New Developments in Iron-Sulphur Proteins from R

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442 Nature Vol. 286 31 July 1980 New developments in iron-sulphur proteins from R. Cammack A NOVEL kind of iron-sulphur cluster has give EPR signals around g = 2.01. However iron atoms are antiferromagnetically been found in a ferrodoxin from the recent results suggest that a novel type of coupled together. The observation that the nitrogen-fixing bacterium Azobacter cluster may also give ag=2.0l signal. cluster gives the EPR signal at g = 2.01 in vinelandii. The cluster is unusual in that it The centre of attention is a ferredoxin the oxidized form, but is non-magnetic in contains a ring of three iron atoms linked from Azotobacter vine/andii. This protein the reduced state, can readily be explained by labile sulphide atoms. The iron-sulphur contains two iron-sulphur clusters with by a three-iron cluster. (This was a proteins are a class of proteins that contain widely different redox potentials, difficulty with the previously proposed clusters of iron atoms bound to cysteine +320mV and -424mV. Both give EPR two-iron cluster). The valence states of the and sulphide sulphur. They are an signals atg= 2.01. The first cluster seems to iron atoms would correspond to 3Fe3 + in important group of biological electron be a conventional [4Fe-4S) type. The the oxidized state, and 2Fe3 + and Fe2 ' in transfer agents. For example, more than a second one, however, seems to be the reduced state, giving net charges for the dozen different types of iron-sulphur different. Chemical analysis (which is, cluster of + 3 and + 2, respectively. clusters have been detected in mito admittedly, notoriously difficult with these If the existence of three-iron clusters is chondria where they are concerned with proteins) indicates that it contains fewer confirmed, there are already a number of energy metabolism. Two years ago, the than four iron atoms. Preliminary X-ray proteins known that may also contain possibility that iron-sulphur clusters can crystallography of the ferredoxin at 4 A them. All of these given the EPR signal at have other functions was raised when F. J. resolution by C. D. Stout of the University g = 2.01. MOssbauer spectra of ferredoxin Ruzicka and H. Beinert (J. Biol. Chem. of Pittsburgh showed that this cluster was II from Desulfovibrio gig as indicate that it 253, 2514; 1978) found that aconitase is an smaller than the other and last year it was contains a [3Fe-3S) cluster on each iron-sulphur protein. This enzyme of the proposed (Stout Nature 279, 83; 1979) that polypeptide chain. Since the protein is a Krebs tricarboxylic acid cycle of it might be a two-iron cluster. A clearer tetramer it appears to contain four such mitochondria has no known electron picture of the cluster has now emerged clusters (Interestingly ferredoxin I from the transfer function. It now seems likely that from an electron density map at 2.5 A same organism contains (4Fe-4S] clusters the iron-sulphur cluster in this enzyme is resolution (Stout, Ghosh, Pattabni & in a trimeric unit of exactly the same unusual in another respect, in that it may be Robbins J. Biol. Chem. 255; 19797; 1980). polypeptide). Other, more complex pro a new type of three-iron cluster, similar to A model (see figure) has now been fitted teins, may also contain the cluster. those recently proposed in certain bacterial to the X-ray data and can be considered as a Emptage et. al. report that they have proteins. two-iron cluster which has opened at one observed similar MOssbauer spectra from In the early l 970's it became established, side to introduce a third iron atom the enzyme glutamate synthase of A. by a combination of X-ray crystallogra (numbered 3) and sulphide atom. Unlike vine/andii and from aconitase of beef heart phy, spectroscopy and synthesis of all other iron atoms in iron-sulphur mitochondria. chemical analogues, that all the iron clusters, atom 3 does not have tetrahedral The presence of this iron-sulphur cluster sulphur proteins contained either I-iron, symmetry but appears to be almost square in aconitase is intriguing. The cluster does (Rd), 2-iron (2Fe-2S) or 4-iron [4Fe-4S) planar. Also, the ligands from the protein not appear to participate in the enzyme clusters (see figure). The I-iron centres are could not all be cysteine sulphurs as there reaction directly, but the enzyme is inactive found only in the rubredoxins of certain are insufficient residues of this amino acid unless the cluster is first reduced. It has bacteria but the {2Fe-2S) and (4Fe-4S) available. therefore been suggested that the cluster clusters, in which iron is attached to The results of the X-ray analysis are still somehow regulates the activity of the sulphide sulphur and cysteine sulphur from preliminary. Refinement of the full enzyme. If this idea becomes accepted, the protein, has been found in many structure of the protein is not expected there are other examples known. Gluta electron-transferring systems and before the end of this year. However mine amidoribosyl transferase is an enzymes. The iron-sulphur proteins are support for the existence of a three-iron enzyme concerned with nucleic acid syn often described by the low-temperature cluster is provided by a totally different thesis. The enzyme, both from human electron-paramagnetic-resonance (EPR) technique, 57Fe MOssbauer spectroscopy. tissue and from Bacillus subtilis has been spectra, since this is a good way of A paper from groups in Wisconsin and found to contain an iron-sulphur cluster detecting them in biological systems. The Minnesota (Emptage, Kent, Huynh, Raw• (possibly of the [4Fe-4S) type). Again this [2Fe-2S) clusters and some of the [4Fe-4S) lings, Orme-Johnson & Miinck J. Biol. has no obvious role in catalysis, but the clusters give EPR signals in the reduced Chem. 255, 17973; 1980) looks at the iron enzyme is only active when the cluster is state at g = 1.94. Other [4Fe-4S) clusters atoms in A. vinelandii ferredoxin. After reduced. subtracting the spectrum of the [4Fe-4S) It seems, as more types of iron-sulphur cluster, the second cluster appears proteins continue to be R. Cammack is a Lecturer in the Department of to discovered, that we Plant Sciences, University of London King's contain two types of iron atom, in the ratio have much to learn about the versatility of College. 2 : 1. As in other iron-sulphur clusters, the iron-sulphur clusters. D Atomic arrangements in the one-iron (rubredoxin, Rd) two-iron and four-iron clusters, and the proposed three-iron cluster.

Cys Cys

[Rd] [2Fe-2S] [4Fe-4S] [3Fe-3S]