Catalyzes the isomerization of citrate to isocitrate via cis-aconitate, a step in the citric acid cycle. Can also provide minor contributions to the reversible dehydration of (R)-homocitrate to cis-homoaconitate, a step in the alpha-aminoadipate pathway for lysine biosynthesis. Also plays an essential role in mtDNA maintenance. May directly protect mtDNA from accumulation of point mutations and ssDNA breaks as a component of mitochondrial nucleoids, or by preventing accumulation of iron citrate thereby alleviating its detrimental effects in mitochondria. citrate = D-threo-isocitrate Binds 1 [4Fe-4S] cluster per subunit. Subject to catabolite regulation. Carbohydrate metabolism; tricarboxylic acid cycle; isocitrate from oxaloacetate: step 2/2. Monomer. Binds to mitochondrial DNA (mtDNA) and identified as component of mitochondrial nucleoids. Mainly mitochondrial, small amounts are also detected in the cytosol with a ratio of 94:6. Highly induced in the absence of glutamate. Induction is further increased when both glutamate and lysine are missing. Essential for growth on nonfermentable carbon sources and for biosynthesis of glutamate. Causes a dramatic increase in cellular citrate levels. The fermenting yeast S.cerevisiae has 2 aconitases, ACO1 essential for the citric acid cycle, and ACO2 specifically and exclusively contributing to lysine biosynthesis. In contrast, in respiring filamentous fungi the ACO2 homologs (acoB) seem enzymatically inactive and the ACO1 homolog (acoA) is solely responsible for these functions. Present with 96700 molecules/cell in log phase SD medium. Belongs to the aconitase/IPM isomerase family.