J. Biol. Chem., Vol. 263, Issue 33, 17418-17421, 11, 1988

Identification of free and [Fe2S2]-bound cysteine residues of adrenodoxin [published erratum appears in J Biol Chem 1989 May 5;264(13):7760]

JR Cupp and LE Vickery
Department of Physiology and Biophysics, University of California, Irvine 92717.

Bovine adrenodoxin was labeled with 5-iodoacetamidofluorescein to determine which of the five cysteines is free and which participate in iron coordination. Native protein was labeled at two stoichiometries, 0.15:1 and 1:1, both of which produced a single fluorescent product. Labeled tryptic peptides were isolated from both preparations and identified as residues 90-98 with 5-acetamidofluorescein cysteine at residue 95. From the preparation labeled at 0.15:1 stoichiometry, the fraction of tryptic peptide containing nonlabeled cysteines 92 and 95 was isolated and identified; this peptide was shown to be absent in the sample labeled at 1:1 stoichiometry. 5-Acetamidofluorescein-labeled adrenodoxin supported electron transport with adrenodoxin reductase and cytochromes P-450sec and P-45011 beta, demonstrating that labeling occurred without disruption of the iron-sulfur center. These results identify cysteine 95 as the most reactive and single free thiol in native adrenodoxin and imply the role of cysteine residues 46 [corrected], 52, 55, and 92 in iron-sulfur coordination.
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