Quinones prosthetic groups

A recently characterized class of dehydrogenases are the quinoproteins which contain a pyrroloquinolene quinone prosthetic group and do not require a separate co-factor Electron transfer mediators such as phenazine ethosulphate 2,6-dichloroindophenol and ferricenium ions have been used to recycle the quinoprotein the reduce mediator is detected amperometrically. 

PQQ and the other quinone prosthetic groups described here all function in reactions that would be possible for pyridine nucleotide or flavin coenzymes. All of them, like the flavins, can exist in oxidized, half-reduced semiquinone and fully reduced dihydro forms. The questions to be asked are the same as we asked for flavins. How do the substrates react How is the reduced cofactor reoxidized In nonenzymatic reactions alcohols, amines, and enolate anions all add at C-5 of PQQ to give adducts such as that shown for methanol in Eq. 15-51, step a 444,449,449a Although many additional reactions are possible, this addition is a reasonable first step in the mechanism shown in Eq. 15-51. An enzymatic base could remove a proton as is indicated in step b to give PQQH2. The pathway for reoxidation (step c) might involve a cytochrome b, cytochrome c, or bound ubiquinone.445 446... 

The electrochemistry of the PQQ (pyrrolo-quinoline quinone) prosthetic group has been investigated at poly(pyrrole)-coated electrodes, and PQQ has been entrapped within poly(pyrrole) films " good electrochemistry was observed in both cases. Poly(pyrrole) has also been used to entrap adenosine triphosphate (ATP) anions, again by growing the film in the presence of the anion, " and as an electrode material for oxidizing ascorbate. " In the latter case the oxidation... 

Oxidation of P-nicotinamide adenine dinucleotide (NADH) to NAD+ has attracted much interest from the viewpoint of its role in biosensors reactions. It has been reported that several quinone derivatives and polymerized redox dyes, such as phenoxazine and phenothiazine derivatives, possess catalytic activities for the oxidation of NADH and have been used for dehydrogenase biosensors development [1, 2]. Flavins (contain in chemical structure isoalloxazine ring) are the prosthetic groups responsible for NAD+/NADH conversion in the active sites of some dehydrogenase enzymes. Upon the electropolymerization of flavin derivatives, the effective catalysts of NAD+/NADH regeneration, which mimic the NADH-dehydrogenase activity, would be synthesized [3]. 

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. 

Peroxidases are found in milk and in leukocytes, platelets, and other tissues involved in eicosanoid metabolism (Chapter 23). The prosthetic group is protoheme. In the reaction catalyzed by peroxidase, hydrogen peroxide is reduced at the expense of several substances that will act as electron acceptors, such as ascorbate, quinones, and cytochrome c. The reaction catalyzed by peroxidase is complex, but the overall reaction is as follows ... 

Fructose dehydrogenase (FDH) having pyrroloquinoline quinone (PQQ) as a prosthetic group is an redox enzyme to catalyze the oxidation of fructose. A... 

Fig. 1. Prosthetic groups in oxidases (A FAD B Thio-Tyrosine C NAD(P) + D 6-Hydroxy-DOPA E Methoxanthin (Pyrroloquinoline quinone PQQ) F Tryptophane-Tryptophan quinone)...

Once the fungicides penetrate to the cell membrane or into the cytoplasm they may operate by devious means to disrupt vital functions. There is substantial evidence that the quinones immobilize the sultliydryl and imino prosthetic group of enzymes. The 8-hydroxyquinoline and dithiocarbamate compounds are active against copper and other metallic members of an enzyme system, presumably hy their ability to chelate metals. Heavy metals such as mercury alTect certain enzymes such as amylases and may serve as general protein precipitants. 

Pyrroloquinolinequinone (PQQ, 1) is one of a number of o-quinones which serve as prosthetic groups in quinoproteins, and has been the subject of intense interest because of its growth-stimulating, pharmaceutical and nutritional activities (c.f. Problem number 41). There is also considerable interest in the in vitro reactions of PQQ with biomolecules, and in this context the reaction of PQQ with L-tryptophan in phosphate buffer (pH 6.5) under aerobic conditions was investigated. This gave three products 2, 3 and 4, with 2 the major component. Compounds 2 and 4 showed a much more pronounced growth-stimulating effect than PQQ, while 3 also showed "a marked effect". 

Prosthetic groups, such as tetrapyrroles (Figure 1) and cofactors such as flavin-adenine dinucleotide (FAD) and pyrroloquinoline quinone (PQQ) (see Chapter 16). 

Mediators/redox dyes Phenolics/quinones Redox proteins Prosthetic groups Thiols/disulfides... 

MDH is a soluble quinoprotein which has pyrroloquinoline quinone (PQQ) as its prosthetic group and it uses a specifie eytoehrome, cytoehrome Cl as eleetron aeeeptor. It is usually assayed in a dye-linked system at high pH when ammonia is required as activator. It has an structure eaeh a... 

PYRROLO-QUINOLINE QUINONE (PQQ) THE PROSTHETIC GROUP OF METHANOL DEHYDROGENASE... 

Fig. 4. Arrangement of the prosthetic groups in the Rp. viridis. reaction center, redrawn from Ref. 101. Ob is shown at the site identified by Deisenhofcr et al. [102], but the orientation of the quinone in this site is drawn arbitrarily the exact orientation of Ob in the crystal structure has not been described. The four hemes at the top of the figure are in the cytochrome subunit the other components are in the L-M complex. As in Fig. 3, the normal to the chromatophore membrane is approximately vertical and the periplasmic side of the complex is at the top.

Figure 18.15. Structure of Q-Cytochrome C Oxidoreductase (Cytochrome BC j). This enzyme is a homodimer with "jljfo, 11 distinct polypeptide chains. The major prosthetic groups, three hemes and a 2Fe-2S cluster, mediate the electron-transfer reactions between quinones in the membrane and cytochrome c in the intermembrane space.

The electron carriers in the respiratory assembly of the inner mitochondrial membrane are quinones, flavins, iron-sulfur complexes, heme groups of cytochromes, and copper ions. Electrons from NADH are transferred to the FMN prosthetic group of NADH-Q oxidoreductase (Complex I), the first of four complexes. This oxidoreductase also contains Fe-S centers. The electrons emerge in QH2, the reduced form of ubiquinone (Q). The citric acid cycle enzyme succinate dehydrogenase is a component of the succinate-Q reductase complex (Complex II), which donates electrons from FADH2 to Q to form QH2.This highly mobile hydrophobic carrier transfers its electrons to Q-cytochrome c oxidoreductase (Complex III), a complex that contains cytochromes h and c j and an Fe-S center. This complex reduces cytochrome c, a water-soluble peripheral membrane protein. Cytochrome c, like Q, is a mobile carrier of electrons, which it then transfers to cytochrome c oxidase (Complex IV). This complex contains cytochromes a and a 3 and three copper ions. A heme iron ion and a copper ion in this oxidase transfer electrons to O2, the ultimate acceptor, to form H2O. 

Figure 19.9 Bacterial photosynthetic reaction center The core of the reaction center from Rhodopseudomonas viridls consists of two simitar chains L (red) and M (blue). An H chain (white) and a cytochrome subunit (yellow) complete the structure. Notice that the L and M subunits are composed largely of helices that span the membrane. Notice also that a chain of electron-carrying prosthetic groups, beginning with a special pair of bacteriochlorophylls and ending at a bound quinone. runs through the structure from top to bottom in this view. [Drawn from IPRC.pdb.]...

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