data fitting problems

rlk rlk at intrepid.chm.jhu.edu
Wed Aug 28 13:48:53 EST 1996

I've run into a problem trying to use the Grafit software package,
and was wondering if anyone had any suggestions.

I am studying an enzyme which exhibits a ping-pong bi-bi
kinetic mechanism.  However, the first substrate is a
competitive inhibitor for the second substrate, and thus
I am trying to fit my kinetic data to this eqn:

v(i)= (Vmax*[A]*[B])/(Kma*[B]+Kmb*(1+[A]/Kia)*[A]+[A]*[B])

When I perform a global fit of this data I get an obviously incorrect
answer independent of my initial estimates or weighting method
(simple or robust.)  Not only is this result incorrect just
from visual inspection, it disagrees strongly with the values
I achieve using more traditional data fitting methods (replots
of slopes and intercepts.)

Furthermore, if I rearrange the eqn somewhat, replacing the
1/Ki with just an 'X' which i manually reconvert to a
Ki value, I get a markedly different set of results.  This, I
think, should definately not happen if I got the eqn
entered correctly.



'If I can't dance, I don't want to be a part of
   your revolution'   Emma Goldman

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