Interplay between Oxygen and Fe-S Cluster Biogenesis: Insights from the Suf Pathway

Iron sulfur (Fe-s) cluster metalloproteins conduct essential functions in nearly all contemporary forms of life. The nearly ubiquitous presence of Fe-s clusters and the fundamental requirement for Fe-s clusters in both aerobic and anaerobic Archaea, Bacteria, and Eukarya suggest that these clusters were likely integrated into central metabolic pathways early in the evolution of life prior to the widespread oxidation of Earth's atmosphere. Intriguingly, Fe-s cluster-dependent metabolism is sensitive to disruption by oxygen because of the decreased bioavailability of ferric iron as well as direct oxidation of sulfur trafficking intermediates and Fe-s clusters by reactive oxygen species. This fact, coupled with the ubiquity of Fe-s clusters in aerobic organisms, suggests that organisms evolved with mechanisms that facilitate the biogenesis and use of these essential cofactors in the presence of oxygen, which gradually began to accumulate around 2.5 billion years ago as oxygenic photosynthesis proliferated and reduced minerals that buffered against oxidation were depleted. This review highlights the most ancient of the Fe-s cluster biogenesis pathways, the Suf system, which likely was present in early anaerobic forms of life. Herein, we use the evolution of the Suf pathway to assess the relationships between the biochemical functions and physiological roles of Suf proteins, with an emphasis on the selective pressure of oxygen toxicity. Our analysis suggests that diversification into oxygen-containing environments disrupted iron and sulfur metabolism and was a main driving force in the acquisition of accessory Suf proteins (such as SufD, SufE, and SufS) by the core SufB-SufC scaffold complex. This analysis provides a new framework for the study of Fe-s cluster biogenesis pathways and Fe-s cluster-containing metalloenzymes and their complicated patterns of divergence in response to oxygen.