\newdimen\Width \newdimen\Height
\define[3]\Chromato{%
  \setbox\scratchbox\hbox{\externalfigure[#1][width=0.6\makeupwidth]}%
  \setuppositioning[unit=pt] \Width=\wd\scratchbox \Height=\ht\scratchbox
  \placefigure[][fig:#1]{#2}{\startpositioning
    \position(0,0){\externalfigure[#1][width=0.6\makeupwidth]}#3%
  \stoppositioning}}
\define[3]\LRposition{\edef\YPos{\withoutpt{\the\dimexpr(\withoutpt{%
        \the\dimexpr(#1pt/100)}\Height - 0.1\baselineskip)}}%
  \position(#2,\YPos){\vbox to 2pt{\vss{\hbox to 0pt{#3}\vss}}}}
\define[4]\Lbrace{\LRposition{#1}0{\hss\rotate[rotation=90]{%
      $\overbrace{\hskip#2\Height}^{\rotate[rotation=270]{#4}}$}\hskip#3em}}
\define[4]\Rbrace{\LRposition{#1}{\withoutpt{\the\Width}}{\hskip#3em
    \rotate[rotation=270]{$\overbrace{\hskip#2\Height}^{%
        \rotate[rotation=90]{#4}}$}\hss}}
\define[2]\Lpfeil{\LRposition{#1}0{\hss{\tx#2} $\rightarrow$}}
\define[2]\Rpfeil{\LRposition{#1}{\withoutpt{\the\Width}}{$\leftarrow$ \tx#2}}
\define[3]\Oposition{\edef\XPos{\withoutpt{\the\dimexpr(\withoutpt{%
      \the\dimexpr(#1pt/100)}\Width)}}%
    \position(\XPos,-\withoutpt{\the\dimexpr(#2\baselineskip)}){%
    \hbox to 0pt{\hss#3\hss}}}
\define[2]\Oben{\Oposition{#1}{0.8}{\tx#2}}
\define[3]\Obrace{\Oposition{#1}{2.5}{$\overbrace{\hskip#2\Width}^{\hbox{#3}}$}}

\starttext
\Chromato{ecolizeit}{Expression der rekombinanten Fucosyltransferase~V in
  {\it E.~coli}.}{%
\Oben5{BRS} \Oben{15}1 \Oben{26}2 \Oben{38}3 \Oben{50}4
\Oben{61}5 \Oben{72}6 \Oben{83}7 \Oben{94}8
\Obrace{49}{0.68}{\tx1--7}
\Lpfeil{12}{97\,kDa} \Lpfeil{20}{66\,kDA}
\Lpfeil{32}{45\,kDa} \Lpfeil{52}{31\,kDa}
\Lbrace{40}{0.8}{4}A \Lbrace{90}{0.2}{0.1}B
\Rpfeil{29}{FucT~V} \Rpfeil{87}{FucT~V}
\Rpfeil{40}1 \Rpfeil{80}2 \Rbrace{60}{0.4}3{\txx1--2}}
\stoptext