documentclass{article}
usepackage{chemfig}
setarrowdefault{15,3,thick}
begin{document}
schemestart
 $A_{1}$
arrow($A_{2}$--){<=>[$k_{1}$][$k_{2}$]}[35] $A_{2}$
arrow($A_{2}$--){<=>[$k_{3}$][$k_{4}$]}[-35] $A_{3}$
schemestop
end{document}

documentclass{article}
usepackage{chemfig}
usepackage{tikz}
usetikzlibrary{decorations.markings,arrows}

setarrowdefault{15,3,thick}
begin{document}
makeatletter
tikzset{
    ddbond/.style n args={4}{
        draw=none,
        decoration={%
            markings,
            mark=at position 0 with {
                coordinate (CF@startdeloc) at (0,dimexpr#1CF@double@sep)
                coordinate (CF@startaxis) at (0,dimexpr-#1CF@double@sep);
            },
            mark=at position 1 with {
                coordinate (CF@enddeloc) at (0,dimexpr#1CF@double@sep)
                coordinate (CF@endaxis) at (0,dimexpr-#1CF@double@sep);
                draw[-{Stealth[left]}, line width=0.4mm] (CF@startdeloc)--node[rotate=#2, above = 0.1cm]{#3}(CF@enddeloc);
                draw[{Stealth[left]-}, line width=0.4mm] (CF@startaxis)--node[rotate=#2, below = 0.1cm]{#4}(CF@endaxis);
        }
     },
  postaction={decorate}
 }
}
makeatother
chemfig{A_1-[-135,5,,,ddbond={+1.5}{45}{$k_1$}{$k_2$}]A_2-[135,5,,,ddbond={+2.5}{-45}{$k_3$}{$k_4$}]A_3}

end{document}