Crystallization and Subunit Structure of Histidine Decarboxylase

Histidine decarboxylase from Lactobacillus 30u has been crystallized in a variety of forms which together indicate a revised subunit structure for the native particle. Octahedral crystals of the wild type enzyme obtained at room temperature from ammonium sulfate solutions in microdiffusion cells belong to tetragonal space group 14122 with a = b = 222 A and c = 107.5 A. Trigonal and hexagonal plates of prohistidine decarboxylase and activated proenzyme obtained at 4°C from polyethyleneglycol solutions by vapor equilibration using the hanging drop technique belong to the trigonal space group P321 with a = b = 100 A and c = 164 A. The space group symmetries and unit cell contents of these crystals indicate 32 point group symmetry for the subunit structure of these enzymes. Sedimentation coefficients of wild type enzyme measured as a function of ionic strength at pH 7.0 indicate a rapid equilibrium between species varying from 6.9 S to 9.4 S. Sedimentation equilibrium analysis demonstrated the existence of a nearly homogeneous particle with M, = 208,000 at ionic strengths above I = 0.20, while an additional species of approximately one-half that molecular weight is observed at very low ionic strengths (I = 0.02). At the pH optimum of the enzyme (pH 4.8), the larger species is dominant at all ionic strengths tested. Electron micrographs of native wild type enzyme show a dominant tetrahedral particle approximately 60 A on an edge while similar micrographs of enzyme cross-linked with glutaraldehyde show a dumbbell-shaped particle approximately 60 A in width and 120 A in length. These results establish that: (a) the native enzyme has a M, = 208,000 and a subunit composition (afi)6; (b) the proenzyme has a subunit composition (77)s; and (c) stable (afi)s and ( T ) ~ particles exist under certain conditions.