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Journal of Bacteriology, December 1998, p. 6689-6696, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Selectivity of Ferric Enterobactin Binding and
Cooperativity of Transport in Gram-Negative Bacteria
Padmamalini
Thulasiraman,1
Salete M. C.
Newton,1
Jide
Xu,2
Kenneth N.
Raymond,2
Christine
Mai,1
Angela
Hall,1
Marjorie A.
Montague,1 and
Phillip
E.
Klebba1,*
Department of Chemistry and Biochemistry,
University of Oklahoma, Norman, Oklahoma 73019,1
and
Department of Chemistry, University of California,
Berkeley, Berkeley, California 947202
Received 10 August 1998/Accepted 8 October 1998
The ligand-gated outer membrane porin FepA serves Escherichia
coli as the receptor for the siderophore ferric enterobactin. We
characterized the ability of seven analogs of enterobactin to supply
iron via FepA by quantitatively measuring the binding and transport of
their 59Fe complexes. The experiments refuted the idea that
chirality of the iron complex affects its recognition by FepA and
demonstrated the necessity of an unsubstituted catecholate coordination
center for binding to the outer membrane protein. Among the compounds we tested, only ferric enantioenterobactin, the synthetic, left-handed isomer of natural enterobactin, and ferric TRENCAM, which substitutes a
tertiary amine for the macrocyclic lactone ring of ferric enterobactin but maintains an unsubstituted catecholate iron complex, were recognized by FepA (Kd
20 nM). Ferric
complexes of other analogs (TRENCAM-3,2-HOPO; TREN-Me-3,2-HOPO;
MeMEEtTAM; MeME-Me-3,2-HOPO; K3MECAMS; agrobactin A) with
alterations to the chelating groups and different net charge on the
iron center neither adsorbed to nor transported through FepA. We also
compared the binding and uptake of ferric enterobactin by homologs of
FepA from Bordetella bronchisepticus, Pseudomonas
aeruginosa, and Salmonella typhimurium in the native
organisms and as plasmid-mediated clones expressed in E. coli. All the transport proteins bound ferric enterobactin with
high affinity (Kd
100 nM) and transported it
at comparable rates (
50 pmol/min/109 cells) in their own
particular membrane environments. However, the FepA and IroN proteins
of S. typhimurium failed to efficiently function in
E. coli. For E. coli, S. typhimurium, and P. aeruginosa, the rate of ferric
enterobactin uptake was a sigmoidal function of its concentration,
indicating a cooperative transport reaction involving multiple
interacting binding sites on FepA.
*
Corresponding author. Mailing address: Department of
Chemistry and Biochemistry, The University of Oklahoma, 620 Parrington Oval, Room 208, Norman, OK 73019. Phone: (405) 325-4969. Fax: (405)
325-6111. E-mail: peklebba{at}ou.edu.
Journal of Bacteriology, December 1998, p. 6689-6696, Vol. 180, No. 24
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
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