Aconitase

• OMIM: 100880
• UniProt: P21399
• MGI: 87879
• CAS Number: 9024-25-3

Aconitase (ACO ) (more precisely, aconitate hydratase ) hot enzymes that catalyze the conversion of citrate or iso-citrate in aconitate, and vice versa. It is an essential enzyme of the citric acid cycle and the glyoxylate cycle. Aconitase is present in all eukaryotes and bacteria in the cytosol. Multicellular animals have an extra copy of the ACO in the mitochondrion. In mammals and bacteria Finally, the cytosolic ACO has evolved and assumed the additional role of a translational repressor that plays a central role in iron metabolism. Mutations in the iron-sulfur cluster scaffold protein ( ISCU ) can lead to rare disease Aconitasemangel because the ACO function by the presence of the cluster depends.

The catalyzed equilibrium is:

Citrate and aconitate be converted into each other (1), such as isocitrate aconitate and (2). The control of the overall reaction of citrate to isocitrate is determined by how the Aconitatmolekül binds to the enzyme, for which there are two possibilities (see picture below).

Aconitase -1 and -2 are the obsolete names for the two Aconitasen in animals ( cytosolic, CAC, and the mitochondria, mAc ). Bacterial Aconitasen are designated ACNA and acnB. CAC and ACNA are orthologous to each other. In all there are thus three phylogenetic categories.

The presence of the iron -sulfur cluster - cofactor determines the function of the aconitase -1. In the citric acid cycle as it catalyzes the reversible rearrangement of citrate via the intermediate enzyme-bound cis- aconitate to isocitrate. Aconitase - 1 without the iron cluster binds to iron response element (IRE ) in several RNAs and regulates the translation of ferritin, the δ - Aminolävulinatsynthase and the transferrin receptor (Fig. below).

Biosynthesis

Human ACO1 gene is located on chromosome 9 and extends over 66,220 base pairs and 21 exons. The mRNA is 3,533 bases long and translational and post-translational modification of the protein is produced with 889 amino acids.

ACO2 the gene is located on chromosome 22, and the final protein contains 753 amino acids.

Structure

The protein folds into four domains, three of which are closely linked. The fourth with the other three forms a pocket in which the catalysis occurs. For the catalytic activity of the specific conformation of the enzyme with the [ 4Fe- 4S ] cluster and several amino acid residues responsible, which stereospecifically from achiral citrate exclusively for the (1S, 2R)- isocitrate allows the reaction to proceed.

Contains Mitochondrial aconitase bound to the cysteine ​​-385, -458 and -461, an iron -sulfur cluster [ 4Fe -4S], which is crucial for the catalytic activity. In the inactive state the clusters is missing, the fourth iron atom, this is only loosely bound and initially has a coordination number of 4, three sulfur atoms and a hydroxide ion ( water ) as a binding partner ( see Figure 2). In the catalytic phase, it increases to 6, it will then also be linked isocitrate and another water molecule.

Figure 1 shows the X-ray crystallographically determined tertiary structure of mAc with bound α - Methylisocitrat ( an ACO inhibitor). In the active site is the [ 4Fe- 4S ] cluster that binds together with side chains of polar amino acids, mainly arginine residues, the substrate.

Catalysed equilibrium

↔ H2O ↔

Citrate is converted to cis- aconitate, and to this L-threo- isocitrate, (1S, 2R isocitrate ), and vice versa. The last step is stereoselective. Under standard conditions are 91 % citrate, 3% cis- aconitate and isocitrate 6% in balance. The accompanying notes to the reaction sequence were indeed obtained exclusively from studies of mitochondrial aconitase, however, the structure on the cytosolic ACO due to the similarity transferable.

Assume the isocitrate side, isocitrate is first by two of its hydroxyl groups bound to the fourth iron atom of the cluster ( see Figure ). The most iron -bound hydroxide previously changes its hydrogen bonding of Asp -100 to His- 167th Asp -100 now forms a bridge to the hydroxyl of isocitrate. The hydrogen atoms of the hydroxide and the hydroxy group in turn, form bonds with the oxygen atom of Asp- 165th

By the deprotonated Ser -642 is a sprawling electron transfer towards instead to His- 101. Here, Ser -642 is replaced by a proton and the iron atom of a second hydroxyl group (see figure), which are in the second part of the reaction, the hydration of aconitate reused. Asp -165 is crucial for the binding of the hydroxyl groups at the iron atom.