Cytochrome substrate

However, there are certain properties for which no modeling attempts have been made due to the lack of the appropriate experimental data. Cytochrome substrate specificity or phase II metabolism are two examples. 

Under certain circumstances cytochromes P-450 produces hydrogen peroxide. This seems to be when the cycle becomes uncoupled and the oxygenated P-450 complex breaks down differently to give the oxidized cytochrome-substrate complex and hydrogen peroxide. 

CYP oxidation reactions involve a complex series of steps that have been well defined Rose and Hodgson, 2004). The initial step involves the binding of substrate to oxidized CYP, followed by a one-electron reduction catalyzed hy NADPH cytochrome P450 reductase to form a reduced cytochrome-substrate complex. The next several steps involve interaction with molecular oxygen, the acceptance of a second electron from NADPH cytochrome P450 reductase or NADH cytochrome b reductase, followed by the subsequent release of water and the oxygenated product of the reaction. This complicated reaction sequence results in the transfer of one atom of molecular oxygen to the substrate while the other atom is reduced to water. 

The other common avenue of investigation is through mechanistic studies - through the application of basic science in the laboratory, can we understand the mechanism Knowing, for example, that an adverse effect mostly occurs in people who are poor metab-olisers of specific hepatic cytochrome substrates could be important in developing preventive measures. 

Heme-coupled monooxygenases contain cytochrome P-450. They are present in microsomes and are responsible for many hydroxylation reactions, e.g. llp-hydroxylation of steroids in the adrenal cortex, 2-hydroxylation of estrc ens in the liver the liver system is especially important in the hydroxylation of drugs and xenobiotics, thus rendering them water soluble, capable of conjugation and easily excreiable. A cytochrome P-450 system responsible for the hydroxylation of camphor (a 5-exo-hydroxylase) has been purified from Pseudomonas putida, and named putidaredoxin it contains FAD, an Fc2S2CyS4 center, and a P-4S0 cytochrome substrate hydroxylation is coupled to the oxidation of NADPH. 

Liidemann et al., 1997] Liidemann, S. K., Carugo, O., and Wade, R. C. Substrate access to cytochrome P450cam A comparison of a thermal motion pathway analysis with moleculM dynamics simulation data. J. Mol. Model. 3 (1997) 369-374... 

Metabolism is still a barrier to be overcome. Some QSAR, pharmacophore, protein, and rule-based models are available to predict substrates and inhibitors of a specific cytochrome P450 isoenzyme [47-55]. 

N—Fe(IV)Por complexes. Oxo iron(IV) porphyrin cation radical complexes, [O—Fe(IV)Por ], are important intermediates in oxygen atom transfer reactions. Compound I of the enzymes catalase and peroxidase have this formulation, as does the active intermediate in the catalytic cycle of cytochrome P Q. Similar intermediates are invoked in the extensively investigated hydroxylations and epoxidations of hydrocarbon substrates cataly2ed by iron porphyrins in the presence of such oxidizing agents as iodosylbenzene, NaOCl, peroxides, and air. 

S Modi, MI Paine, MI Sutcliffe, L-Y Lian, WU Pnmi-ose, CR Wolfe, GCK Roberts. A model for human cytochrome P450 2d6 based on homology modeling and NMR studies of substrate binding. Biochemistry 35 4540-4550, 1996. 

In the endoplasmic reticulum of eukaryotic cells, the oxidation of the terminal carbon of a normal fatty acid—a process termed ch-oxidation—can lead to the synthesis of small amounts of dicarboxylic acids (Figure 24.27). Cytochrome P-450, a monooxygenase enzyme that requires NADPH as a coenzyme and uses O, as a substrate, places a hydroxyl group at the terminal carbon. Subsequent oxidation to a carboxyl group produces a dicarboxylic acid. Either end can form an ester linkage to CoA and be subjected to /3-oxidation, producing a... 

This impressive reaction is catalyzed by stearoyl-CoA desaturase, a 53-kD enzyme containing a nonheme iron center. NADH and oxygen (Og) are required, as are two other proteins cytochrome 65 reductase (a 43-kD flavo-protein) and cytochrome 65 (16.7 kD). All three proteins are associated with the endoplasmic reticulum membrane. Cytochrome reductase transfers a pair of electrons from NADH through FAD to cytochrome (Figure 25.14). Oxidation of reduced cytochrome be, is coupled to reduction of nonheme Fe to Fe in the desaturase. The Fe accepts a pair of electrons (one at a time in a cycle) from cytochrome b and creates a cis double bond at the 9,10-posi-tion of the stearoyl-CoA substrate. Og is the terminal electron acceptor in this fatty acyl desaturation cycle. Note that two water molecules are made, which means that four electrons are transferred overall. Two of these come through the reaction sequence from NADH, and two come from the fatty acyl substrate that is being dehydrogenated. 

