Aconitase iron—sulfur cluster function

It is now clear that in addition to their widespread involvement in electron transfer pathways, iron-sulfur clusters function as catalytic centers in a wide variety of enzymes. The first example of such an enzyme is aconitase. It was at first thought that the role of the iron-sulfur group was regulatory, but it is now clear that in this enzyme the iron-sulfur group is part of the catalytic site. One of the iron atoms can coordinate water or hydroxyl and plays a key role in the isomerization catalyzed by the enzyme (Emptage et al., 1983). 

To successfully describe the structure and function of nitrogenase, it is important to understand the behavior of the metal-sulfur clusters that are a vital part of this complex enzyme. Metal-sulfur clusters are many, varied, and usually involved in redox processes carried out by the protein in which they constitute prosthetic centers. They may be characterized by the number of iron ions in the prosthetic center that is, rubredoxin (Rd) contains one Fe ion, ferredoxins (Fd) contain two or four Fe ions, and aconitase contains three Fe ions.7 In reference 18, Lippard and Berg present a more detailed description of iron-sulfur clusters only the [Fe4S4] cluster typical of that found in nitrogenase s Fe-protein is discussed in some detail here. The P-cluster and M center of MoFe-protein, which are more complex metal-sulfur complexes, are discussed in Sections 6.5.2. and 6.5.3. 

Proteins containing iron-sulfur clusters are ubiquitous in nature, due primarily to their involvement in biological electron transfer reactions. In addition to functioning as simple reagents for electron transfer, protein-bound iron-sulfur clusters also function in catalysis of numerous redox reactions (e.g., H2 oxidation, N2 reduction) and, in some cases, of reactions that involve the addition or elimination of water to or from specific substrates (e.g., aconitase in the tricarboxylic acid cycle) (1). 

Aconitase exists as both mitochondrial and cytosolic isoenzyme forms of similar structure. However, the cytosolic isoenzyme has a second function. In its apoenzyme form, which lacks the iron-sulfur cluster, it acts as the much-studied iron regulatory factor, or iron-responsive element binding protein (IRE-BP). This protein binds to a specific stem-loop structure in the messenger RNA for proteins involved in iron transport and storage (Chapter 28).86/9°... 

The role of the iron-sulfur clusters in many of the proteins that we have just considered is primarily one of single-electron transfer. The Fe-S cluster is a place for an electron to rest while waiting for a chance to react. There may sometimes be an associated proton pumping action. In a second group of enzymes, exemplified by aconitase (Fig. 13-4), an iron atom of a cluster functions as a Lewis acid in facilitating removal of an -OF group in an a,P dehydration of a carboxylic acid (Chapter 13). A substantial number of other bacterial dehydratases as well as an important plant dihydroxyacid dehydratase also apparently use Fe-S clusters in a catalytic fashion.317 Fumarases A and B from E. coli,317 L-serine dehydratase of a Pepto-streptococcus species,317-319 and the dihydroxyacid... 

Aconitase was the first protein to be identified as containing a catalytic iron-sulfur cluster [24-26]. It was also readily established that the redox properties of the [4Fe-4S](2+ 1+) cluster do not play a role of significance in biological functioning the 1 + oxidation state has some 30% of the activity of the 2+ state [25], Since then several other enzymes have been identified or proposed to be nonredox iron-sulfur catalysts. They are listed in Table 2. It appears that all are involved in stereospecific hydration reactions. However, these proteins are considerably less well characterized than aconitase. In particular, no crystal structural information is available yet. Therefore, later we summarize structural and mechanistic information on aconitase, noting that many of the basic principles are expected to be relevant to the other enzymes of Table 2. 

The stmctural, electronic, biological, and magnetic properties of iron sulfur clusters in metalloproteins has been reviewed focussing particularly on [4Fe-4S ]"+ (n = 1-3) centres, and on synthetic [Fe3S4] and heterometallic [MFe3 S4] clusters. The chemistry and properties of aconitase have been reviewed in the context of its spectroscopic properties, structure, mechanistic function, and its relationship to the iron-regulatory protein. ... 

Together, the biochemical, spectroscopic, and structural data suggest a mechanism for aconitase in which the unique iron site of the [4Fe S] cluster serves as a Lewis acid in catalysis, binding and polarizing substrate to facilitate the dehydration/rehydration reactions. The use of an iron-sulfur cluster in a nonredox role, as well as its function in binding substrates, was novel and unexpected, and was the first indication of the remarkable diversity of these clusters in functions beyond electron transfer. 

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