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J. Bacteriol., 06 1996, 3133-3139, Vol 178, No. 11
H Jiang, RE Parales, NA Lynch and DT Gibson
The terminal oxygenase component of toluene dioxygenase from Pseudomonas
putida F1 is an iron-sulfur protein (ISP(TOL)) that requires mononuclear
iron for enzyme activity. Alignment of all available predicted amino acid
sequences for the large (alpha) subunits of terminal oxygenases showed a
conserved cluster of potential mononuclear iron-binding residues. These
were between amino acids 210 and 230 in the alpha subunit (TodC1) of
ISP(TOL). The conserved amino acids, Glu-214, Asp-219, Tyr-221, His-222,
and His-228, were each independently replaced with an alanine residue by
site-directed mutagenesis. Tyr-266 in TodC1, which has been suggested as an
iron ligand, was treated in an identical manner. To assay toluene
dioxygenase activity in the presence of TodC1 and its mutant forms,
conditions for the reconstitution of wild-type ISP(TOL) activity from TodC1
and purified TodC2 (beta subunit) were developed and optimized. A mutation
at Glu-214, Asp-219, His-222, or His-228 completely abolished toluene
dioxygenase activity. TodC1 with an alanine substitution at either Tyr-221
or Tyr-266 retained partial enzyme activity (42 and 12%, respectively). In
experiments with [14C]toluene, the two Tyr-->Ala mutations caused a
reduction in the amount of Cis-[14C]-toluene dihydrodiol formed, whereas a
mutation at Glu-214, Asp-219, His-222, or His-228 eliminated cis-toluene
dihydrodiol formation. The expression level of all of the mutated TWO
proteins was equivalent to that of wild- type TodC1 as judged by sodium
dodecyl sulfate-polyacrylamide gel electrophoresis and Western blot
(immunoblot) analyses. These results, in conjunction with the predicted
amino acid sequences of 22 oxygenase components, suggest that the conserved
motif Glu-X3-4,-Asp-X2-His-X4-5- His is critical for catalytic function and
the glutamate, aspartate, and histidine residues may act as mononuclear
iron ligands at the site of oxygen activation.
Copyright © 1996, American Society for Microbiology
Site-directed mutagenesis of conserved amino acids in the alpha subunit of toluene dioxygenase: potential mononuclear non-heme iron coordination sites
Department of Microbiology, The University of Iowa, Iowa City, 52242, USA.
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