Joyce E. Morningstar

Electron paramagnetic resonance and magnetic circular dichroism studies of electron-transfer flavoprotein-ubiquinone oxidoreductase from pig liver.

Pig liver electron‐transfer flavoprotein‐ubiquinone oxidoreductase has been investigated by room temperature UV‐visible, low‐temperature electron paramagnetic resonance and low‐temperature magnetic circular dichroism spectroscopies. The results provide unambiguous evidence for the presence of a single [4Fe‐4S] cluster that is diamagnetic in the isolated enzyme and becomes paramagnetic with an S = , ground state on reduction with dithionite or enzymatically with the physiological electron donor. The EPR data for samples at pH 7.8 indicate that FAD is reduced by one electron to the anionic semiquinone form in the enzymatically reduced enzyme, and by two electrons to the hydroquinone form by excess dithionite. The possibility of weak spin‐spin interaction between the FAD semiquinone and the [4Fe‐4S]1+ center is discussed in the light of the observation of a small increase in the linewidth of the Fe‐S EPR in enzymatically reduced samples.

The Iron-Sulfur Clusters in Succinate Dehydrogenase

Succinate-ubiquinone oxidoreductase (Complex II) is a membrane-bound enzyme that couples the oxidation of succinate to fumarate to the reduction of ubiquinone in the mitochondrial respiratory chain. It is composed of four subunits (1); two hydrophilic subunits, a flavoprotein containing covalently bound FAD (Mr = 70,000) and a smaller iron-sulfur protein (Mr. = 27,000), that together constitute the enzyme succinate dehydrogenase (2), and two small hydrophobic subunits which are associated with a b-type cytochrome (3). The two small peptides are required for anchoring the enzyme to the membrane and for the reduction of ubiquinone, but are not required for the catalytic oxidation of succinate in the presence of artificial electron acceptors (4,5).