Figure 7
Fragment of the core streptavidin amino acid sequence. Panel A – the amino acid sequence of the fragment (see also Figure 6). The amino acid sequence loop (GTTEANAWK) links two antiparallel β-sheets (fragments underlined). Panel B – same amino acid sequence fragment with its secondary structure shown. The 9 amino acid loop could be modified and other protein fragments, peptides or tags could be inserted without destabilising the secondary structure of the core streptavidin sequence. Stabilisation of the secondary structure could be achieved by substituting the circled pairs of amino acids (dashed lines) with Cysteines. The seven pairs of amino acids are especially suitable due to their proximity to the loop and molecular architecture. The distances between corresponding Cβ atoms in amino acid pairs (indicated on the panel B in Angstroms) are sufficient to accommodate two sulfhydryl groups and the resulting disulphide bond without major disturbances of the SAPV folding. The (Trp + Gly) pair is less suitable for (Cys + Cys) substitution due to Trp involvement in biotin binding.