Iron-Sulfur Cluster Biosynthesis

Abstract

Iron-sulfur clusters are among the most complex metal-containing prosthetic centers in biology. Most if not all of the proteins involved in the biosynthesis of "simple" Fe-S clusters have been identified. The structural and functional chemistry of these proteins has been the subject of intense research efforts, and many of the key details are now understood in structural and mechanistic detail. The fact that Fe-S cluster-binding proteins can be reconstituted in vitro with no accessory proteins provides an important indicator of the intracellular roles for many proteins on the Fe-S cluster assembly pathway. Indeed, such proteins are more correctly viewed as carrier proteins, rather than as catalysts for the reaction, that both avoid the toxicity associated with free iron and sulfide and allow delivery at lower intracellular concentrations of these species. The IscU (or ISU) family of proteins serves a key role as scaffolding proteins on which [2Fe-2S] building blocks are assembled prior to transfer to final apo target proteins. IscU in particular exhibits highly unusual conformational flexibility that appears critical to its function.

NifS-Mediated Assembly of [4Fe−4S] Clusters in the N- and C-Terminal Domains of the NifU Scaffold Protein

“…However, the presence of D37 does affect the properties of both [4Fe-4S] 2+ and [2Fe-2S] 2+ clusters assembled on NifU-1. This is evident by the differences in the Mo ¨ssbauer parameters for both the [4Fe-4S] 2+ and [2Fe-2S] 2+ clusters in purified samples of wild-type and D37A NifU-1 (see Table 1) and the resonance Raman spectra of the [2Fe-2S] 2+ clusters in wild-type and D37A NifU-1 (58). The structural origin of these spectroscopic differences are not known at present, but they could be interpreted in terms of D37, providing a cluster ligand and/or controlling cluster solvent exposure, in light of the proximity of D37 to the three conserved cysteine residues in the solvent-exposed cluster-binding site (30).…”

A Conformational Mimic of the MgATP-Bound “On State” of the Nitrogenase Iron Protein,

“…The [4Fe-4S] 2+,+ clusters that mediate reductive cleavage of S -adenosyl- l -methionine to yield 5‘-deoxyadenosyl radicals are known to undergo oxidative degradation via a [2Fe-2S] 2+ cluster with resonance Raman properties almost identical to those of the semistable [2Fe-2S] 2+ intermediate observed during oxidative degradation of the Fe protein in the ATP-bound conformation ( − ). Reductive coupling of adjacent [2Fe-2S] clusters has been invoked to explain the biosynthesis of [4Fe-4S] clusters on apo Fe-S proteins and on proteins such as IscU, IscA, and NifU that function as scaffolds for Fe-S cluster biosynthesis ( − ). The current study provides the first structurally characterized example of this coupling mechanism occurring in a biological system.…”

Iron–sulfur cluster biosynthesis

“…Furthermore, structural and mutagenesis data implicate the rigorously conserved aspartate, (D39 in Av IscU) as the non-cysteinyl ligand. Substitution of this aspartate with alanine results in major changes in the resonance Raman spectrum of the [Fe 2 S 2 ] 2+ center in both the N-terminal domain of NifU [32] and in IscU (H. Gao and M. K. Johnson, unpublished results) and the crystal structure of Zn-bound S. pyogenes SufU places this residue in the cluster binding pocket [29]. However, whereas substitution of a cluster-ligating residue by alanine generally destabilizes Fe-S clusters, the D39A substitution stabilizes the cluster in IscU and the N-terminal domain of NifU as judged by the ability to purify the one [Fe 2 S 2 ] 2+ cluster-bound form with the cluster intact [10] and the dramatic decrease in the rates of cluster transfer to acceptor proteins [33].…”