It's Strong, It's Stable, It's Streptavidin

Abstract

Many proteins bind to substrates, so what makes streptavidin binding biotin so amazing? Perhaps, the fact that its dissociation constant (Kd = 10‐15 M) is the lowest in nature. The bacterium Streptomyces avidinii has taken advantage of this incredibly low Kd to develop a defense mechanism with which it can withhold valuable biotin (vitamin B7) from competing bacteria. This high affinity binding is facilitated by the structure of tetrameric streptavidin, consisting of a dimer of dimers each formed by the interaction between monomers that involves a single amino acid, Trp120. Each subunit consists of an 8 antiparallel beta strand barrel forming the biotin binding site which is capped by a flexible loop between strands 3 and 4 which essentially acts as a lid over the binding site on the end of the barrel. Tight biotin binding results from multiple hydrogen bonding interactions, and a hydrophobic Trp‐lined binding site that allows for extensive van der Waal interactions. The strong streptavidin‐biotin interaction has been exploited in biotechnology for processes such as protein identification, purification, isolation, and enrichment; as well as, nucleic acid detection involving binding of biotinylated targets by immobilized proteins. The Ironwood Ridge High School SMART Team (Students Modeling a Research Topic) has modeled streptavidin bound with biotin and bound with a Strep‐tag using JMOL and 3‐D fabrication to speculate on the mechanism, and to highlight the residues, which facilitate streptavidin's record breaking dissociation constant.