FIGURE 25.14 The conversion of stearoyl-CoA to oleoyl-CoA in eukaryotes is catalyzed by stearoyl-CoA desaturase in a reaction sequence that also involves cytochrome -65 and cytochrome -65 reductase. Two electrons are passed from NADH through the chain of reactions as shown, and two electrons are also derived from the fatty acyl substrate. 

Like the examples above, dihydroxyacetanilide epoxidase (DHAE) uses an olefin as the substrate for epoxidation. Its mechanism, however, is fundamentally different from those of cytochrome P450 or flavin-dependent enzymes. Dihydroxyacetanilide is an intermediate in the biosynthesis of the epoxyquinones LL-C10037a, an antitumor agent produced by the actinomycete Streptomyces LL-C10037 [75, 76], and MM14201, an antibiotic produced by Streptomyces MPP 3051 (Scheme 10.20) [77]. The main structural difference between the two antibiotics lies in the opposite stereochemistry of the oxirane ring. 

Atovaquone, a hydroxynaphthoquinone, selectively inhibits the respiratory chain of protozoan mitochondria at the cytochrome bcl complex (complex III) by mimicking the natural substrate, ubiquinone. Inhibition of cytochrome bcl disrupts the mitochondrial electron transfer chain and leads to a breakdown of the mitochondrial membrane potential. Atovaquone is effective against all parasite stages in humans, including the liver stages. 

Active caspases 8, 9 and 10 can convert caspase-3, the most abundant effector caspase from its pro-form to its active cleaved form. Cleavage of a number of different substrates by caspase-3 and also by caspase-6 and -7 which are two other executioner caspases besides caspase-3 then results in the typical morphology which is characteristic of apoptosis. Yet, the activation of caspase-3 and also of caspase-9 can be counteracted by IAPs, so called inhibitor of apoptosis proteins. However, concomitantly with cytochrome C also other proteins are released from mitochondria, including Smac/DIABLO. Smac/DIABLO and potentially other factors can interact with IAPs and thereby neutralize their caspase-inhibitory activity. This releases the breaks on the cell death program and allows apoptosis to ensue. 

Cytochrome P450 (CYP) mono-oxygenases, also called mixed function oxidases, are versatile hemoprotein enzymes that catalyze the cleavage of molecular oxygen to incoiporate one oxygen atom into a substrate molecule and one atom into water [1]. The general stoichiometry of the reaction is as follows (S-H, substrate) ... 

The numerous biotransformations catalyzed by cytochrome P450 enzymes include aromatic and aliphatic hydroxylations, epoxidations of olefinic and aromatic structures, oxidations and oxidative dealkylations of heteroatoms and as well as some reductive reactions. Cytochromes P450 of higher animals may be classified into two broad categories depending on whether their substrates are primarily endogenous or xenobiotic substances. Thus, CYP enzymes of families 1-3 catalyze... 

Although only two protons are pumped out of the matrix, two others from the matrix are consumed in the formation of H2O. There is therefore a net translocation of four positive charges out of the matrix which is equivalent to the extrusion of four protons. If four protons are required by the chemiosmotic mechanism to convert cytosolic ADP + Pj to ATP, then 0.5 mol ATP is made for the oxidation of one mol of ubiquinol and one mol ATP for the oxidation of 2 mols of reduced cytochrome c. These stoichiometries were obtained experimentally when ubiquinol was oxidized when complexes I, II, and IV were inhibited by rotenone, malonate, and cyanide, respectively, and when reduced cytochrome c was oxidized with complex III inhibited by antimycin (Hinkle et al., 1991). (In these experiments, of course, no protons were liberated in the matrix by substrate oxidation.) However, in the scheme illustrated in Figure 6, with the flow of two electrons through the complete electron transport chain from substrate to oxygen, it also appears valid to say that four protons are extmded by complex I, four by complex III, and two by complex 1. 

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