Modifying arrow{-U>} with makeatletter

I made a reaction scheme like in the code and modified the arrow[-U>] so that I can add an additional reactant that catalyses the reaction by makeatletter. Unfortunately, the text is too close to the arrow and I want some space between it. All my efforts with the usal spacing commands were unsuccesful. Any ideas?
Thanks

documentclass{article}
usepackage{chemfig}

begin{document}
    begin{figure}[h]
        schemestart
        A
        arrow{-U>[X][Y]}
        B
        makeatletter
        chemmove{node[below] at (Uarrow@arctangent){textbf{Protein}};}
        makeatother
        schemestop
    end{figure}
end{document}