Iron-sulfur enzymes redox-catalytic

The existence of hydrolytic iron-sulfur enzymes is well established. Con-trarily, redox-catalytic iron-sulfur centers are presently still hypothetical at least... 

Table 3 Possible Redox-Catalytic Iron-Sulfur Enzymes, Their Substrates, and the Putative Structures of Their Catalytic Center...

The many redox reactions that take place within a cell make use of metalloproteins with a wide range of electron transfer potentials. To name just a few of their functions, these proteins play key roles in respiration, photosynthesis, and nitrogen fixation. Some of them simply shuttle electrons to or from enzymes that require electron transfer as part of their catalytic activity. In many other cases, a complex enzyme may incorporate its own electron transfer centers. There are three general categories of transition metal redox centers cytochromes, blue copper proteins, and iron-sulfur proteins. 

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 next two entries to Table 3 are cited for completeness. Nitrogenase is treated in Chapter 7 and CO dehydrogenase in Chapter 9. Nitrogenase contains a very complex iron-sulfur cluster that includes another metal, molybdenum or vanadium. The crystal structure of the Mo variant has been determined. There is a third variant, alternative nitrogenase [92], whose cluster apparently does not contain any heterometal. That cluster would thus be a perfect candidate for our definition of a redox-catalytic iron-sulfur cluster. Unfortunately, this third nitrogenase has thus far been characterized to a much lesser extent than the other two forms. For all nitrogenases holds that the binding of N2 to the cluster has not been established [53] therefore, formally these enzymes have not yet been positively identified as redox iron-sulfur catalysts. 

This review is a survey of the research on the direct electron transfer (DET) between biomolecules and electrodes for the development of reagentless biosensors. Both the catalytic reaction of a protein or an enzyme and the coupling with further reaction have been used analytically. For better understanding and a better overview, this chapter begins with a description of electron transfer processes of redox proteins at electrodes. Then the behaviour of the relevant proteins and enzymes at electrodes is briefly characterized and the respective biosensors are described. In the last section sensors for superoxide, nitric oxide and peroxide are presented. These have been developed with several proteins and enzymes. The review is far from complete, for example, the large class of iron-sulfur proteins has hardly been touched. Here the interested reader may consult recent reviews and work cited therein [1,19]. 

A mediator would not be needed if the enzymes used in BFCs are capable of DET via the active site of the enzyme. Several enzymes capable of DET have been reported [18,19]. Many of these enzymes contain redox-active metal centers, such as iron-sulfur groups, heme groups, and metallic centers, that perform the catalytic transfer of electrons. These enzymes convert the chemical signal directly to an electrical signal through the transfer of charge to the redox center, which is in mm... 

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