Cysteine sulfurate

Cub and the iron atom of cytochrome Og are also situated close to each other and are thought to share a ligand, which may be a cysteine sulfur (Figure 21.19). This closely associated pair of metal ions is referred to as a buiuclear center. 

Figure 25.9 Some non-haem iron proteins (a) rubredoxin in which the single Fe is coordinated, almost tetra-hedrally, to 4 cysteine-sulfurs, (b) plant ferredoxin, [Fe2S2(S-Cys)4], (c) [Fe4S4(S-Cys)4] cube of bacterial ferredoxins. (This is in fact distorted, the Fe4 and S4 making up the two interpenetrating tetrahedra, of which the latter is larger than the former).

Cytochromes. A cytochrome is a protein containing a heme with an iron cation bonded to four donor nitrogen atoms in a square planar array. Figure 20-29a shows the structure of cytochrome c, in which a histidine nitrogen atom and a cysteine sulfur atom occupy the fifth and sixth coordination sites of the octahedral iron center. 

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. 

LADH contains a tetrahedral zinc which is coordinated by one histidine nitrogen and two cysteine sulfurs. One aim is to make synthetic analogs of this coordination sphere that also feature the catalytically important water or hydroxide ligand in the fourth coordination site. Analogs containing bound substrates such as alcohols are also of interest. 

A one-electron transfer is thus suggested from the cysteine sulfur to the oxygen molecule within the complex. 

Poisoning and sometimes death from eating (unidentified) mushrooms is well known. In particular, Amanita sp. are particularly dangerous, with much emphasis on the death cap fungus , Amanita phalloides.24 The best known toxins are the amatoxins and phallotoxins, which are complex, bicyclic peptides. An unusual feature relates to sulfur a tryptophan (or substituted tryptophan) unit is linked to a cysteine sulfur at the carbon atom next to the NH group of the pyrrole ring, forming the unit, -CH2-S-C(NH)=C, e.g. in... 

Chapter 6). Other iron-sulfur proteins, so named because they contain iron sulfur clusters of various sizes, include the rubredoxins and ferredoxins. Rubredoxins are found in anaerobic bacteria and contain iron ligated to four cysteine sulfurs. Ferredoxins are found in plant chloroplasts and mammalian tissue and contain spin-coupled [2Fe-2S] clusters. Cytochromes comprise several large classes of electron transfer metalloproteins widespread in nature. At least four cytochromes are involved in the mitrochondrial electron transfer chain, which reduces oxygen to water according to equation 1.29. Further discussion of these proteins can be found in Chapters 6 and 7 of reference 13. 

The mechanism of zinc deprivation by 3-nitrosobenzamide was elucidated most recently. When the reconstituted nucleocapsid protein p7 of HIV-1 (15 i-M) was incubated with 3-nitrosobenzamide (300 iM) at pH 7.5, three disulfide bonds per protein molecule were formed while 3-nitrosobenzamide was reduced to the hydroxylamine. Molecular masses of p7 adducts augmented by one or two 3-nitrosobenzamide residues were observed by electrospray ionization MS, consistent with covalent bond formation between cysteine sulfur and the nitroso nitrogen atom127. 

In this case also, the Cu(II) ion is coordinated to two histidine nitrogen atoms, a cysteine sulfur atom and a methionine sulfur atom with a distorted tetrahedral geometry. Since this protein has an overall positive charge (p/ = 10.6), it gives a well formed response for the Cu(II)/Cu(I) reduction ( ,o =+0.32 V vs. NHE) simply using a glassy carbon... 

Fig. 58. Stereo drawing of the rubredoxin backbone with the iron (filled circle) and its cysteine sulfur ligands and all the water molecules (open circles) identified during refinement of the structure at 1.2 A resolution. Adapted from Watenpaugh et al. (1979), Fig. 11, with permission.

K V. A sulfiirtransferase is required in the transfer of cysteine sulfur in the in vitro synthesis of molybdopterin from precursor Z in Escherichia coli, J Biol Chem 2001, 276, 22024-22031. 

Iron-sulfur proteins serve predominantly as electron carriers (28,29). The best understood examples are those proteins with IFe, 2Fe, and 4Fe centers. The environment of the mononuclear iron center, rubredoxin, is shown in structure C (17). It consists of a distorted tetrahedral array of sulfur atoms from cysteine residues at nearly equal distances from the iron atom. Crystal structures are available for 2Fe-2S ferredoxins from Spindina plantensis (19) and Aphanothece sacrum (20). A representation of the geometry of this site is given in structure D. The 2Fe-2S core is anchored to the polypeptide by ligation to 4 cysteine sulfur atoms, yielding distorted tetrahedral geometry for both iron atoms. Crystal... 

Arrangement of subunits in nitrite reductase from Achromohacter cycloclastes. Domains are denoted D1 and D2, copper sites are shaded spheres, and Cu ligands are denoted by one-letter abbreviations C for cysteine sulfur and H for histidine imidazole nitrogen. From Fenderson et al. (1991). 

The theoretical assignments of the first two peaks are the same as those suggested by Helton et al. (172). A number of transitions were found to contribute to the higher energy part of the spectrum between 24,000 and 30,000 cm-1. The negative MCD at the high end of the spectrum is assigned to another LMCT transition from the cysteine sulfur. 

Tests for carboxylic acids were made by paper chromatography using 95% ethanol (100 ml.) and concentrated ammonium hydroxide (1 ml.) as solvent and aqueous bromothymol blue as indicator (1). Tests for pyridines were made on their hydrochlorides using butanol-.acetic acid water (4 1 5) as solvent and ammoniacal iron chloride or cysteine-sulfuric acid as indicators. Phenol tests were made using the same solvent and ammoniacal silver nitrate as indicator. Preliminary separations by gas chromatography were attempted using a 15-foot silicone gum column and a hydrogen flame attachment. 

For +2 cations such as zinc(ll) and cadmiunXIl) each metallothionem molecule contains up to seven metal atoms. X-ray studies indicate that the metal atoms are in approximately tetrahedral sites bound to the cysteine sulfur atoms. The soft mer-cury(JI) ion has a higher affinity for sulfur and will displace cadmium from mefallothio-nein. At first the mercury ions occupy tetrahedral sites but as the number increases, the geometries of the metal sites and protein change until about nine Hg(II) atoms are bound in a linear (S—Hg—S) fashion.92 Up to twelve + I cations such as copper(l) find silverfUcan bind per molecule, indicating fi coordination number lower than four, probably three (see Problem 12.34). 

The N-terminal domain (residues 1-29) contains one Cd2+ and two Zn2 and nine cysteine sulfurs which bind the metal ions. Three of the sulfur atoms form bridges between pairs of metals. The second cluster contains four Cd2+ held by 11 cysteine sulfur atoms,... 

Another group of related electron carriers, the high-potential iron proteins (HIPIP) contain four labile sulfur and four iron atoms per peptide chain 261-266 X-ray studies showed that the 86-residue polypeptide chain of the HIPIP of Chromatium is wrapped around a single iron-sulfur cluster which contains the side chains of four cysteine residues plus the four iron and four sulfur atoms (Fig. 16-15D)261 This kind of cluster is referred to as [4Fe-4S], or as Fe4S4. Each cysteine sulfur is attached to one atom of Fe, with the four iron atoms forming an irregular tetrahedron with an Fe-Fe... 

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