Sulfur-accepting protein

In Paracoccus vertusus, thiosulfate is oxidized directly to sulfate by the catalysis of an enzyme complex containing several cytochromes c but not cytochrome b (Kelly, 1989). Although sulfite-cytochrome c oxidoreductase occurs in the enzyme complex, the enzyme is thought not to participate in the oxidation of thiosulfate, because the rhodanese activity is not observed with the complex. However, as already indicated, it could be that as the enzyme complex contains a thiosulfatecleaving enzyme strongly bound to both the sulfur-accepting protein and sulfite-cytochrome c oxidoreductase, thiosulfate appears to be oxidized directly to sulfate. 

Thus, the thiosulfate-cleaving enzyme does not show rhodanese activity in the presence of the sulfur-accepting protein (Fukumori et al., 1989). 

A third type of trinuclear cluster duplicates the Fe3S stoichiometry that is most accepted for protein-bound 3Fe clusters and in fact appears to be a structural isomer of the 3Fe cluster present in aconitase at least. The [Fe3S (SR) ]ion (3) is obtained by reaction of [Fe(SR) ] " with 1.4 equivalents of sulfur in MeCN, as indicated above (13, 16). It contains a central Fe(III) ligated tetrahedrally by four sulfides that connect it to two terminal Fe(IIl)(SR)2 units. The spectroscopic and magnetic properties of 3 are virtually identical with those of the 3Fe form of aconitase at high pH, but substantially different from those of aconitase under normal physiological conditions (23). Possible structures for the latter that are consistent with available data are shown in Figure 7. 

The sulfur atom is a favorable zinc ligand because of its size and polarizability. The thiol side chain of cysteine (p/Cg —8.5) is negatively charged as it complexes a metal ion in a protein in addition to metal coordination, the cysteine thiol may simultaneously accept hydrogen bonds from other protein residues (Adman et al, 1975 Ippolito et al, 1990). Hydrogen bond networks with cysteine metal ligands are discussed further in Section I11,B. 

The iron responsive element, a critical factor in the control of proteins involved in iron utilization, has been identified as the cytoplasmic form of the iron-sulfur protein aconitase (Kennedy et al., 1992). Activated macrophages have been shown to activate this element, presumably by attack of the iron-sulfur cluster by NO (Drapier et al., 1993). It has been claimed that this attack is mediated by peroxynitrite (Castro et al., 1994 Hausladen and Fridovich, 1994, but this conclusion is not universally accepted. 

In addition to NAD and flavoproteins, three other types of electron-carrying molecules function in the respiratory chain a hydrophobic quinone (ubiquinone) and two different types of iron-containing proteins (cytochromes and iron-sulfur proteins). Ubiquinone (also called coenzyme Q, or simply Q) is a lipid-soluble ben-zoquinone with a long isoprenoid side chain (Fig. 19-2). The closely related compounds plastoquinone (of plant chloroplasts) and menaquinone (of bacteria) play roles analogous to that of ubiquinone, carrying electrons in membrane-associated electron-transfer chains. Ubiquinone can accept one electron to become the semi-quinone radical ( QH) or two electrons to form ubiquinol (QH2) (Fig. 19-2) and, like flavoprotein carriers, it can act at the junction between a two-electron donor and a one-electron acceptor. Because ubiquinone is both small and hydrophobic, it is freely diffusible within the lipid bilayer of the inner mitochondrial membrane and can shuttle reducing equivalents between other, less mobile electron carriers in the membrane. And because it carries both electrons and protons, it plays a central role in coupling electron flow to proton movement. 

Mitochondrial subunit II contains two Cu ions com-plexed with the —SH groups of two Cys residues in a binuclear center (CuA Fig. 19-13b) that resembles the 2Fe-2S centers of iron-sulfur proteins. Subunit I contains two heme groups, designated a and a3, and another copper ion (CuB). Heme a3 and CuB form a second binuclear center that accepts electrons from heme a and transfers them to 02 bound to heme a3. 

Synthetic iron-sulfur clusters have weakly basic properties273 and accept protons with a pKa of from 3.9 to 7.4. Similarly, one clostridial ferredoxin, in the oxidized form, has a pKa of 7.4 it is shifted to 8.9 in the reduced form 295 If we designate the low-pH oxidized form of such a protein as HOx+ and the reduced form as HRed, we can depict the reduction of each Fe4S4 cluster as follows. 

This enzyme releases S° from cysteine with formation of alanine464-466 as is shown in Eq. 14-34 for release of Se° from selenocysteine. As with rhodanese an active site cysteine accepts the departing S° of cysteine to form an enzyme-bound persulfide. This protein may in turn transfer the sulfur into the forming Fe-S or Fe-S-Mo clusters.464 Three PLP-dependent persulfide-forming sulfurtransferases related to the NifS protein have been found in E. coli. Similar enzymes are present in other organisms.4663 1 A sulfur atom may be transferred from the bound persulfide anion to acceptor proteins involved in metal cluster formation. Some members of the nifS-like family act on cystine to release free thiocysteine (cysteine peroxide), which may also serve as a sulfur atom donor 466e... 

We now see that mitochondria contain a variety of molecules—cytochromes, flavins, ubiquinone, and iron-sulfur proteins—all of which can act as electron carriers. To discuss how these carriers cooperate to transport electrons from reduced substrates to 02, it is useful to have a measure of each molecule s tendency to release or accept electrons. The standard redox potential, E°, provides such a measure. Redox potentials are thermodynamic properties that depend on the differences in free energy between the oxidized and reduced forms of a molecule. Like the electric potentials that govern electron flow from one pole of a battery to another, E° values are specified in volts. Because electron-transfer reactions frequently involve protons also, an additional symbol is used to indicate that an E° value applies to a particular pH thus, E° refers to an E° at pH 7. 

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