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Drugs: From Molecules to Man - Lecture 5 (12/02/2018)
Imagine there is a 1 m2 board with a series of holes drilled in it.
If we tilt the board and roll marbles down it, it is obvious that only some kinds of marbles will lodge in the holes.
Those that are too big will roll over and away, and those that are too small will simply fall through, this displays that in order to bind a ligand must be complementary to its binding site.
The more marbles rolled, the more will stay bound to the board. However, the board has a fixed number of holes so no matter how many marbles we use, no more than this number will lodge on it. This is similar to the situation with a cell.
The cell has a certain number of drug binding sites. If we can increase the amount of the drug, the amount of binding site occupancy will also increase, but it will never exceed the number of sites on the cell as the binding is said to be saturable.
Plotting these Results
If we plotted out these results, a Rectangular Hyperbola would be observed.
To find the maximum number of sites we would have to use a very wide range of ball numbers. This will cause problems with the plotting as many of the attempts with smaller numbers of balls will be scrunched up at the beginning of the graph.
Note that "number of balls" here refers to the number of balls per square meter (eg:balls actually on the board as they are rolled down).
It is therefore analogous to concentration in a solution.
A better way to plot the results would be to use a logarithmic scale, this spreads the scale out and produces a Sigmoid Curve (S-shaped curve).
However, in real life this will not be the case. There will clearly be an optimum size, and as you move away from the optimum the chances of sticking will decrease.
This again relates to the concept of complementarity but shows that it is more a graded phenomenon. How tight something binds is termed its affinity.
Affinity is not something you can quantify by looking at the maximum occupancy, as this is a feature of the board not the marbles.
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