An unusual phylogenetic variation in the metal ion binding sites of porphobilinogen synthase

Porphobilinogen synthase (PBGS), which catalyzes the first common step in tetrapyrrole biosynthesis contains a unique phylogenetic variation in the use of metal ions. Using sequence, structure, and enzymological information, this work codifies the phylogenetic segregation of metal utilization in PBGS from archaea, bacteria, and eucarya. All PBGS contain an active site metal binding sequence, determined herein to be either DXCXCX(Y/F)X(3)G(H/Q)CG or DXALDX(Y/F)X(3)G(H/Q)DG. The former dictates a requirement for zinc. Most PBGS that do not require zinc require magnesium and/or potassium instead. Most PBGS are also found to contain the binding determinants for an allosteric magnesium that resides outside the active site. The phylogenetic distribution of PBGS metal ion utilization suggests that the primordial PBGS required zinc and supports a hypothesis that the loss of the zinc site was concurrent with the advent of oxygenic photosynthesis.