Antimycin A (antimycin), one of the first known and most potent inhibitors of the mitochondrial respiratory chain, binds to the quinone reduction site of the cytochrome bc 1 complex. Structureactivity-relationship studies have shown that the N-formylamino-salicylamide group is responsible for most of the binding specificity, and suggested that a low pK a for the phenolic OH group and an intramolecular H-bond between that OH and the carbonyl O of the salicylamide linkage are important.Two previous X-ray structures of antimycin bound to vertebrate bc 1 complex gave conflicting results. A new structure reported here of the bovine mitochondrial bc 1 complex at 2.28 Å resolution with antimycin bound, allows us for the first time to reliably describe the binding of antimycin and shows that the intramolecular hydrogen bond described in solution and in the small-molecule structure is replaced by one involving the NH rather than carbonyl O of the amide linkage, with rotation of the amide group relative to the aromatic ring. The phenolic OH and formylamino N form H-bonds with conserved Asp228 of cyt b, and the formylamino O H-bonds via a water molecule to Lys227. A strong density the right size and shape for a diatomic molecule is found between the other side of the dilactone ring and the αA helix.

The toxin 3-nitropropionic acid 5 is produced by certain plants and fungi. It is a specific inhibitor of mitochondrial respiratory complex II. Fatalities after eating moldy sugarcane have been linked to 3-NP toxicity (1, 2). Ruminants grazing in regions with 3-NP-producing plants acquire resistance because of reduction of the nitro group to an amine by ruminal bacteria (3).The effectiveness of 3-NP in vivo after injection or oral administration has made it useful in studies involving tissues or whole animals. Ingestion of 3-NP results in neurodegeneration with symptoms resembling those of Huntington disease (4), and conversely Huntington disease results in a loss of complex II activity (5); thus 3-NP has been used to produce an animal model for the study of Huntington disease (6, 7). Symptoms also include convulsions, and 3-NP is being looked at for inducing a model of epilepsy (8). Prior subacute 3-NP poisoning seems to provide resistance to ischemic damage to nervous tissue by a preconditioning effect (9) similar to that resulting from mild ischemia.The target of 3-NP is Complex II, which is both a member of the Krebs tricarboxylic acid cycle (oxidizing succinate to fumarate) and an entry point for electrons into the respiratory chain at the level of ubiquinol. It consists of a large flavoprotein subunit containing covalently bound FAD, an iron-sulfur protein (IP) with three different iron-sulfur clusters, and two small membrane anchor subunits (chains C and D) ligating a single low spin heme of type B. Human genetic defects in the IP subunits or chains C or D lead to development of paragangliomas (10, 11). A mutation in chain C leads to premature aging in nematodes, presumably through excessive production of free radicals (12). Bacterial homologs succinate:quinone oxidoreductase (SQR) and menaquinol: fumarate oxidoreductase (FRD) in Escherichia coli have been studied as genetically manipulable models for the mitochondrial protein. Recent reviews cover this family of enzymes (13-18). X-ray crystallographic structures are available for a number of members of the family. The available mitochondrial structures and representative bacterial examples are listed in Table 1.The toxin 3-NP, structurally similar to and isoelectronic with the substrate succinate, is believed to be a suicide inactivator of Complex II. Alston et al. (19) proposed, based on previous observations and on their own experience with another flavoprotein, that the normal reaction pathway involves a temporary adduct with the N-5 nitrogen of flavin, which in the case of 3-NP collapses to a stable adduct resulting in permanent inactivation. Irreversible covalent modification of the flavin was ruled out by later work (20) examining the spectral change induced and showing that unmodified flavin peptide could be isolated from the inactivated complex by mild proteolysis. It was proposed that 3-NP is oxidized to 3-nitroacrylate, an unstable molecule that then reacts with some residue in the active site. A cysteine that was believed to be in the active ...

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