Sulfur biological function

Rapid and reversible making and breaking of the sulfur-sulfur bond is essential to the biological function of a lipoic acid... 

PS Brereton, FJM Verhagen, ZH Zhou, MWW Adams. Effect of iron-sulfur cluster environment m modulating the thermodynamic properties and biological function of ferredoxm from Pyrococcus furiosus. Biochemistry 37 7351-7362, 1998. 

There exist a number of homo metal-sulfur clusters possessing an M3S core (M = Fe, Co, Ni, Mo, W n = l l)la, but the most represented assemblies are M3S2 and M3S4, respectively. The latter is present in a few biological functions as an Fe3S4 unit2 (see also Chapter 12, Section 3). 

Most mechanisms which control biological functions, such as cell respiration and photosynthesis (already discussed in Chapter 5, Section 3.1), are based on redox processes. In particular, as shown again in Figure 1, it is evident that, based on their physiological redox potentials, in photosynthesis a chain of electron carriers (e.g. iron-sulfur proteins, cytochromes and blue copper proteins) provides a means of electron transport which is triggered by the absorption of light. 

The recent progress in the area of biological functions and molecular properties of iron-sulfur proteins has been phenomenal, and comprehensive summaries of these proteins have appeared in many reviews (2,3,4,5) and books (6,7,8). In this paper, the properties of some of the well understood iron-sulfur proteins will be briefly described. An effort will be made then to relate these properties to their possible participation in degradation reactions on organic chemicals, and particularly on pesticide chemicals. 

An excellent, still up-to-date survey on chemical properties, structures and biological functions of iron-sulfur clusters has been given by Beinert.174 As a more general survey of EPR properties of iron-sulfur proteins we recommend a reference which was also quoted in our earlier report.175... 

It is interesting to speculate on the chemical properties of molybdenum which make it suitable for its biological function. Obvious features in the chemistry of molybdenum are (a) a range of oxidation states which can be stabilized in aqueous solution by the common ligands of biology (b) the formation of oxo compounds and the sulfur analogue (c) the ability to participate in atom-transfer reactions and (d) the possibility of higher coordination numbers. 

Several other possible biological functions of iron-sulfur clusters have been proposed as summarized in Table 1. Let us briefly discuss this list in detail to indicate why the majority of the entries are excluded from our definition of catalytic iron-sulfur clusters. 

Table 1 Established, Putative, and Proposed Biological Functions of 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. 

Since the protein environment around the iron site of the iron sulfur protein is related to the protein s biological function, peptide sequences nearest to the iron core are of utmost importance. Based on the peptide sequences reported for various iron-sulfur proteins, some examples of invariant sequences are discernible. For example, Cys-Gly-X-Cys sequences are involved in coordination at the active sites of bacterial ferredoxins (9). This macro-ring chelation is believed to be a feature and conveys specific chemical properties to the iron core. 

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