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Petrovich RM , Litwin S , Jaffe EK
Bradyrhizobium japonicum porphobilinogen synthase uses two Mg(II) and monovalent cations
Journal of Biological Chemistry. 1996 Apr 12;271(15) :8692-8699
PMID: ISI:A1996UE73000032   
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Abstract
Bradyrhizobium japonicum porphobilinogen synthase (B. japonicum PBGS) has been purified and characterized from an overexpression system in an Escherichia coli host (Chauhan, S., and O'Brian, M. R. (1995) J. Biol. Chem. 270, 19823-19827). B. japonicum PBGS defines a new class of PBGS protein, type IV (classified by metal ion content), which utilizes a catalytic Mg-A present at a stoichiometry of 4/octamer, an allosteric Mg- C present at a stoichiometry of 8/octamer, and a monovalent metal ion, K+. However, the divalent Mg-B or Zn-B present in some other PBGS is not present in B. japonicum PBGS. Under optimal conditions, the K-d for Mg-A is < 0.2 mu M, and the K-d for Mg-C is about 40 mu M. The response of B. japonicum PBGS activity to monovalent and divalent cations is mutually dependent and varies dramatically with pH. B. japonicum PBGS is also found to undergo a dynamic equilibrium between active multimeric species and inactive monomers under assay conditions, a kinetic characteristic not reported for other PBGSs. B. japonicum PBGS is the first PBGS that has been rigorously demonstrated to lack a catalytic Zn-A. However, consistent with prior predictions, B. japonicum PBGS can bind Zn(II) (presumably as Zn-A) at a stoichiometry of 4/octamer with a K-d of 200 mu M; but this high concentration is outside a physiologically significant range.
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Times Cited: 18 English Article UE730 J BIOL CHEM