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Protocatechuate 3,4-dioxygenase (EC 1.13.11.3) from Pseudomonas aeruginosa has been investigated by EPR and Mössbauer spectroscopy. Low temperature Mössbauer data on the native enzyme (Fe3+, S = 5/2) yields a hyperfine field Hsat=-525 kG at the nucleus. This observation is inconsistent with earlier suggestions, based on EPR data of a rubredoxin-like ligand environment around the iron, i.e. a tetrahedral sulfur coordination. Likewise, the dithionite-reduced enzyme has Mössbauer parameters unlike those of reduced rubredoxin. We conclude that the iron atoms are in a previously unrecognized environment. The ternary complex of the enzyme with 3,4-dihydroxyphenylpropionate and O2 yields EPR signals at g = 6.7 and g = 5.3; these signals result from an excited state Kramers doublet. The kinetics of the disappearance of these signals parallels product formation and the decay of the ternary complex as observed in the optical spectrum. The Mössbauer and EPR data on the ternary complex establish the iron atoms to be a high-spin ferric state characterized by a large and negative zero-field splitting, D = approximately -2 cm-1.
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E Münck, and
N R Orme-Johnson, and
W H Orme-Johnson
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