Question

1. Aconitase is an enzyme that displays moonlighting behavior dependent on cytosolic [iron). When the concentration of iron is low, aconitase undergoes a shift towards mRNA binding activities. Please speculate as to the structural basis for this [iron)-dependent activity switch. What is the role and function of the iron-sulfur cluster in aconitase? What is the physiologic relevance of aconistase's functional dependence on iron)?

please no hand write note. only type the anwer

Answer 1

Aconitase, displayed in the structures in the right margin of this page, has two slightly different structures, depending on whether it is activated or inactivated. in the inactive form, its structure is divided into four domains. Counting from the N-terminus, only the first three of these domains are involved in close interactions with the [3Fe-4S] cluster, but the active site consists of residues from all four domains, including the larger C-terminal domain. The Fe-S cluster and a SO42− anion also reside in the active site.When the enzyme is activated, it gains an additional iron atom, creating a [4Fe-4S] cluster.

Aconitase has an active [Fe4S4]2+ cluster, which may convert to an inactive [Fe3S4]+ form. Three cysteine (Cys) residues have been shown to be ligands of the [Fe4S4] centre. In the active state, the labile iron ion of the [Fe4S4] cluster is not coordinated by Cys but by water molecules.

The iron-responsive element-binding protein(IRE-BP) and 3-isopropylmalate dehydratase(α-isopropylmalate isomerase; an enzyme catalysing the second step in the biosynthesis of leucine, are known aconitase homologues. Iron regulatory elements (IREs) constitute a family of 28-nucleotide, non-coding, stem-loop structures that regulate iron storage, heme synthesis and iron uptake. They also participate in ribosome binding and control the mRNA turnover (degradation). The specific regulator protein, the IRE-BP, binds to IREs in both 5' and 3' regions, but only to RNA in the apo form, without the Fe-S cluster. Expression of IRE-BP in cultured cells has revealed that the protein functions either as an active aconitase, when cells are iron-replete, or as an active RNA-binding protein, when cells are iron-depleted. Mutant IRE-BPs, in which any or all of the three Cys residues involved in Fe-S formation are replaced by serine, have no aconitase activity, but retain RNA-binding properties.