documentclass[12pt]{article}
usepackage{chemfig,chemmacros}
usepackage{amsmath}
begin{document}
schemestart
Earrow(enzyme--){0}[0,0]+Sarrow{<=>[$k_{1}$][$k_{2}$]}ESarrow{->[$ k_{3} $][]}E+P
arrow(@enzyme--){0}[-90,0.15]+arrow{0}[-90,0.15]Iarrow{<=>[*{0}$ k_{4} $][*{0}$ k_{5} $]}[-90]EI
schemestop
end{document}

documentclass[12pt]{standalone}
usepackage{chemfig}

makeatletter
definearrow{2}{p}{%
    expandafterpathexpandafter[CF@arrow@current@style,-CF](CF@arrow@start@node)--(CF@arrow@end@node)
    node[midway, anchor=center]{+};
    CF@arrow@display@label{#1}{0.5}{+}{CF@arrow@start@node}{#2}{0.5}{-}{CF@arrow@end@node}
}
makeatother

begin{document}
schemestart
Earrow(enzyme--){p}[0,0.5]Sarrow{<=>[$k_{1}$][$k_{2}$]}ESarrow{->[$ k_{3} $][]}Earrow{p}[,0.5]P
arrow(@enzyme--){p}[-90,0.5]Iarrow{<=>[*{0}$ k_{4} $][*{0}$ k_{5} $]}[-90]EI
schemestop
end{document}