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. 2011 Mar 15;435(Pt 1):55–63. doi: 10.1042/BJ20101593

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© 2011 The Author(s) The author(s) has paid for this article to be freely available under the terms of the Creative Commons Attribution Non-Commercial Licence (http://creativecommons.org/licenses/by-nc/2.5/) which permits unrestricted non-commercial use, distribution and reproduction in any medium, provided the original work is properly cited.

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(A) The electron density for biotin and Ser⁴⁵ surrounds a stick model of the atoms involved, for Tr (left-hand panel) and two different streptavidin structures (1SWD in the middle panel and 1SWE in the right-hand panel). Hydrogen bonds are indicated by a broken line. (B) The rival hydrogen bonding by Ser⁴⁵. An overlay of the L3/4 region, with key residues highlighted in stick format. For Tr (left-hand panel) with and without biotin L3/4 is closed, with no hydrogen bond between Ser⁴⁵ and Gly⁵². For SA (right-hand panel) without biotin and with an undefined L3/4 (1SWC, chain B), Ser⁵² forms hydrogen bonds to Ser⁴⁵ and Ala⁴⁶ (indicated by broken lines). When biotin is bound and the loop is closed (1SWE, chain D), Ser⁴⁵ now forms a hydrogen bond to biotin.