The aconitase family: three structural variations on a common theme.

The aconitase family contains a diverse group of iron-sulphur (Fe-S) isomerases and two types of iron regulatory protein (IRP). Structural comparisons have revealed three architecturally distinct variants in which one of the four structural domains is covalently linked at either the amino- or carboxy-terminal end of a single polypeptide or else this domain exists as an independent subunit.

Spectroscopic characterisation of an aconitase (AcnA) of Escherichia coli.

A spectroscopic study of an aconitase, AcnA, from Escherichia coli is presented. The amino acid sequence of AcnA has 53% identity with mammalian cytosolic aconitase (c-aconitase) which is the translational regulator known as iron regulatory factor (IRF). In the [3Fe-4S](+)-containing, inactive state, AcnA displays an EPR signal which is not unlike the corresponding signal from mammalian mitochondrial aconitase (m-aconitase) but is even more similar to the signal from c-aconitase. This is perhaps related to the greater similarity of the AcnA amino acid sequence with c-aconitase. Magnetic circular dichroism (MCD) spectroscopy has revealed that the electronic structure of the [3Fe-4S] cluster of AcnA must be similar to, but not identical to that of m-aconitase. Whilst the [Fe-4S] clusters from both of these enzymes display some features in their MCD spectra common to [3Fe-4S] clusters in general, their spectra overall are unique and indicate that the Fea atom of the [4Fe-4S] form is not the only unusual feature of the [Fe-S] clusters of aconitases. Active [4Fe-4S]-containing AcnA can be reduced to yield an EPR signal due to a [4Fe-4S]+ cluster which is indistinguishable from the signals from the [4Fe-4S]+ cluster in the mammalian enzymes. However, in contrast to the mammalian enzymes, the EPR signals of the cluster in AcnA are not significantly perturbed upon the addition of substrate. Furthermore, the catalytic activity of [Fe-4S](2+)-containing AcnA is fivefold higher than that of m-aconitase. The mechanistic implications of these data are discussed. A novel S = 1/2 EPR signal with g approximately 2 was observed in AcnA upon treatment with EDTA. The species giving rise to this signal is proposed to be an intermediate in cluster deconstruction.

Two genetically-distinct and differentially-regulated aconitases (AcnA and AcnB) in Escherichia coli.

An acnA mutant of Escherichia coli was constructed by replacing the chromosomal acnA gene by an internally deleted derivative containing a kanR cassette. Southern and Western blotting confirmed that the acnA gene had been replaced by the disrupted gene and that the aconitase A protein was no longer expressed. However, the mutant failed to exhibit the anticipated glutamate auxotrophy and it retained a residual aconitase activity. This activity was due to an analogous unstable enzyme(s) designated aconitase B. Studies on the regulation of aconitase A synthesis using an acnA-lacZ translational fusion showed that the acnA gene resembles other citric acid cycle genes in being subject to CRP-mediated catabolite repression and ArcA-mediated anaerobic repression. In addition to being activated by the SoxRS oxidative stress regulatory system, the acnA gene appeared to be activated by the ferric uptake regulator (Fur). It was concluded that the acnA gene belongs to at least four global regulatory networks, crp, arcA, fur and soxRS. In contrast, the aconitase B activity decreased after exposure to oxidative stress and was less affected by anaerobiosis. Comparable studies with the fumarase genes (fumA, B and C) indicated that fumA (encoding the unstable aerobic iron-sulphur-containing fumarase) is activated by the ferric uptake regulator (Fur) and fumC (encoding the stable fumarase) is activated by the SoxRS oxidative stress regulatory system.