logKm or logVmax dependence on pH

[GVE]

I have information Km and Vmax dependence on pH: 

pH	6.0	6.3	6.5	7.0	7.5	8.0
KM (mM)	0.120	0.163	0.216	0.250	0.210	0.160
Vmax	0.430	0.480	0.526	0.530	0.450	0.313

and I have to determine which amino acids are in the active site. Which is a better way to find pKa and determine the amino acids? To look at logKm dependence on pH, or at logVmax dependence on pH? Thank you.

[BabcockHall]

Have you thought about looking at V_max/K_M?  It turns out that each parameter that you examine tells a slightly different story.  

[GVE]

20 hours ago, BabcockHall said:

Have you thought about looking at V_max/K_M?  It turns out that each parameter that you examine tells a slightly different story.  

I created pH dependence on log(Vmax/Km) graph, but I'm not sure how I can get pKa values from it, because it's form is strange.

 Then I created pH dependence on log(Vmax) graph:

and painted lines to find pKa values. Am I doing it right?

[BabcockHall]

Your estimates do not look bad, but it might be possible to improve them.  Ideally one would use nonlinear regression.

Provided certain assumptions are correct, pK_a values in the V_max graph pertain to the E•S complex, whereas pK_a values in the V/K graph pertain to free E or to free S.  CRC Critical Reviews in Biochemistry (Knowles JR 1976, p. 165) has a good intermediate-level discussion of this topic.  In your case the graph of V/K vs. pH has an unusual shape, which I find a bit puzzling.   It may be that the data do not span as a large range of velocities as one might like.