[limonade]

Hi
I would like to know if I am approaching the following problem correctly.  In particular, I would like to verfy that my assumption KM = Keq is correct for calculating gibbs free energy. Also , what is considered a "large" Kd value vs a small one for determining if an enzyme substrate complex is in its dissociated or associated form at equilibrium? Since Kd=1/Keq, would I just use Kd = 1/Km?

Thank you

[limonade]

whoops. I think I made a mistake. the problem says k2<<k-1

sooo I think I should have my Km= Kd
then just take the point where Km= 1/2 Vmax = 1/2 *9.6 from the table I suppose. If anyone can confirm I would appreciate it

[mjc123]

I'm not a biochemist, but there are some obvious problems here.
There are not one but three equilibrium constants here (two independent); one for each of the separate equilibria and one for the overall reaction:
K₁ = [ES]/[E][S ]; K₂ = [E][P]/[ES]; K_eq = [P]/[S ] = K₁K₂
You are asked for ΔG° for the first step, not the overall reaction, so you need -RTlnK₁, not -RTlnK_eq.
The statement K_M = K_eq for k₂>>k_-1 is wrong; K_M and K_eq have different units.
For k₂<<k_-1, K_M = k_-1/k₁ = 1/K₁
so ΔG° = -RTln(1/K_M)

[Babcock_Hall]

When I work with a Lineweaver Burk plot, I find 1/V_max from the y-intercept of the graph.  Then I calculate K_M using the slope of the L-B plot and the value of V_max.  The Al Gebra is quite easy.  Just don't do it on an American Airlines flight.
EDT
K_M is approximately equal to K_d under the conditions that the problem specified.  But K_d is the dissociation constant of the substrate from the enzyme, not the equilibrium constant.  The relationships between the kinetic constants and the equilibrium constant for the reaction are called the Haldane relationships, after JBS Haldane.

[limonade]

Thanks to all for your help