Pseudomonas aeruginosa Contains a Novel Type V Porphobilinogen Synthase with No Required Catalytic Metal Ions†

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Abstract
Porphobilinogen synthases (PBGS) are metalloenzymes that catalyze the first common step in tetrapyrrole biosynthesis. The PBGS enzymes have previously been categorized into four types (I−IV) by the number of Zn2+ and/or Mg2+ utilized at three different metal binding sites termed A, B, and C. In this study Pseudomonas aeruginosa PBGS is found to bind only four Mg2+ per octamer as determined by atomic absorption spectroscopy, in the presence or absence of substrate/product. This is the lowest number of bound metal ions yet found for PBGS where other enzymes bind 8−16 divalent ions. These four Mg2+ allosterically stimulate a metal ion independent catalytic activity, in a fashion dependent upon both pH and K+. The allosteric Mg2+ of PBGS is located in metal binding site C, which is outside the active site. No evidence is found for metal binding to the potential high-affinity active site metal binding sites A and/or B. P. aeruginosa PBGS was investigated using Mn2+ as an EPR probe for Mg2+, and the active site was investigated using [3,5-13C]porphobilinogen as an NMR probe. The magnetic resonance data exclude the direct involvement of Mg2+ in substrate binding and product formation. The combined data suggest that P. aeruginosa PBGS represents a new type V enzyme. Type V PBGS has the remarkable ability to synthesize porphobilinogen in a metal ion independent fashion. The total metal ion stoichiometry of only 4 per octamer suggests half-sites reactivity.

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