Superoxide Inhibits 4Fe-4S Cluster Enzymes Involved in Amino Acid Biosynthesis

• Published in: The Journal of biological chemistry 2004-07, Vol.279 (31), p.32055-32062
• Main Authors: Wallace, Matthew Alan, Liou, Lee-Loung, Martins, Jacob, Clement, Matthew H.S., Bailey, Sasaneh, Longo, Valter D., Valentine, Joan Selverstone, Gralla, Edith Butler
• Format: Article
• Language: English
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M403590200

Among the phenotypes of Saccharomyces cerevisiae mutants lacking CuZn-superoxide dismutase (Sod1p) is an aerobic lysine auxotrophy; in the current work we show an additional leaky auxotrophy for leucine. The lysine and leucine biosynthetic pathways each contain a 4Fe-4S cluster enzyme homologous to aconitase and likely to be superoxide-sensitive, homoaconitase (Lys4p) and isopropylmalate dehydratase (Leu1p), respectively. We present evidence that direct aerobic inactivation of these enzymes in sod1 Δ yeast results in the auxotrophies. Located in the cytosol and intermembrane space of the mitochondria, Sod1p likely provides direct protection of the cytosolic enzyme Leu1p. Surprisingly, Lys4p does not share a compartment with Sod1p but is located in the mitochondrial matrix. The activity of a second matrix protein, the tricarboxylic acid cycle enzyme aconitase, was similarly lowered in sod1 Δ mutants. We measured only slight changes in total mitochondrial iron and found no detectable difference in mitochondrial “free” (EPR-detectable) iron making it unlikely that a gross defect in mitochondrial iron metabolism is the cause of the decreased enzyme activities. Thus, we conclude that when Sod1p is absent a lysine auxotrophy is induced because Lys4p is inactivated in the matrix by superoxide that originates in the intermembrane space and diffuses across the inner membrane.