Hemes electron transfer

For the identification of low-spin iron(Il), a low value of the quadrupole splitting is generally not enough. As an example, the Mossbauer spectrum of the reduced low-spin ferrous form of cytochrome C552 is shown in Figure 10. The reduced heme iron has 5 = 0.46 nun s and AEq = -Fl.30mms . These values are characteristic for the low-spin ferrous forms of several electron transfer heme proteins like cytochromes c and... 

Complex heme proteins which contain redox centers other than just electron transfer hemes are normally associated with enzymatic catalysis. In this section, we will refer to some of the best studied complex heme proteins sulfite reductase, nitrite reductase, formate dehydrogenase, fumarate reductase and rubredoxin-oxygen oxidoreductase. 

The hemes are located in different enzyme domains with structures similar to those of class I cytochromes c. The LP heme or peroxidatic heme is in the N-terminal domain (between residues 17 and 164). In the oxidized state (the only state for which crystallographic data are available) the LP heme is in a low-spin configuration with (His 57-His 71) coordination. The residues 51 to 55 form the heme binding sequence (Cys-X-Y-Cys-His). The HP heme (the electron transfer heme) is included in the C-terminal domain, which covers residues 165 to 302. This heme... 

The model for calcium binding proposed by Gilmour et al. [17] (Figure 6-3) anticipates that the purified enzyme is a mixture of oxidized and inactive species A and B. Species A has all the binding sites for Ca " " filled. In order to activated this species it is only necessary to reduce the electron transfer heme. Species B, monomeric, has lost calcium from site II and needs two activation steps binding of the... 

In the diamagnetic region it is possible to identify a resonance with three-proton intensity at -3.09 ppm which was attributed to the axial methionine. This result is once more compatible with a Met-His coordination for the HP heme. The H-NMR spectrum of the oxidized and calcium-depleted CCP shows important modifications both at the level of the electron transfer heme as well as the peroxidatic heme (Figure 6-5A). The resonances of both hemes are at different positions from those observed in the native enzyme, and very broad resonances are also present and not resolved underneath the resonances of both hemes. These results reflect the possibility that when Ca " " is removed, the environment of both hemes is perturbed and probably the global conformation of the enzyme as well. 

In the case of horse heart cytochrome c, the effects of CCP binding on the heme methyl resonances are much smaller (Figure 7-11) and resemble those observed in the presence of a polyanion such as polyglutamate (not shown). In the reciprocal experiment, the methyl resonances of the electron transferring heme are perturbed by addition of horse heart cytochrome c while those of the peroxidatic heme are not. However, the observed pattern differs from that observed with cytochrome... 

Cytochrome (cyt b ) is a 134 amino acid membrane-bound electron transfer heme protein that is anchored to the ER membrane by its COOH... 

Molybdenum. Molybdenum is a component of the metaHoen2ymes xanthine oxidase, aldehyde oxidase, and sulfite oxidase in mammals (130). Two other molybdenum metaHoen2ymes present in nitrifying bacteria have been characteri2ed nitrogenase and nitrate reductase (131). The molybdenum in the oxidases, is involved in redox reactions. The heme iron in sulfite oxidase also is involved in electron transfer (132). 

The most conspicuous use of iron in biological systems is in our blood, where the erythrocytes are filled with the oxygen-binding protein hemoglobin. The red color of blood is due to the iron atom bound to the heme group in hemoglobin. Similar heme-bound iron atoms are present in a number of proteins involved in electron-transfer reactions, notably cytochromes. A chemically more sophisticated use of iron is found in an enzyme, ribo nucleotide reductase, that catalyzes the conversion of ribonucleotides to deoxyribonucleotides, an important step in the synthesis of the building blocks of DNA. 

The electron on the bj heme facing the cytosolic side of the membrane is now passed to the bfj evcie on the matrix side of the membrane. This electron transfer occurs against a membrane potential of 0.15 V and is driven by the loss of redox potential as the electron moves from bj = — O.IOOV) to bn = +0.050V). The electron is then passed from bn to a molecule of UQ at a second quinone-binding site, Q , converting this UQ to UQ . The result-... 

FIGURE 21.17 The electron transfer pathway for cytochrome oxidase. Cytochrome c binds on the cytosolic side, transferring electrons through the copper and heme centers to reduce O9 on the matrix side of the membrane. 

Electron Transfer in Complex IV Involves Two Hemes and Two Copper Sites... 

Cytochrome c oxidase contains two heme centers (cytochromes a and %) as well as two copper atoms (Figure 21.17). The copper sites, Cu and Cug, are associated with cytochromes a and respectively. The copper sites participate in electron transfer by cycling between the reduced (cuprous) Cu state and the oxidized (cupric) Cu state. (Remember, the cytochromes and copper sites are one-electron transfer agents.) Reduction of one oxygen molecule requires passage of four electrons through these carriers—one at a time (Figure... 

FIGURE 22.18 Model of the R. viridis reaction center, (a, b) Two views of the ribbon diagram of the reaction center. Mand L subunits appear in purple and blue, respectively. Cytochrome subunit is brown H subunit is green. These proteins provide a scaffold upon which the prosthetic groups of the reaction center are situated for effective photosynthedc electron transfer. Panel (c) shows the spatial relationship between the various prosthetic groups (4 hemes, P870, 2 BChl, 2 BPheo, 2 quinones, and the Fe atom) in the same view as in (b), but with protein chains deleted. 

In the ci positional state, fast electron transfer from the Rieske protein to cytochrome Ci will he facilitated hy the close interaction and by the hydrogen bond between His 161 of the Rieske protein and a propionate group of heme Ci, but the Rieske cluster is far away from the quinone binding site. 

In the b positional state, The Rieske cluster can interact with quinone bound in the reaction pocket, but the distance to heme Ci is too large (>30 A) to allow fast electron transfer. 

After the second electron transfer from semiquinone to heme 6l (step 4), the interaction between the Rieske cluster and the resulting quinone is weakened so that the reduced Rieske protein can now occupy the preferred ci positional state (E), which allows rapid electron transfer from the Rieske cluster to heme Ci (step 5). 

It has always been assumed that these simple proteins act as electron-transfer proteins. This is also a fair conclusion if we take in account that different proteins were isolated in which the Fe(RS)4 center is in association with other non-heme, non-iron-sulfur centers. In these proteins the Fe(RS)4 center may serve as electron donor/ac-ceptor to the catalytic site, as in other iron-sulfur proteins where [2Fe-2S], [3Fe-4S], and [4Fe-4S] clusters are proposed to be involved in the intramolecular electron transfer pathway (see the following examples). 

These enzymes may contain other redox-active sites (iron-sulfur centers, hemes, and/or flavins), either in distinct domains of a single polypeptide or bound in separate subunits. These additional cofactors perform electron transfer from the molybdenum center to an external electron acceptor/donor. 

The electron transfer properties of the cytochromes involve cycling of the iron between the +2 and +3 oxidation states (Cytochrome)Fe + e" (Cytochrome)Fe ° = -0.3Vto+ 0.4V Different cytochromes have different side groups attached to the porphyrin ring. These side groups modify the electron density in the delocalized iz system of the porphyrin, which in turn changes the redox potential of the iron cation in the heme. 

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