Stearoyl-acyl carrier protein Δ⁹ desaturase from Ricinus communis is a diiron-oxo protein

A gene encoding stearoyl-acyl carrier protein Δ⁹ desaturase (EC 1.14.99.6) from castor was expressed in Escherichia coli. The purified catalytically active enzyme contained four atoms of iron per homodimer. The desaturase was studied in two oxidation states with Mössbauer spectroscopy in applied fields up to 6.0 T. These studies show conclusively that the oxidized enzyme contains two (identical) clusters consisting of a pair of antiferromagnetically coupled (J > 60 cm^-1, H = JS₁·S₂) Fe³⁺ sites. The diferric cluster exhibited absorption bands from 300 to 355 nm; addition of azide elicited a charge transfer band at 450 nm. In the presence of dithionite, the clusters were reduced to the diferrous state. Addition of stearoyl-CoA and O₂ returned the clusters to the diferric state. These properties are consistent with assigning the desaturase to the class of O₂-activating proteins containing diiron-oxo clusters, most notably ribonucleotide reductase and methane monooxygenase hydroxylase. Comparison of the primary structures for these three catalytically diverse proteins revealed a conserved pair of the amino acid sequence -(Asp/Glu)-Glu-Xaa-Arg-His- separated by ≈100 amino acids. Since each of these proteins can catalyze O₂-dependent cleavage of unactivated C-H bonds, we propose that these amino acid sequences represent a biological motif used for the creation of reactive catalytic intermediates. Thus, eukaryotic fatty acid desaturation may proceed via enzymatic generation of a high-valent iron-oxo species derived from the diiron cluster.