Cytosolic Aconitase

Abstract

Cytosolic aconitases (cAcns) have the remarkable property that they can function either as an enzyme or as a messenger (m) RNA‐binding protein. In the first case they contain a [4Fe–4S] cluster that is directly involved in the catalysis of the isomerization between citrate and isocitrate via a cis ‐aconitate intermediate. Upon loss of the cluster the enzymatic function is lost, the protein undergoes a substantial structural rearrangement and becomes capable of recognizing, with very high affinity, the so‐called iron‐responsive elements (IREs) in mRNAs. The latter code for proteins that are either dependent on Fe or are involved in the regulation of iron levels in the cell. The nonenzymatic form of cAcn is known as iron‐regulatory protein 1 or IRP1. Here we review the most recent structural data on the two forms, cAcn and IRP1, and correlate these with their respective functional properties. In addition, a comparison is made with closely related proteins like IRP2 and mitochondrial and bacterial aconitases.

Metalloproteins in the Biology of Heterocysts

“…This non-liganded Fe atom plays an essential role in the citrate–isocitrate isomerization reaction binding citrate and promoting the removal of a proton and a hydroxyl group, a step that generates the cis-aconitate intermediate, followed by rehydration of cis-aconitate to form isocitrate [238,459]. This labile Fe atom is easily oxidized and released as Fe 2+ in oxidizing conditions and in the presence of ROS, generating an enzymatically inactive [3Fe–4S] cluster [348,460]. Importantly, Fe 2+ ions released in this process can exacerbate the oxidative stress and ROS generation via Fenton chemistry (see Section 5).…”