Aconitase - WikiHQ

Aconitase (aconitate hydratase; ) is an enzyme that catalyses the stereo-specific isomerization of citrate to isocitrate via cis-aconitate in the tricarboxylic acid cycle, a non-redox-active process.

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Structure

Aconitase has two slightly different structures, depending on whether it is activated or inactivated. In the inactive form, its structure is divided into four domains. However, the structure of the rest of the enzyme is nearly unchanged; the conserved atoms between the two forms are in essentially the same positions, up to a difference of 0.1 angstroms. The iron–sulfur cluster is highly sensitive to oxidation by superoxide.

Mechanism

thumb|none|upright=2.5|Aconitase arrow-pushing mechanism

Aconitase employs a dehydration-hydration mechanism. His-101 protonates the hydroxyl group on C3 of citrate, allowing it to leave as water, and Ser-642 concurrently abstracts the proton on C2, creating a double bond between C2 and C3, and forming the so-called cis-aconitate intermediate (the two carboxyl groups on the double bond are cis). The carbon atom from which the hydrogen is removed is the one that came from oxaloacetate in the previous step of the citric acid cycle, not the one that came from acetyl CoA, even though these two carbons are equivalent except that one is "pro-R" and the other "pro-S" (see Prochirality). At this point, the intermediate is rotated 180°. Because of this flip, the intermediate is said to move from a "citrate mode" to a "isocitrate mode."

How exactly this flip occurs is debatable. One theory is that, in the rate-limiting step of the mechanism, the cis-aconitate is released from the enzyme, then reattached in the isocitrate mode to complete the reaction. Another hypothesis is that cis-aconitate stays bound to the enzyme while it flips from the citrate to the isocitrate mode. As the fruit matures, citric acid is returned back to the cytosol where an increase in cytosolic aconitase activity reduces its levels in the fruit.