Cytochromes apoprotein

In the case of the hemochromogen, cytochrome c, the iron coordinates probably with two imidazole N atoms of histidine so that O2 cannot attach to the iron atom. The imidazole bonds to the iron are relatively weak so that it is necessary to stabilize these bonds. Theorell found that there is a direct chemical thioether linkage from the side chains of the porphyrin to the cytochrome apoprotein this would appear to constrain the two N atoms of imidazoles to lie in a correct spatial configuration for coordination with the iron, when the iron is in the ferrous or in the ferric state. Since the imidazole rings are probably in resonance with the porphyrin ring it would seem likely that electron transfer may occur on collision of a 1-electron donor or acceptor with either a portion of the imidazole ring or a portion of the porphyrin ring. 

Mitochondrial DNA is inherited maternally. What makes mitochondrial diseases particularly interesting from a genetic point of view is that the mitochondrion has its own DNA (mtDNA) and its own transcription and translation processes. The mtDNA encodes only 13 polypeptides nuclear DNA (nDNA) controls the synthesis of 90-95% of all mitochondrial proteins. All known mito-chondrially encoded polypeptides are located in the inner mitochondrial membrane as subunits of the respiratory chain complexes (Fig. 42-3), including seven subunits of complex I the apoprotein of cytochrome b the three larger subunits of cytochrome c oxidase, also termed complex IV and two subunits of ATPase, also termed complex V. 

Hutzler, J.M., Steenwyk, R.C., Smith, E.B., Walker, G.S. and Wienkers, L.C. (2004) Mechanism-based inactivation of cytochrome P450 2D6 by l-[(2-ethyl-4-methyl-1 H-imidazol-5-yl) methyl]-4-[4-(trifluoromethyl)-2-pyridinyl]piperazine kinetic characterization and evidence for apoprotein adduction. Chemical Research in Toxicology, 17 (2), 174—184. 

The analysis of the cytochrome crystal structures reveals extensive differences among the mammalian and the human CYPs that may reflect the structural flexibility of these enzymes and therefore the broad substrate specificity observed. In general, the secondary elements and the overall structure of the CYP are conserved. Nevertheless, it seems that in most enzymes, the most flexible regions are located between the F-G helixes and the B-C loop. For example in the CYP2B4 apoprotein (lpo5) the... 

The cytochromes P-450 monooxygenase system is actually a collection of isoenzymes, all of which possess an iron protoporphyrin IX as the prosthetic group. The monomer of the enzyme has a molecular weight of 45,000 to 55,000. The enzyme is membrane bound within the endoplasmic reticulum. Cytochromes P-450 are closely associated with another vital component of the system, NADPH cytochrome P-450 reductase. This is a flavoprotein, which has 1 mol of FAD and 1 mol of FMN per mol of apoprotein. The monomeric molecular weight of the enzyme is 78,000. The enzyme transfers two electrons to cytochromes P-450, but one at a time. There only seems to be one reductase, which serves a group of isoenzymes of cytochromes P-450, and consequently, its concentration is 1/10 to 1/30 that of cytochromes P-450. 

As well as induction of the synthesis of the apoprotein portion of cytochrome P-450, there is also induction of the synthesis of the heme portion. Clearly, it is also necessary to have an increased amount of heme if there is an increase in the amount of the enzyme apoprotein being synthesized. Thus, the rate-limiting step in heme synthesis, the enzyme 5-aminolaevulinate synthetase, is inducible by both phenobarbital and TCDD. This is the result of transcriptional activation of the gene, which codes for the S-aminolaevulinate synthetase. It may be that the decrease in the heme pool, which results from incorporation of heme into the newly synthesized apoprotein, leads to derepression of the gene and hence increased mRNA synthesis. The gene repression could be heme-mediated, or heme may modulate P-450 genes. 

Zgoda VG, Karuzina II, Archakov AI (1999) Heme and apoprotein modification of cytochrome P450 2B4 during its oxidative inactivation in monooxigenase reconstituted system. Free Radic Biol Med 26 620-632... 

Keeping in mind that all three x-rayed flavodoxins (7, 8, 9, 10, 11) exhibit C(7,8) as the only possible point of direct outer contact while all the rest of the flavin molecule is buried in the protein, we must finally admit that chemically C(8) is the reasonable site of orbital overlap between flavin and secondary heteroaromatic le -donor-acceptor molecules such as flavin itself and, above all, cytochrome. Flavodoxins represent the case of flavoproteins scheduled for le -only transfer by a hydrogen bond directed from the apoprotein towards N(5), stabilizing the radical Hp. The bond is strong enough to maintain the active protein reduced in the lower le -shuttle between Flred and Fl. 

Cytochrome oxidase contains two haem groups and two protein-bound coppers per minimal functional unit, i.e., the monomer (Table 3.4). Apparently a single copy of most polypeptides is present in this monomer. The haems are chemically identical (haem A), but are bound quite differently to the protein, which gives them widely different functions and spectroscopic properties (Table 3.4). The same is true of the two coppers, Cu and Cug. The haems will be called a and a, respectively. As discussed below, it is not certain whether they have different apoprotein parts, i.e., whether they are formally different cytochromes . 

The sersatility of cytochrome P-450 in carrying out a vari-ciy of oxidation reactions on a multitude of substrates may be altrihutahle to the multiple forms of the enzyme. Con.se-quently. the student must realize that the biotransformation Ol a parent xenohiotic to several oxidized metabolites is carried out not just by one form of P-450 but, more likely, hy several different forms. Extensive. studies indicate that the jpoprutein portions of various cytiK-hrome P-450s differ Ihinione another in their tertiary structure (because of differences in amino acid. sequence or the makeup of the polypeptide chain). Because the apoprotein portion is im-... 

These cytochromes each contain heme as a prosthetic group but have different apoproteins. 

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