Avidin

The discovery of avidin is closely related to the discovery of biotin, because the egg white that was fed to the rats only contains biotin that is bound to avidin and the metabolism of rats is not able to separate those. Avidin accounts for maximal 0.05% of the protein that is found in eggs and oviducts of many species of birds [57].

Figure 1.2: Three-dimensional structure of avidin, acquired with x-ray diffraction methods [106]

0.5

Avidin is very soluble in water and salt solutions between pH 5-7 and has its isoelectric point at pH 10 [57]. The basic tetrameric glycoprotein consists of 256 amino acids with an overall dimension of
$72 \times 80 \times44$Å$^3$ [106] and has a molecular weight of 57kDa [98]. A comprehensive overview about avidin
was already published in 1963 by
GREEN and MELAMED [56]. Figure 1.2 shows the three-dimensional structure of avidin, which was acquired by x-ray diffraction methods.

Although the high affinity to biotin was known from early on, the interest in avidin was low until the discovery of the coenzyme function of covalently bound biotin. The very high affinity between biotin and
avidin together with the possibility to bind coenzymes covalently to the biotin made clear that avidin could be very useful to characterise new classes of enzymes [57]. Furthermore, the bond between biotin and avidin can resist dissociation in the presence of detergents, high and low pH values, protein denaturants and high temperatures. Today, many different macromolecules (e.g. proteins, polysaccharides or nucleic acids) can easily be linked to biotin without serious affect on their biochemical or physical properties. The generic nature of the biotinylation process and the high strength of the avidin-biotin bond makes this technology easily accessible and, therefore, interesting for scientific and industrial research [100].

Section 1.1.4 presents a more in-depth study of ligand-receptor bonds like the avidin-biotin bond.