The X-ray three-dimensional structure of avidin

Avidin is a basic, highly stable, homotetrameric protein, isolated from bir d egg-white, binding up to four molecules of D-biotin with extremely high a ffinity (K-d similar to 10(-15) M). The protein has been the object of diff erent crystallographic investigations. In all the crystal structures, the f our avidin subunits display almost exact 222 symmetry. Each avidin chain (1 28 amino acids) is arranged in a eight-stranded antiparallel beta-barrel, w hose inner region defines the D-biotin binding site. The molecular bases of D-biotin affinity can be recognised in a fairly rigid binding site, which is sterically complementary to the shape and polarity of the incoming vitam in, and is readily accessible in the apoprotein structure. Avidin displays remarkable structural and functional relationships to the acidic protein sr etpavidin, isolated from Streptomyces avidinii. (C) 1999 Elsevier Science B .V. All rights reserved.