Redox sulfur

Iron-sulfur proteins. In an iroinsulfiir protein, the metal center is surrounded by a group of sulfur donor atoms in a tetrahedral environment. Box 14-2 describes the roles that iron-sulfur proteins play in nitrogenase, and Figure 20-30 shows the structures about the metal in three different types of iron-sulfur redox centers. One type (Figure 20-30a l contains a single iron atom bound to four cysteine ligands. The electron transfer reactions at these centers... 

There are three general types of iron-sulfur redox centers (a) a single iron atom (brown) surrounded by four cysteine sulfur atoms (yellow) (b) two iron atoms bound to two cysteines and a pair of bridging sulfide ligands and (c) a cubelike stracture consisting of four irons and four sulfiirs. 

The most noteworthy multistage element cycles in which bacteria play important roles are the nitrogen and sulfur redox cycles. The fixation of nitrogen is a reductive process that provides organisms with nitrogen in a form usable for the synthesis of amino acids, nucleic acids, and other cell constituents. In essence, the overall conversion to the key intermediate, ammonia, can be represented as ... 

In the transfer of reducing equivalents from the pyridine nucleotide pool, flavoproteins carry out a central role of mediating the conversion of the obligatory 2-electron reductant to 1-electron receptors such as hemes and iron-sulfur redox centers. In such a role, the semiquinone form of the flavin serves as a pivotal intermediate. The reduction of flavins and flavoproteins by reduced pyridine nucleotides has been extensively studied since the initial work of Singer and Kearney which showed that flavin reduction can occur in a non-enzyme catalyzed manner. The reduction proceeds as a 2-electron process since the formation of a nicotinamide semiquinone (a necessary intermediate in a 1-electron process) has been... 

Thus, the hybrid cluster is a putative iron-sulfur redox catalyst. It is, however, a very uncommon cluster (perhaps only comparable to the nitrogenase active site) in two aspects (1) it is a hybrid cluster i.e., it contains intrinsic building blocks that are distinctly strange to iron-sulfur clusters and (2) it can exist in more than two (in fact, four [63]) oxidation states. 

Sulfur redox reactions seem to be more reversible than those of nitrogen. Intermediate compounds in the reaction series from sulfate to sulfur or sulfide, and vice versa, do not appear in soils. Sulfur also differs from nitrogen in that little sulfur volatilizes from soils. Although H2S is a gas, apparently any HjS formed in soils reacts rapidly with Fe and other transition metal oxides to form sulfides. Some organic... 

These iron-sulfur redox proteins contain a cagelike cluster of iron and sulfur linked to cysteine residues of the peptide chain. 

Finally, a two electron reduction of cMEPP forms 2-methyl-2-( )-butenyl diphosphate followed by conversion to IPP and DMAPP. These steps are catalyzed by 1 -hydroxy-2-methyl-2-( )-butenyl-4-diphosphate synthase (IspG) [26] and 4-hydroxy-3-methyl-2-( )-butenyl-4-diphosphate reductase (IspH) [27], respectively. IspG and IspH enzymes possess [4Fe-4S] iron-sulfur redox cluster in their C-terminal domains, which represents the active center of the enzymes. The newest cocrystal structures of IspH from E. coli revealed the presence of a [4Fe-4S] cluster... 

---