For quite a while now there has been a question on {TeX}.SE about how to modify the ratio of the two harpoons indicating an equilibrium reaction.
There is also a nice solution (by Matthew Leingang) to the problem using mhchem. Since the question predates the chemformula part of chemmacros the following solution couldn’t yet have been provided:
[cce lang=”latex”]\documentclass{article}
\usepackage{chemmacros}
\begin{document}
\ch{CH2OH + ATP <=>>[Hexokinase] “glucose~6-phosphate” + ADP + H+}
\ch[arrow-ratio=.2]{CH2OH + ATP <=>>[Hexokinase] “glucose~6-phosphate” + ADP + H+}
\end{document}[/cce]
Note: The quotation marks around [cce inline=”true” lang=”latex”]”glucose~6-phosphate”[/cce] prevent it from being treated as a formula.