The structure of the bifunctional, pyridoxal phosphate-dependent enzyme dialkylglycine decarboxylase was determined to 2.1-angstrom resolution. Model building suggests that a single cleavage site catalyzes both decarboxylation and transamination by maximizing stereoelectronic advantages and providing electrostatic and general base catalysis. The enzyme contains two binding sites for alkali metal ions. One is located near the active site and accounts for the dependence of activity on potassium ions. The other is located at the carboxyl terminus of an alpha helix. These sites help show how proteins can specifically bind alkali metals and how these ions can exert functional effects.