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 Jiron)-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]?
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Answer 1

Aconitases are enzymes that catalyze the stereospecific isomerization of citrate to isoicitrtae in the tricarboxylic cycle. These enzymes are iron-sulfur proteins that contain 4Fe--4S clusters. There are two different aconitases that a mammalian tissue encodes, Mitochondrial aconitases and cytosolic aconitases. cytosolic aconitase is a bifunctional protein which upon the loss of its Fe-S cluster is converted from an enzyme to iron regulatory protein -1(IRP1). hence it is very important to understand the structural integrity of the Fe-S cluster for knowing the physilogical role of aconitases.

The iron-sulfur cluster in aconitase is unstable and it is to be replaced occasionally when it falls out. when iron levels in the cell get low there is not enough iron to regenerate the cluster and hence the protein shifts its second function. the protein open up and grips hairpin loops in a few specific messenger RNA molecules. These include a hairpin at the start of the mRNA for ferritin and five similar hairpins at the end off MRNA for the transferrin receptor. When the iron regulatory protein 1 binds it inhibits the formation of ferri, so the cell can pick up more transferrin out of the blood.