3-DIMENSIONAL STRUCTURES OF AVIDIN AND THE AVIDIN-BIOTIN COMPLEX

The crystal structures of a deglycosylated form of the egg-white glyco protein avidin and of its complex with biotin have been determined to 2.6 and 3.0 angstrom, respectively. The structures reveal the amino ac id residues critical for stabilization of the tetrameric assembly and for the exceptionally tight binding of biotin. Each monomer is an eigh t-stranded antiparallel beta-barrel, remarkably similar to that of the genetically distinct bacterial analog streptavidin. As in streptavidi n, binding of biotin involves a highly stabilized network of polar and hydrophobic interactions. There are, however, some differences. The p resence of additional hydrophobic and hydrophilic groups in the bindin g site of avidin (which are missing in streptavidin) may account for i ts higher affinity constant. Two amino acid substitutions are proposed to be responsible for its susceptibility to denaturation relative to streptavidin. Unexpectedly, a residual N-acetylglucosamine moiety was detected in the deglycosylated avidin monomer by difference Fourier sy nthesis.