*\circleatom[1]{\tikz\node[circle,fill=green!30]{\printatom{#1}};}
\setatomsep{2em}
\setcompoundsep{7em}
%
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\begin{document}
\schemestart
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-\circleatom{OH}} % I want to circle this last OH
\+
\chemfig{CH(-[4]\circleatom{H}_2N)(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)} % I want to circle the H of the H_2N near the beginning
\arrow{<=>}[-90]
\chemfig{CH(-[4]H_2N)(-[6]CH_2(-[6]C(=[5]O)(-[7]OH)))-C(=[2]O)-N(-[2]H)-CH(-[6]CH_2(-[6]*6(=-=-=-)))-C(=[1]O)(-[7]O-CH_3)}
\+
\chemfig{\circleatom{H_2O}}
\schemestop
\end{document}
