Chemistry · the experiment
Unmask the fake cooperativity
A streptavidin is full when its four sites each carry a biotin. Fill half of the streptavidins the proper way, then see how a drop that is too small creates a cooperativity that does not exist.
1 · Four independent sites
Beginner
If the four sites are independent, how does the share of full streptavidins, carrying four biotins, rise as biotin is added?
Add dilute, well-mixed biotin until half of the streptavidins are full.
2 · Faster than the stir
Intermediate
A drop reacts inside a sphere of radius 1.5 R + 1 mm, where R is the radius of the drop. A 4 µL drop has a radius of about 1 mm. What is the radius of its reaction zone?
Still 1.2 biotins per protein, but more or less diluted: change the volume of the drop and find the one that gives the most full streptavidins.
3 · Biotin at 25 µM
Expert
Biotin at 25 µM, in drops of a few microlitres. According to the article's map, at which biotin-to-streptavidin ratios do small drops multiply the number of full proteins the most, compared with good mixing?
4 · The right dilution
Expert
You want 0.5 biotin per streptavidin, in 200 µL at 200 nM, with no artefact: the drop must be at least 15% of the volume. What is the highest biotin concentration the solution to pipette may have?
Illusion unmasked
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