aconitate hydratase
Illustration of pig aconitase in complex with the [Fe4S4] cluster. The protein is colored by secondary structure, and iron atoms are blue and the sulfur red.[1]
Identifiers
EC no.	4.2.1.3
CAS no.	9024-25-3
Databases
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway
PRIAM	profile
PDB structures	RCSB PDB PDBe PDBsum
Gene Ontology	AmiGO / QuickGO
Search PMC articles PubMed articles NCBI proteins

Aconitase family (aconitate hydratase)
Structure of aconitase.[2]
Identifiers
Symbol	Aconitase
Pfam	PF00330
InterPro	IPR001030
PROSITE	PDOC00423
SCOP2	1aco / SCOPe / SUPFAM
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

Aconitase (aconitate hydratase; EC 4.2.1.3) 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.^[3][4][5]

• 
  Citric acid
• 
  Aconitic acid
• 
  Isocitric acid

Structure

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.^[6][7] In the inactive form, its structure is divided into four domains.^[6] 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.^[6] The Fe-S cluster and a SO²⁻
₄ anion also reside in the active site.^[6] When the enzyme is activated, it gains an additional iron atom, creating a [4Fe-4S] cluster.^[7][8] 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.^[7]

Function

In contrast with the majority of iron-sulfur proteins that function as electron carriers, the iron-sulfur cluster of aconitase reacts directly with an enzyme substrate. 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; EC 4.2.1.33), 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.

Aconitase is inhibited by fluoroacetate, therefore fluoroacetate is poisonous. Fluoroacetate, in the citric acid cycle, can innocently enter as fluorocitrate. However, aconitase cannot bind this substrate and thus the citric acid cycle is halted. The iron sulfur cluster is highly sensitive to oxidation by superoxide.^[9]

Mechanism

Aconitase arrow-pushing mechanism ^[10][11]

Citrate and the Fe-S cluster in the active site of aconitase: dashed yellow lines show interactions between the substrate and nearby residues^[12]

Aconitase employs a dehydration-hydration mechanism.^[10] The catalytic residues involved are His-101 and Ser-642.^[10] 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).^[10][13] 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).^[14]: 393 At this point, the intermediate is rotated 180°.^[10] This rotation is referred to as a "flip."^[11] Because of this flip, the intermediate is said to move from a "citrate mode" to a "isocitrate mode."^[15]

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.^[15] This rate-limiting step ensures that the right stereochemistry, specifically (2R,3S), is formed in the final product.^[15][16] Another hypothesis is that cis-aconitate stays bound to the enzyme while it flips from the citrate to the isocitrate mode.^[10]

In either case, flipping cis-aconitate allows the dehydration and hydration steps to occur on opposite faces of the intermediate.^[10] Aconitase catalyzes trans elimination/addition of water, and the flip guarantees that the correct stereochemistry is formed in the product.^[10][11] To complete the reaction, the serine and histidine residues reverse their original catalytic actions: the histidine, now basic, abstracts a proton from water, priming it as a nucleophile to attack at C2, and the protonated serine is deprotonated by the cis-aconitate double bond to complete the hydration, producing isocitrate.^[10]

Isocitrate and the Fe-S cluster in the active site of aconitase^[12]PDB: 1C97​;

Family members

Aconitases are expressed in bacteria to humans. Humans express the following two aconitase isozymes:

Interactive pathway map

Click on genes, proteins and metabolites below to link to respective articles. ^{[§ 1]}

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|alt=TCACycle_WP78 edit]]

TCACycle_WP78 edit

1. ↑ The interactive pathway map can be edited at WikiPathways: "TCACycle_WP78".

References

Further reading

External links

• Aconitase at the U.S. National Library of Medicine Medical Subject Headings (MeSH)
• Proteopedia Aconitase - the Aconitase structure in interactive 3D