Abstact

BACKGROUND: Catalases are important antioxidant metalloenzymes that catalyze disproportionation of hydrogen peroxide, forming dioxygen and water. Two families of catalases are known, one having a heme cofactor, and the other, a structurally distinct family containing nonheme manganese. We have solved the structure of the mesophilic manganese catalase from Lactobacillus plantarum and its azide-inhibited complex. RESULTS: The crystal structure of the native enzyme has been solved at 1.8 A resolution by molecular replacement, and the azide complex of the native protein has been solved at 1.4 A resolution. The hexameric structure of the holoenzyme is stabilized by extensive intersubunit contacts, including a beta zipper and a structural calcium ion crosslinking neighboring subunits. Each subunit contains a dimanganese active site, accessed by a single substrate channel lined by charged residues. The manganese ions are linked by a mu1,3-bridging glutamate carboxylate and two mu-bridging solvent oxygens that electronically couple the metal centers. The active site region includes two residues (Arg147 and Glu178) that appear to be unique to the Lactobacillus plantarum catalase. CONCLUSIONS: A comparison of L. plantarum and T. thermophilus catalase structures reveals the existence of two distinct structural classes, differing in monomer design and the organization of their active sites, within the manganese catalase family. These differences have important implications for catalysis and may reflect distinct biological functions for the two enzymes, with the L. plantarum enzyme serving as a catalase, while the T. thermophilus enzyme may function as a catalase/peroxidase.