Cytochrome reductases

NADH-cytochrome reductase NADH NAD + FAD Fe respira-tory chain cytochrome b ... 

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. 

The iron of Hb must be maintained in the ferrous state ferric iron is reduced to the ferrous state by the action of an NADH-dependent methemoglobin reductase system involving cytochrome reductase and cytochrome b. ... 

Reduced cytochrome is then regenerated by the action of cytochrome reductase ... 

Jaffe ER, Hultquist DE Cytochrome reductase deficiency and enzymopenic hereditary methemoglobinemia. In The Metabolic and Molecular Bases of Inherited Disease, 8th ed. Scriver CR et al (editors). McGraw-Hill, 2001. 

Improving functionality may involve a complex biocatalytic system including more than one biocatalyst, as it is the case in BDM reactions. Additionally to the BDS for instance, another biocatalyst active for BDM [395,406], which consists of a heme oxygenase or a Cytochrome reductase could be used to widen up the functionality. 

In a recent investigation to develop novel cytochrome P450 biocatalysts, DNA shuffling was used to produce chimeric cytochrome P450s mutants with enhanced biocatalytic activities, which were then co-expressed with NADPH-cytochrome reductase in E. coli to form an efficient system, in this case demonstrated to be effective for indole oxidation [69]. 

Based on crystallographic observations it was suggested that the HA intermediate is bound to the cytochrome reductase via the iron atom, Fe(II)—NH2OH, and undergoes subsequent reduction to produce the NH3 that then dissociates from the protein ". It is of interest that the specific activity of cytochrome c-nitrite reductase from S. deleyianum in the conversion of N02 to NH3 is only 2-fold greater than that recorded for the conversion of HA to ammonia by the same enzyme, an observation that strongly supports the involvement of HA as an intermediate in the catalytic reduction of nitrite to NH3 . 

Oxidation of a-amino acids to keto acids catalysed by D- and L-amino acid oxidases Oxidation of NADH via the cytochrome system catalyzed by cytochrome reductase Energy production via the TCA or Krebs cycle catalyzed by succinate dehydrogenase Fatty acid oxidation catalyzed by acyl-coenzyme A dehydrogenases Synthesis of fatty acids from acetate (80,81)... 

Synthesis in mammalian tissues of arachidonic acid from linoleic acid. The A5 and A6 desaturases are separate enzymes and are also different from the A9 desaturase (fig. 18.16). The mechanisms, however, seem to be the same, involving cytochrome b5 and cytochrome reductase. The enzymes for elongation of unsaturated fatty acid such as 18 3 to 20 3 occur on the endoplasmic reticulum. 

Figure 3. Changes of hepatic cytochrome reductase content and hexobarbital sleeping time with fibrosarcoma growth.

Microsomes contain, in addition to the two cytochrome reductases just discussed, a flavoprotein which catalyzes the mixed function oxidation of secondary and tertiary amines to the hydroxylamines and amine oxides, respectively (333, 334). This flavoprotein, which contains about 2 moles of phospholipid and 1 mole of FAD per 70,000 g of protein, is specific for NADPH (333, 334) The enzyme is also able to catalyze the further oxidation of the hydroxylamines to nitrones (336). The reactions... 

Hovm ller, S., Leonard, K., and Weiss, H., 1981, Membrane crystals of a subunit complex of mitochondrial cytochrome reductase containing the cytochromes b and cl, FEBS Lett. 123 118nl22. 

Karlsson, B., Hovmoller, S., Weiss, H., and Leonard, K., 1983, Structural studies of cytochrome reductase. Subunit topography determined by electron microscopy of membrane crystals of a subcomplex, J. Mol. Biol. 165 2878302. 

The second of the three proton pumps in the respiratory chain is Q-cytochrome c oxidoreductase (also known as Complex III and cytochrome reductase). A cytochrome is an electron-transferring protein that contains a heme prosthetic group. The iron ion of a cytochrome alternates between a reduced ferrous (+2) state and an oxidized ferric (+3) state during electron transport. The function of Q-cytochrome c oxidoreductase is to catalyze the transfer of electrons from QH2 to oxidized cytochrome c (cyt c), a water-soluble protein, and concomitantly pump protons out of the mitochondrial matrix. 

Like cytochrome reductase in mitochondria, this complex runs die Q-cycle, which delivers the two electrons from one PQH2 to two plastocyanin molecules, which only carry one electron each. For each molecule of PQH2 that arrives at the complex, one electron is sent back to another PQ, whilst the other is passed on down the chain, to a copper-containing protein called plastocyanin. Two rounds of this pump across 4 protons, and produce two reduced plastocyanin molecules. 

Two dehydrogenases and a cytochrome carry the electrons from cellular NADPH and NADH to the terminal oxidase, cytochrome P-450. This cytochrome catalyzes the hydroxylation of a substrate. The reaction scheme has been described by Peterson et al. (1973). Fpj and Fpa (see Fig. 28) are the two flavoproteins NADPH cytochrome P-450 reductase and NADH cytochrome reductase. 

KC1 gradient (right ordinate). In these fractions, neither NADPH cytochrome reductase activity, nor cytochrome bs could be detected. From Balny et al. (1975). Reprinted with permission of Analytical Biochemistry. Copyright by Academic Press. 

The successful solution of the many bacterial cytochrome crystal structures was largely due to the simple fact that they are soluble. Mammalian microsomal P450s, on the other hand, are membrane bound and hence insoluble. The A -terminal region of the microsomal isoforms is believed to form a single membrane-spanning a-helix that tethers it close to the other protein in the system, the NADP/H dependent cytochrome reductase. This reductase is the source of electrons for the catalytic mechanism described earlier. 

The mechanism for NADH-cytochrome reductase described in Section VII,B was worked out with the soluble enzyme, and the question of its applicability to the interaction of the amphipathip proteins can now be considered. 

Methemoglobinemia has long been associated with the absence of an NADH-linked diaphorase (363, 363). However, flavoproteins isolated from the red cell were never sufficiently active to account for the methemoglobin reductase activity calculated to be necessary. It has now been shown that a methemoglobin reductase system of high activity is composed of soluble forms of cytochrome reductase and cytochrome bs... 

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