Adviser: Brian W. Matthews. xii, 211 p. ; ill. (some col.) A print copy of this title is available through the UO Libraries under the call number: SCIENCE QP609.G3 J84 2000 / An atomic-level description of the presumed catalytic action of β-galactosidase is described. This large enzyme, from E. coli , carries out two reactions which allow the bacterium to live on the disaccharide lactose. First, it breaks down lactose to the two monosaccharides galactose and glucose. Second, it converts lactose into another disaccharide, allolactose, which is the inducer for the lac operon, and thus is the signal to the bacterium to produce more β-galactosidase. The work is based on high resolution x-ray crystallography and enzyme kinetics. A crystal form of β-galactosidase was isolated that permits data collection up to 1.5 à resolution. Using this crystal form, the structures of several ligands bound to the enzyme were determined. These ligands were chosen to mimic various points in the reaction: binding of substrate, covalent intermediates, transition states, and products. Together these complexes suggest a reaction coordinate for β-galactosidase which clarifies and enhances previous ideas about the reaction mechanism. The reaction includes a conformational change triggered by the progression of the substrate towards the transition state. Additional investigation suggests that this conformational change is involved in determining whether the enzyme carries out its hydrolysis or isomerization reaction. Considerations of the structure in the context of other related enzymes suggest an evolutionary path for β-galactosidase. It is suggested that a progenitor enzyme which catalyzed the hydrolysis of long polysaccharide substrates recruited additional domains which permit β-galactosidase to act on smaller substrates and produce the inducer, allolactose. This dissertation includes both my previously published and co-authored material.