A structural view of the action of Escherichia coli (lacZ) beta-galactosidase

The structures of a series of complexes designed to mimic intermediates alo ng the reaction coordinate for beta -galactosidase are presented. These com plexes clarify and enhance previous proposals regarding the catalytic mecha nism. The nucleophile, Glu537, is seen to covalently bind to the galactosyl moiety. Of the two potential acids, Mg2+ and Glu461, the latter is in bett er position to directly assist in leaving group departure, suggesting that the metal ion acts in a secondary role. A sodium ion plays, a part in subst rate binding by directly ligating the galactosyl 6-hydroxyl. The proposed r eaction coordinate involves the movement of the galactosyl moiety deep into the active site pocket. For those ligands that do bind dee ly there is an associated conformational change in which residues within loop 794-804 move up to 10 Angstrom closer to the site of binding. In some cases this can be inhibited by the binding of additional Ligands. The resulting restricted a ccess to the intermediate helps to explain why allolactose, the natural ind ucer for the lac operon, is the preferred product of transglycosylation.