How to make a function inside the tikzpicture behave like outside

I have a makro which generates a structure(rule) and a reference to that structure. It does not behave when inside a tikzpicture. How to make the function robust to reference the structure even when inside tikzpicture? (I also want to get rid of the error message "mevp not defined")

\documentclass[12pt,a4paper]{book}
\makeatletter
%\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{ifthen}
\usepackage{tikz}
\newcounter{strctz}
\DeclareRobustCommand{\strct}[1]{%
%  \ifthenelse{\isundefined{\c#1}} %
%     {
       \refstepcounter{strctz}%
       \textbf{#1}\textbf{\Large({\thestrctz})}%
       \\[2mm]%
       \rule{2cm}{5mm}%
       \begingroup
       \expandafter\protected@edef\csname c#1\endcsname{\thestrctz}%
       \expandafter\endgroup
       \expandafter\DeclareRobustCommand
       \csname c#1\expandafter\expandafter\expandafter\endcsname
       \expandafter\expandafter\expandafter{\csname c#1\endcsname}%
 %    }
 %    {#1%
 %    \\[2mm]%
 %    \rule{2cm}{5mm}%
 %      (\c#1)%
 %    }
 }
 \makeatother
 \begin{document}
 \strct{mevalonate}
 (\cmevalonate) and

  \strct{ohpregnenolone}

  Structure (\cohpregnenolone) and structure (\cmevalonate) and
  Structure (\cohpregnenolone)

 \strct{alanine}  and (\calanine) (\cmevalonate) (\cohpregnenolone)

 \strct{mevalonate}

  %Isoprenyl-Bildung, Dimethyl-Allyl-Bildung aus Mevalonat
 %
  \tikzstyle{prenPP}= [rectangle,minimum size=2.4cm,text width=3.8cm,text  centered,inner sep=.05cm]
  \begin{tikzpicture}
  \node [prenPP%
         ,text width=4.6cm%
        ]      (mev)        at (0,8) {\strct{mevalonate}};
   node [prenPP%
         ,text width=4.6cm%
   ] (mevP)       at (8,8) {\strct{mevalonateP}};
   \draw[->,shorten >=0.5cm,shorten <=0.5cm,very thick]
     (mev)   -- node[below=0.5pt] {\parbox{2.5cm}{%
       \begin{center}%
         enzym{mevalonate kinase}
       \end{center}}}%
     (mevP) ;
   \end{tikzpicture}
 \cmelonalonateP
 \end{document}