Kcat determination

Kresten kresten at my-dejanews.com
Mon May 17 09:48:50 EST 1999


In article <373D7F50.6B1D66D1 at sciborg.uwaterloo.ca>,
  Michael Allen <m3allen at sciborg.uwaterloo.ca> wrote:
> Hello all
> I apologize in advance for the naivete of this question.
> When you are determining the Kcat of an enzyme which is a homodimer in
> its active state, do you use the molecular mass of the monomer or the
> dimer?

Depends on what you are doing. If the dimerisation equilibrium is:

2M = D         where M is the monomer and D the dimer

(and = here and in the rest does not mean equilibrium)

and almost all of the protein is in D, then you can go ahead as if D
were your enzyme E in the simple MM mech: E+S = ES -> E+P
or whatever system you are working with. You must however be aware that
if you solve for [D] using K_dimerisation you cannot use:
[M]+2[D]=[enzyme]_total since there is also some Michaelis-complex
present.

What you really ought to do is to solve the equations of the system e.g.
under steady-state conditions. If you are sure that only D and not M has
activity the system could look something like:

 D + S = DS -> D+P
 II
 2M

(sorry about the drawing, it's an ASCII world)

You can probably find the solution for this problem in an enz. kinetics
textbook, but I would think it is faster to solve the equations rather
than to go to the library.

If you are not sure whether M also has activity you certainly have to
do more experiments e.g. not only varying [S] but also [enzyme]_tot.

I would guess you again could find info in an enzyme kinetics textbook
and again recommend to do the algebra yourself.

Hope this helps
Kresten

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Kresten Lindorff Larsen, Dept. Yeast Genetics
Carlsberg Laboratory, Denmark


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