The Iron−Sulfur Cluster of Pyruvate Formate-Lyase Activating Enzyme in Whole Cells: Cluster Interconversion and a Valence-Localized [4Fe-4S]2+ State

Pyruvate formate-lyase activating enzyme (PFL-AE) catalyzes the generation of a catalytically essential glycyl radical on pyruvate formate-lyase (PFL). Purified PFL-AE contains an oxygen-sensitive, labile [4Fe-4S] cluster that undergoes cluster interconversions in vitro, with only the [4Fe-4S]+ cluster state being catalytically active. Such cluster interconversions could play a role in regulating the activity of PFL-AE, and thus of PFL, in response to oxygen levels in vivo. Here we report a Mössbauer investigation on whole cells overexpressing PFL-AE following incubation under aerobic and/or anaerobic conditions and provide evidence that PFL-AE undergoes cluster interconversions in vivo. After 2 h aerobic induction of PFL-AE expression, approximately 44% of the total iron is present in [4Fe-4S]2+ clusters, 6% in [2Fe-2S]2+ clusters, and the remainder as noncluster FeIII (29%) and FeII (21%) species. Subsequent anaerobic incubation of the culture results in approximately 75% of the total iron being present as [4Fe-4S]2+ clusters, with no detectable [2Fe-2S]2+. Ensuing aerobic incubation of the culture converts the iron species nearly back to the original composition (42% [4Fe-4S]2+, 10% [2Fe-2S]2+, 19% FeIII, and 29% FeII). The results provide evidence for changes in cluster composition of PFL-AE in response to the redox state of the cell. Furthermore, the Mössbauer spectra reveal that the [4Fe-4S]2+ cluster of PFL-AE in whole cells contains a valence-localized FeIIIFeII pair which has not been previously observed in the purified enzyme. Addition of certain small molecules containing adenosyl moieties, including 5′-deoxyadenosine, AMP, ADP, and methylthioadenosine, to purified PFL-AE reproduces the valence-localized state of the [4Fe-4S]2+ cluster. It is speculated that the [4Fe-4S]2+ cluster of PFL-AE in whole cells may be coordinated by a small molecule, probably AMP, and that such coordination may protect this labile cluster from oxidative damage.