Question

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]?

Answer 1

Answer:

Aconitase is an iron-sulfur protein that is involved in the first step of the tricarboxylic acid (TCA) cycle, and oxidative inactivation of its iron-sulfur cluster by superoxide leads to iron toxicity and peroxide production.

Two aconitase isozymes are present in mammalian cells:

1. m-aconitase: the mitochondrial enzyme that functions in the TCA cycle and also essential for mtDNA maintenance.

2. c-aconitase/IRP1: the cytosolic enzyme

- plays a role in the regulation of iron metabolism.

In mammalian cell , IRP1(Iron regulatory protein 1) switches between aconitase and RNA-binding functions, whereas IRP2( Iron regulatory protein 2) has no aconitase activity and functions solely as an RNA-binding protein. Intracellular iron homeostasis is largely achieved by the iron-dependent regulation of transferrin receptor(TfR) and and iron storage protein ferritin. The mRNAs of TfR and ferritin contain IREs (Iron responsive elements) in their untranslated regions. In iron-depleted cells in cytosol, binding of the IRP1 and IRP2 to the IREs leads to stabilization of the TfR mRNA and translationali inhibitionof ferritin mRNAs, resulting in increased TfR-dependent iron uptake and decreased iron sequestration into ferritin.

The role and function of the iron-sulfur cluster in aconitase

• In the absence of the Fe–S cluster, IRP1 loses aconitase activity and binds to IREs with high affinity. When iron is abundant,the Fe–S cluster is efficiently repaired/regenerated, and IRP1 functions mainly as an aconitase.
• In the cytosol, Fe–S cluster biogenesis is important for the switch between cytosolic aconitase and IRE-binding activities of IRP1.
• In the mitochondria, Fe–S cluster biogenesisis needed not only to support respiratory functions and heme biosynthesis, but also for sensing and regulation of mitochondrial iron homeostasis.

What is the physiologic relevance of aconistase’s functional dependence on iron ?

• Aconitase is an essential enzyme in the citric acid cycle and iron regulatory protein 1 interacts with messenger RNA to control the levels of iron inside cells.
• Iron deficiency can decrease aconitase protein levels and limit the assembly of the Fe–S clusters required for their enzymatic activities. As a result, iron deficiency can potentially affect the citric acid cycle, lipid biosynthesis,carbohydrate metabolism and many other biological pathways that involve citrate.
• Iron deficiency and suppression of Fe–S cluster biogenesis may hamper the functions of many other Fe–S proteins in intermediarymetabolism, respiratory complexes,heme biosynthesis and DNA repair.
• The selective suppressionof iron-containing proteins in the mtochondria can also lead to decoupling of the electron transport chain and increased oxidative stress.

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