Steve Ealick's Research Group
Human Phosphopantothenoylcysteine Decarboxylase
Phosphopantothenoylcysteine (PPC) decarboxylase is an essential enzyme in the biosynthesis of coenzyme A and catalyzes the decarboxylation of PPC to phosphopantetheine. The Laue class of the diffraction data appears to be 3bar m suggesting space group R32 with two monomers per asymmetric unit; however, the crystals belong to the space group R3 and the asymmetric unit contains four monomers. The structure was solved by molecular replacement and refined to a R-factor of 29%. The crystal packing can be considered as two interlaced lattices, each consistent with space group R32 and with the corresponding twofold axes parallel to each other but separated along the threefold axis. The true space group is R3 with four monomers per asymmetric unit.
|Each of the four monomers in the asymmetric unit is related to two of the others by a pseudo-twofold rotation about the face diagonal a+b followed by a translation along the c axis and is related to the third monomer by a pure translation along the c axis. The crystal packing can be described as two almost independent R32 crystal patterns that are interlaced.|
Three kinds of crystal packing interaction are observed in the refined structure. A total of 2500 Å2 of surface area is buried between the A trimer and the B trimer. Each monomer A and B contributes six residues with crystal contacts less than 3.6 Å. A total of 2135 Å2 of surface area is buried between the C trimer and The D trimer. Each monomer C and D contributes five residues with contacts less than 3.6 Å. The AB and CD trimer pairs form columns parallel to the c axis. The columns are linked by the C-terminal helix of each monomer packed against the equivalent helix in an adjacent monomer related by pseudo-twofold symmetry.
Manoj N and Ealick SE. Unusual space group pseudo-symmetry in crystals of human phosphopantothenoylcysteine decarboxylase. Acta Crystallogr. D59:1762-1766 (2003).