# Best graph for Km?

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#1
Which of the following graphs are going to give me an accurate Km and why??
1. Rate (V) against substrate concentration [S]
2. 1/V against 1/[S]
3. [S]/V against [S]
0
1 year ago
#2
Hi Debbie,

If you plot (1), the graph levels out after a certain substrate conc-n, but it is not possible to work out Km accurately, as it requires the measurement of the gradient of the graph at a precise point, and the gradient is changing all the time, AS THE GRAPH IS A CURVE.

When you plot (2) i.e. the reciprocals of reaction velocity (1/V - y axis) against reciprocal of substrate conc-n (I/[S] - x axis), then the point where the straight line you then get, crosses the y axis (the intercept) gives you the EXACT value of 1/Km, which you can PRECISELY read off and calculate the reciprocal to get Km.

The maths of it is a bit complex, but if you would like to know, please post a request here. (cannot promise I will recall it after 10 years, but will try - after all, even Sheldon is only human!).
Last edited by macpatgh-Sheldon; 1 year ago
0
#3
I'd like the mathematical explanation too please.
Also, why not the Hanes-Woolf plot (3) ? It is also a linear graph?
(Original post by macpatgh-Sheldon)
Hi Debbie,

If you plot (1), the graph levels out after a certain substrate conc-n, but it is not possible to work out Km accurately, as it requires the measurement of the gradient of the graph at a precise point, and the gradient is changing all the time, AS THE GRAPH IS A CURVE.

When you plot (2) i.e. the reciprocals of reaction velocity (1/V - y axis) against reciprocal of substrate conc-n (I/[S] - x axis), then the point where the straight line you then get, crosses the y axis (the intercept) gives you the EXACT value of 1/Km, which you can PRECISELY read off and calculate the reciprocal to get Km.

The maths of it is a bit complex, but if you would like to know, please post a request here. (cannot promise I will recall it after 10 years, but will try - after all, even Sheldon is only human!).
0
1 year ago
#4
Hi again, just remembered that this Q has been asked before (phrased differently), so I fished it out, and here it is (check out my answer for a brief general idea, then the detailed answer by randombiochemist (who seems to be the guru of Sheldon himself ):

https://www.thestudentroom.co.uk/sho...d%20reciprocal
0
1 year ago
#5
I am just about to post a pic of the graph to make it easier for u - give me a couple of minutes - need to do it with phone.
0
1 year ago
#6
Here we are:

Sry you will have to do a headstand haha! (and you look at the x-intercept [not y] which gives -1/Km)
Last edited by macpatgh-Sheldon; 1 year ago
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