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Project Progress

We have made seminal progress in our understanding of complex biological iron-sulfur cluster assembly during this past year. The results of our biochemical work coupled with a parallel phylogenetic analysis of complex iron-sulfur enzymes and their closely relatives has provided for significant advances in our ability to rationalize how at least a subset of complex iron-sulfur enzymes evolved. The main breakthrough from the past year was the structure of an [FeFe]-hydrogenase which was captured prior to insertion of its complex iron-sulfur cluster. In place of the cluster the structure revealed an open cavity and perhaps more importantly the structure of the evolutionary ancestor. We have made strong connections between the biosynthetic pathways for maturation of [FeFe]-hydrogenases and nitrogenases and have brought to light that both systems involved similar simple cluster precursors, radical chemistry, and nucleotides (Figure 1). For nitrogenase it has been known for sometime as well that a form of nitrogenase without its complex FeMo-cofactor can exist in a stable state and recently it was also revealed that its closest evolutionary relative involved in chlorophyll biosynthesis (protocholorophylide reductase) is similar to this nitrogenase intermediate.

The culmination of these results have lead to the proposal of a new unifying theory for the evolutionary origin of nitrogenase and [FeFe]-hydrogenases. We believe now that both complex iron-sulfur enzymes were evolved from an ancestral protein with an open cavity. The cavity was capable of binding metals and metal clusters that could lead to a selectable catalytic activity; for example the ability to catalyze dinitrogen reduction, hydrogen oxidation, or proton reduction. In biology the generation of cluster or modified by an enzyme activity altered by mutation could be selected for. Subsequent modifications could be added in a stepwise fashion in a similar manner. The scheme is to our knowledge the first theory for the evolutionary origin of complex iron-sulfur enzymes marries the current knowledge of iron-sulfur enzyme biochemistry with the details of protein evolution. For the nitrogenase we believe that the ancestor of nitrogenase and protochlorophylide is a protein thought to be involved in nickel cofactor biosynthesis in methanogenesis and thus an open cavity capable of binding and nickel porphyrin (Figure 2).