Aconitases: non-redox iron-sulfur proteins sensitive to reactive species

Abstract

Conspectus Mammalian aconitases (mitochondrial and cytosolic isoenzymes) are unique iron–sulfur cluster-containing proteins in which the metallic center participates in the catalysis of a non-redox reaction. Within the cubane iron–sulfur cluster of aconitases only three of the four iron ions have cysteine thiolate ligands; the fourth iron ion (Fe α ) is solvent exposed within the active-site pocket and bound to oxygen atoms from either water or substrates to be dehydrated. The catalyzed reaction is the reversible isomerization of citrate to isocitrate with an intermediate metabolite, cis -aconitate. The cytosolic isoform of aconitase is a moonlighting enzyme; when intracellular iron is scarce, the complete disassembly of the iron–sulfur cluster occurs and apo-aconitase acquires the function of an iron responsive protein and regulates the translation of proteins involved in iron metabolism. In the late 1980s and during the 1990s, cumulative experimental evidence pointed out that aconitases are main targets of reactive oxygen and nitrogen species such as superoxide radical (O 2 •– ), hydrogen peroxide (H 2 O 2 ), nitric oxide ( • NO), and peroxynitrite (ONOO – ). These intermediates are capable of oxidizing the cluster, which leads to iron release and consequent loss of the catalytic activity of aconitase. As the reaction of the Fe–S cluster with O 2 •– is fast (∼10 7 M –1 s –1 ), quite specific, and reversible in vivo, quantification of active aconitase has been used to evaluate O 2 •– formation in cells. While • NO is modestly reactive with aconitase, its reaction with O 2 •– yields ONOO –, a strong oxidant that readily leads to the disruption of the Fe–S cluster. In the case of cytosolic aconitase, it has been seen that H 2 O 2 and • NO promote activation of iron responsive protein activity in cells. Proteomic advances in the 2000s confirmed that aconitases are main targets of reactive species in cellular models and in vivo, and other post-translational oxidative modifications such as protein nitration and carbonylation have been detected. Herein, we (1) outline the particular structural features of aconitase that make these proteins specific targets of reactive species, (2) characterize the reactions of O 2 •–, H 2 O 2, • NO, and ONOO – and related species with aconitases, (3) discuss how different oxidative post-translational modifications of aconitase impact the different functions of aconitases, and (4) argue how these proteins might function as redox sensors within different cellular compartments, regulating citrate concentration and efflux from mitochondria, iron availability in the cytosol, and cellular oxidant production.