Escherichia coli contains two major aconitases (Acns), AcnA and AcnB. They are distantly related monomeric Fe-S proteins that contain different arrangements of four structural domains. On the basis of the differential expression of the acnA and acnB genes, AcnA has been designated as an aerobic-stationary-phase enzyme that is specifically induced by iron and oxidative stress, whereas AcnB functions as the major citric-acid-cycle enzyme during exponential growth. The biochemical and kinetic properties of the purified enzymes have now shown that AcnA is more stable than AcnB, has a higher affinity for citrate, and operates optimally over a wider pH range, consistent with its role as a maintenance or survival enzyme during nutritional or oxidative stress. In contrast, the better performance at high substrate concentrations and greater instability of AcnB indicate that AcnB is specifically adapted to function as the main catabolic enzyme and, by inactivation, to rapidly modulate energy metabolism in response to oxidative or pH stress, either directly or indirectly by regulating post-transcriptional gene expression. EPR and magnetic-CD spectroscopy showed that the iron-sulphur clusters of the bacterial Acns (and their binding sites) strongly resemble those of the mammalian enzymes. The EPR and MCD spectra of the oxidized inactive form of AcnB confirmed the presence of a [3Fe-4S]1+ (S = 1/2) cluster. Comparisons showed that the EPR spectrum of AcnB more closely resembled that of mammalian mitochondrial Acn (m-Acn), whereas the spectrum of AcnA more closely resembled that of the cytoplasmic enzyme (c-Acn). The MCD spectra revealed spectroscopic signatures similar to that of m-Acn. Reconstitution of the active [4Fe-4S]2+ forms followed by one-electron reduction gave rise to EPR spectra that are almost identical with those reported for the mammalian enzymes.
Present address: Department of Chemistry, University of Wyoming, Laramie, WY 82017-3838, USA.