R. T. Guo, T. P. Ko, A. P. Chen, et al. Crystal structures of undecaprenyl pyrophosphate synthase in complex with magnesium, lsopentenyl pyrophosphate, and farnesyl thiopyrophosphate: Roles of the metal ion and conserved residues in catalysis. The Journal of Biological Chemistry, 2005, 280(21): 20762-20774.