\documentclass[12pt,border=5pt]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{chemfig,chemmacros}
\usepackage{lmodern}

\chemsetup[chemformula]{font-shape=sf,format=\sffamily}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setatomsep{2.5em}
\setdoublesep{.6ex}
\setarrowdefault{,1.2}
\setbondstyle{thick}

\begin{document}
\schemestart
    \chemfig{H_2N-[:-30](-[6]R)-[:30]~[::0]@{a1}N}
    \arrow{<=>[\chemfig{@{a2}H^{\scriptstyle+&#x7D;&#x7D;]}
    \chemfig{H_2N-[:-30](-[6]R)-[:30]@{b1}~[@{b2}::0]@{b3}NH^{\scriptstyle+&#x7D;&#x7D;
    \arrow{<=>[\chemfig{H_2@{b4}O}]}
    \chemfig{H_2N-[:-30](-[6]R)-[:30](-[2]OH_2^{\scriptstyle+})=[:-30]NH}
    \arrow{<=>[][*{0}\parbox{1.5cm}{\small Proton transfer}]}[-90,.9]
    \chemfig{H_2N-[:-30](-[6]R)-[:30]@{c1}(-[2]OH)=[@{c2}:-30]@{c3}NH_2^{\scriptstyle+&#x7D;&#x7D;
    \arrow{->[\chemfig{H_2@{c0}O}]}[180]
    \chemfig{H_2N-[:-30](-[6]R)-[:30](-[2]OH)(-[:-30]OH_2^{\scriptstyle+})-[:-75]NH_2}
    \arrow{<=>[*{0.south}\parbox{1.5cm}{\small\centering Proton transfer}]}[180]
    \chemfig{H_2N-[:-30](-[6]R)-[:30](-[@{d1}2]@{d2}OH)(-[:-30]OH)-[@{d3}:-75]@{d4}NH_3^{\scriptstyle+&#x7D;&#x7D;
    \arrow{<=>[][*{0}$-$\chemfig{NH_3}]}[-90,.9]
    \chemfig{H_2N-[:-30](-[6]R)-[:30](=[2]OH^{\scriptstyle+})(-[:-30]OH)}
    \arrow{->[$-$\chemfig{H^{\scriptstyle+&#x7D;&#x7D;]}
    \chemfig{H_2N-[:-30](-[6]R)-[:30](=[2]O)(-[:-30]OH)}
    \chemmove[->,red, shorten <=3pt, shorten >=3pt,-stealth]{
    	\draw (a1) .. controls +(north east:8mm) and +(north:5mm) .. (a2);
        \draw (b2) .. controls +(south:8mm) and +(south:5mm) .. (b3);
        \draw (b4) .. controls +(north:10mm) and +(north:12mm) .. (b1);
        \draw (c2) .. controls +(south:10mm) and +(south:10mm) .. (c3);
        \draw (c0) .. controls +(north:10mm) and +(north west:10mm) .. (c1);
        \draw (d2) .. controls +(west:5mm) and +(west:5mm) .. (d1);
        \draw (d3) .. controls +(west:5mm) and +(west:5mm) .. (d4);
    }
\schemestop
\end{document}