Refined Structure of the Pyruvoyl-dependent Histidine Decarboxylase from Lactobacillus 30a

The crystal structure of the pyruvoyl-dependent histidine decarboxylase from Lactobacillus 30a has been refined to an R-value of 0.15 (for the 5.0 to 2.5 A resolution shell) and 0.17 (for the 10.0 to 2.5 A resolution shell). A description of the overall structure is presented, focusing on secondary structure and subunit association. The enzyme is a hexamer of alpha beta subunits. Separate alpha and beta-chains arise from an autocatalytic cleavage reaction between two serine residues, which results in the pyruvoyl cofactor. The central core of the alpha beta subunit is a beta-sandwich which consists of two face-to-face three-stranded antiparallel beta-sheets, flanked by alpha-helices on each side. The beta-sandwich creates a stable fold that allows conformational strain to be introduced across an internal cleavage region between the alpha and beta chains and places the pyruvoyl cofactor in a position for efficient electron withdrawal from the substrate. Three alpha beta subunits are related by a molecular three-fold symmetry axis to form a trimer whose interfaces have complementary surfaces and extensive molecular interactions. Each of the interfaces contains an active site and a solvent channel that leads from the active site to the exterior of the molecule. The trimers are related by a crystallographic two-fold symmetry axis to form the hexamer with an overall dumbbell shape. The interface between trimers has few molecular interactions.