The use of bifunctional biotinyl compounds to determine the arrangement of subunits in avidin.

A series of bisbiotinyl diamines was synthesized with between 9 and 25 bonds between the carboxyl groups of the two biotin residues. It was found that only one of the two biotin residues could combine with avidin when there were fewer than 12 bonds between the biotin residues. Compounds with longer chains behaved in a bifunctional manner and gave rise to linear polymers of avidin, which were characterized by electron microscopy and by gel filtration. The polymers formed with the shorter-chain reagents (12, 13 or 14 bonds) were relatively unstable and could be depolymerized by weakly bound analogues of biotin. The polymers of longer-chain reagents were not depolymerized under these conditions and were only slowly affected by added biotin. When the chain length of the reagent reached 23 bonds the polymers became much shorter, suggesting that the reagent was now able to link two subunits within the same avidin molecule. From the morphology of the polymers it could be concluded that the four subunits of the avidin molecules were arranged with 222 symmetry and that they were grouped in two pairs at opposite ends of the short axis of the molecule whose dimensions were 55Ax55Ax41A.