Heme in cytochrome

Fig. 8. ET pathways from His33 and His39 to the heme in cytochrome c. Edge-edge distances are as follows His39 to the heme, 13.0 His33 to HislS, 11.7 His33 to the heme 13.2 A [47]...

This general approach has, however, serious limitations. The position of the site for attack (and therefore the electron transfer distance involved) is very conjectural. In addition, the vexing possibility, which we have encountered several times, of a dead-end mechanism (Sec. 1.6.4) is always present. One way to circumvent this difficulty, is to bind a metal complex to the protein at a specific site, with a known (usually crystallographic) relationship to the metal site. The strategy then is to create a metastable state, which can only be alleviated by a discernable electron transfer between the labelled and natural site. It is important to establish that the modification does not radically alter the structure of the protein. A favorite technique is to attach (NH3)5Ru to a histidine imidazole near the surface of a protein. Exposure of this modified protein to a deficiency of a powerful reducing agent, will give a eon-current (partial) reduction of the ruthenium(III) and the site metal ion e.g. iron(III) heme in cytochrome c... 

In photosynthesis radical-ions and triplet states of the pigments, radical-pairs and biradicals involving various chlorophylls and quinones, amino acid radicals, hemes in cytochromes, metal clusters of low and higher nuclearity and even coupled metallo-radical species have been observed. Thus the field of photosyn-... 

In contrast to the flavin oxidases, flavin dehydrogenases pass electrons to carriers within electron transport chains and the flavin does not react with 02. Examples include a bacterial trimethylamine dehydrogenase (Fig. 15-9) which contains an iron-sulfur duster that serves as the immediate electron acceptor167 169 and yeast flavocytochrome b2, a lactate dehydrogenase that passes electrons to a built-in heme group which can then pass the electrons to an external acceptor, another heme in cytochrome c.170-173 Like glycolate oxidase, these enzymes bind their flavin coenzyme at the ends of 8-stranded a(i barrels similar... 

Once the Ax values and the molecular directions are obtained, the metal centered pseudocontact shifts can be calculated of nuclei which experience also contact and ligand centered pseudocontact shifts. With this procedure the contact plus ligand centered pseudocontact shift (which is small [100]) have been calculated for several systems. In Table 2.11 the data relative to the low spin iron(III) containing heme in cytochrome b are reported [101]. 

Plate 18 Dotted surface displays of heme in cytochrome b5 (PDB 3b5c). Smaller van der Waals surface encloses heme completely. Small outer dotted surface is the solvent-accessible surface of the heme. Most of the heme surface is buried within the protein. (For discussion, see Chapter 11.) Image SPV/POV-Ray. 

Some enzymes require cofactors for the activities Enzymes that require covalent cofactors (prosthetic groups, e.g., heme in cytochromes) or non-covalent cofactors (coenzymes, e.g., NAD(P)+ in dehydrogenases) for activities are called haloenzymes (or simply enzymes). The protein molecule of a haloenzyme is termed proenzyme. The prosthetic group/coenzyme dictates the reaction type catalyzed by the enzyme, and the proenzyme determines the substrate specificity. 

Figure 14.8 Plot of k2 against the distance of tryptophan residues from the heme in cytochrome P-450. The dotted lines in the figure show the crystallographic distances between tryptophan residues, and lines labeled with rx and show the calculated distances between the heme and different tryptophan residues using all possible values of tc2 (0.0-4.0). The point where these two lines intersect shows the k2 value for the particular residue and is indicated by the arrows. Source of figures 11.22 and 11.23 Khan, K.K., Mazumdar, S., Modi, S., Sutcliffe, M., Roberts, G.C.K. and Mitra, S. (1997). European Journal of Biochemistry, 244, 361-370.

Wang K, Mei H, Geren L, et al. Design of a ruthenium-cytochrome c derivative to measure electron transfer to the radical cation and oxyferryl heme in cytochrome c peroxidase. Biochemistry 1996 35 15107-19. 

As mentioned earlier, the copper containing nitrite reductase is a trimer of identical subunits. In each subunit there is a type 1 copper which acts analogously to the c-type heme in cytochrome cd and thus is the point of entry of electron into the enzyme. The three eatalytic sites have type 2... 

Hoch U, Ortiz De Montellano PR. Covalently linked heme in cytochrome P4504A fatty acid hydroxylases. J. Biol. Chem. 2001 276 11339-11346. 

There are two heme A molecules, called heme a and heme in cytochrome c oxidase. Heme A differs from the heme in cytochrome c and C in three ways (1) a formyl group replaces a methyl group, (2) a Ci 7 hydrocarbon chain replaces one of the vinyl groups, and (3) the heme is not covalently attached to the protein. 

As the protein sequence is covalently attached to the heme in cytochrome c, digestion by pepsin gives rise to a heme undecapeptide, and following by tryptic digestion decreases the length of the attached peptide from eleven residues to eight residues, i.e., to a heme octapeptide [12]. These two peptides aggregate in aqueous... 

Table 4. Order of reduction of the four hemes in cytochrome c3 from several organisms. The numbering of the hemes is done according to their position in the primary structure. Hemes I and II of D. desulfuricans (27774) have very similar reduction potentials, and the same is observed for hemes I and IV of Dsm. baculatum...

The efficiency of heme peroxidase catalysis also depends on the rates of reduction of compounds I and II (equations 2 and 3). For both CCP (16) and HRP (11), the rate-limiting step under optimal conditions is that of Fe =0 reduction. Details of how heme proteins control the reactivity of Fe =0 catalytic intermediates are poorly understood. The Fe =0 catalytic intermediates of cytochrome P450 and cytochrome c oxidase are highly unstable (4, 17). In fact, substrate must already be bound close to the heme in cytochrome P45o(cam) before the catalytic redox cycle begins to ensure that substrate hy-... 

In solution, one MV ion can reduce a single heme site in cytochrome c or one of two in the oxidase. The spectra were recorded after 5-nanoequivalent increments of charge, and they indicate that one of the heme groups in cytochrome c oxidase is reduced first. Then reduces the heme in cytochrome c before it deals with the second heme of the oxidase. 

Cytochromes a and a3 contain heme A, in which two of the side chains are modified (Figure 15.6b). Cytochromes a and a3 evidently represent two identical heme A moieties, attached to the same polypeptide chain. They are within different environments in the inner membrane, however, so they have different reduction potentials. Each of the hemes in cytochromes a and a3 is associated with a copper ion, located close to the heme iron. Cytochromes undergo oxidoreduction through the complexed metal, which cycles between +2 and +3 states for the heme iron and +1 and +2 states for the copper in cytochromes a and a3. Thus, the cytochromes are one-electron carriers. Cytochromes a and a3 form part of mitochondrial complex IV (Figures 15.2 and 15.3). 

Figure 31. An electron transport system and redox potentials in mitochondria. FMN refers to Flavin mononucleotide in NADH2 dehydrogenase, FAD refers to Flavin adenine dinucleotide in succinate dehydrogenase, I, II, and III correspond to the reaction processes which may be involved in phosphorylation, Fe—S non-heme iron, Cyt Heme in cytochromes (after ref. 171).

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