Im having trouble understanding Scatchard plots.

Y Axis = Bound/Free Ligand X Axis = Bound Ligand

The graph has a negative slope.

Why when there is almost no Bound (Y axis = 0) do we get a high positive number for Bound ligand on the X Axis corresponding to Receptor concentration? if there is no bound how is there a positive bound number?
it looks like the less bound ligand on the Y axis, the more bound on the X axis!
enter image description here


The Scatchard eqn can be written as $\displaystyle \frac{Y}{[L]}=\frac{n}{K}-\frac{Y}{K}$ with $[L]$ the free ligand concentration and $Y$ the fraction bound, (i.e. $[L]$ bound /total protein) so the x-axis intercept (when $Y/[L]=0$) is $n$ the number of ligands bound, the slope is $-1/K$ and the y-axis intercept is $n/K$. Your plot confirms this equation, the interpretation of intercepts and slope follow from the equation.

(Your plot seems to plot bound concentration not fraction bound so needs correcting with total protein to make $Y$ and this is presumably why $n$ comes out to be a small value)


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