THE STRUCTURE OF THE COMPLEX BETWEEN AVIDIN AND THE DYE, 2-(4'-HYDROXYAZOBENZENE) BENZOIC-ACID (HABA)

The crystal structure of the complex formed between the egg-white biot in-binding protein, avidin, and the dye, 2-(4'-hydroxyazobenzene) benz oic acid (HABA), was determined to a resolution of 2.5 angstrom. The i nteraction of avidin with the benzoate ring of HABA is essentially ide ntical to that of the complex formed between HABA and streptavidin (th e bacterial analogue of the egg-white protein). This interaction emula tes the definitive high-affinity interaction of both proteins with the ureido moiety of biotin. The major difference between the avidin- and streptavidin-HABA complexes lies in their interaction with the hydrox ypheny) ring of the dye molecule; in avidin, two adjacent amino acid r esidues (Phe72 and Ser73), which are not present in streptavidin, form additional interactions with this ring. These are suggested to accoun t for the higher affinity of avidin for HABA. The characteristic red s hift, which accompanies the interaction of both proteins with the dye, was traced to a proposed charge-transfer complex formed between the h ydroxyphenyl ring of HABA and the indole ring of Trp79 in avidin (Trp7 9 in streptavidin). Comparison of binding site residues of two such si milar proteins versus their markedly different affinities for two such different substrates should eventually contribute to a better design of biomimetic reagents and drugs.