Streptavidin, from the bacteria Streptomyces avidinii, is a tetrameric protein that has a strong affinity for binding biotin (the 3D structure of is now displayed at the right-hand panel).
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You can read more details at the avidin and streptavidin page
This page focuses on the binding site for biotin and how two of the subunits collaborate in binding.
The shows a single subunit of streptavidin with its bound biotin.
What does the binding site look like? Let's display the (in pale blue).
As you see, biotin is embedded inside streptavidin, but a considerable –and hydrophobic– part of the biotin molecule remains exposed.
However, when we add the (residues 116 to 121) that contacts with the first subunit, you will see that it covers most of the biotin molecule.
Now only the carboxylate group of biotin (note the two red oxygens) sticks out.
