Spectroscopic Characterisation of an Aconitase (AcnA) of Escherichia coli

Abstract

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 [3Fe‐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 Fe_a 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 [4Fe‐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≈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.