<p> <taxon tax_id="562">Escherichia coli</taxon> heat-labile enterotoxin is a bacterial protein toxin with an AB5 multimer structure, in which the B pentamer (<db_xref db="INTERPRO" dbkey="IPR001835"/>) has a membrane-binding function and the A chain is needed for enzymatic activity [<cite idref="PUB00003304"/>]. The B subunits are arranged as a donut-shaped pentamer, each subunit participating in ~30 hydrogen bonds and 6 salt bridges with its two neighbours [<cite idref="PUB00003304"/>].</p><p>The A subunit has a less well-defined secondary structure. It predominantly interacts with the pentamer via the C-terminal A2 fragment, which runs through the charged central pore of the B subunits. A putative catalytic residue in the A1 fragment (Glu112) lies close to a hydrophobic region, which packs two loops together. It is thought that this region might be important for catalysis and membrane translocation [<cite idref="PUB00003304"/>].</p>