Streptavidin

The discovery of streptavidin was totally unexpected and happened during a screening of Streptomyces for antibiotics. The antibacterial effects could be reversed by high concentrations of biotin in the medium. A closer look revealed that the high molecular weight component was a biotin binding protein that had remarkably similar physical and chemical characteristics as avidin [57], including a 33% identity in the amino acid sequence [64].

Figure 1.3: Three-dimensional structure of streptavidin, acquired with x-ray diffraction methods [106]

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In 1964, CHAIET and WOLF [21] published the first article about ``The properties of Streptavidin, a Biotin-Binding Protein Produced by Streptomycetes''. This was the first time a biotin-binding protein was isolated from a microbial source and not from egg white.

Streptavidin has its name from the bacterial source of the protein, Streptomyces avidinii, and from egg-white avidin. It is a tetrameric protein that consists of 254 amino acids. It has a size of $46\times 93\times 104$Å$^3$ [106] and a molecular weight of about
60kDa [9]. Figure 1.3 presents the three-dimensional structure of streptavidin that was acquired with x-ray diffraction methods.

The exceptionally high binding affinity to biotin is in the same range as the binding affinity of avidin-biotin ( $K_a \approx 10^{-15}\,M$) [64]. This is the highest known affinity without the formation of covalent bonds [57]. In order to understand these special properties and be able to use them for biotechnologic application, many groups investigated the streptavidin-biotin system.

The following section reviews the current state of the research on ligand-receptor bonds.