Protein quinone reactions

Protein-quinone reactions seem to have little influence on protein digestibility or on the bioavailability of tryptophan. Methionine does not appear to react covalently but may be extensively oxidized to its sulphoxide with some reduction in bioavailability. The reactions of cysteine were not investigated in our study because of the low level of cyst(e)ine in casein. Pierpoint (1969ab) has reported that cysteine reacts like lysine via a substitution into the quinone ring and it is also possible that cyst(e)ine residues may be oxidized (Finley and Lundin, 1980). 

The metabolites 2- and 4-bromophenol can also be metabolized by a further oxidation pathway to yield catechols and quinones, some of which are cytotoxic and potentially hepa to toxic. Thus, in vitro studies have indicated that bromo quinones and bromocatechols may be responsible for some of the covalent binding to protein and reaction with GSH. 

The three-dimensional structures of the reaction centers of purple bacteria (Rhodopseudomonas viridis and Rhodobacter sphaeroides), deduced from x-ray crystallography, shed light on how phototransduction takes place in a pheophytin-quinone reaction center. The R. viridis reaction center (Fig. 19-48a) is a large protein complex containing four polypeptide subunits and 13 cofactors two pairs of bacterial chlorophylls, a pair of pheophytins, two quinones, a nonheme iron, and four hemes in the associated c-type cytochrome. 

Covalent protein adducts of quinones are formed through Michael-type addition reaction with protein sulfhydryl groups or glutathione. Metabolic activation of several toxins (e.g., naphthalene, pentachlorophenol, and benzene) into quinones has been shown to result in protein quinone adducts (Lin et al., 1997 Rappaport et al., 1996 Zheng et al., 1997). Conversion of substituted hydroquinones such as p-aminophenol-hydroquinone and 2-hromo-hydroquinone to their respective glutathione S-conjugates must occur to allow bioactivation into nephrotoxic metabolites (Dekant, 1993). Western blot analysis of proteins from the kidneys of rats treated with 2-bromo-hydroquinone has revealed three distinct protein adducts conjugated to quinone-thioethers (Kleiner et al., 1998). 

Recent approach utilise isolated PSII complexes as biomediators. The PSII reaaion center was isolated in 1987 and the isolated complex was found to maintain its herbicide-binding ability. The binding affinity of the herbicide depends on the amino acid composition of the hydrophilic loop in the D1 protein. It is recognized that these herbicides act by binding D1 protein of reaction center. Two levels of action occun displacement of the secondary quinone electron transfer PQ and inhibition of D1 protein turnover. ... 

An especially important case is H-bonding of a paramagnetic molecule to the protein. In membrane-located proteins, quinone molecules play an important role in electron-transfer reactions. These quinone molecules can be bound to the protein by H-bonds or may diffuse through the membrane. By... 

Secondary photosynthetic electron transfer was detected after addition of artificial electron acceptors and donors, including a quinone dependent activit y on adding decylplastoquinone. An equivalent activity was obtained on addition of the much more hydrophobic plastoquinone-9 molecule but only when a reconstitution procedure was adopted in which a diacyl glycerolipid extract of thylakoids was used. Thermoluminescence measurements showed that reconstitution with plasto-quinone-9 and lipid involved the binding of the quinone to some of the reaction centres in a preparation. Thus a limited reconstitution of quinone-reaction centre interactions could be achieved without proteins other than those already present in the isolated reaction centre. 

The chloroplast bf complex proteins have great similarities of primary amino acid sequences, of predicted secondary structure and of redox centres, although the value of cytochrome f is approx. lOOmV higher, and the two b haems each around lOOmV lower, than their be complex counterparts. The enzyme has been shown to function electrogenically under some conditions with electron transfer behaviour and quinone reaction sites similar to those of the be complexes. 

Breton, J. and Nabedryk, E., Protein-quinone interactions in the bacterial photosynthetic reaction center light-induced FTIR difference spectroscopy of the quinone vibrations, Biochim. Biophys. Acta, 1275, 84, 1996. 

In the bacterial reaction center the photons are absorbed by the special pair of chlorophyll molecules on the periplasmic side of the membrane (see Figure 12.14). Spectroscopic measurements have shown that when a photon is absorbed by the special pair of chlorophylls, an electron is moved from the special pair to one of the pheophytin molecules. The close association and the parallel orientation of the chlorophyll ring systems in the special pair facilitates the excitation of an electron so that it is easily released. This process is very fast it occurs within 2 picoseconds. From the pheophytin the electron moves to a molecule of quinone, Qa, in a slower process that takes about 200 picoseconds. The electron then passes through the protein, to the second quinone molecule, Qb. This is a comparatively slow process, taking about 100 microseconds. 

FIGURE 22.17 The R. viridis reaction center is coupled to the cytochrome h/Cl complex through the quinone pool (Q). Quinone molecules are photore-duced at the reaction center Qb site (2 e [2 hv] per Q reduced) and then diffuse to the cytochrome h/ci complex, where they are reoxidized. Note that e flow from cytochrome h/ci back to the reaction center occurs via the periplasmic protein cytochrome co- Note also that 3 to 4 are translocated into the periplasmic space for each Q molecule oxidized at cytochrome h/ci. The resultant proton-motive force drives ATP synthesis by the bacterial FiFo ATP synthase. (Adapted from Deisenhofer, and Michel, H., 1989. The photosynthetic reaction center from the purple bac-terinm Rhod.opseud.omoaas viridis. Science 245 1463.)... 

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. 

Quinone methides have been shown to be important intermediates in chemical synthesis,1 2 in lignin biosynthesis,3 and in the activity of antitumor and antibiotic agents.4 They react with many biologically relevant nucleophiles including alcohols,1 thiols,5-7 nucleic acids,8-10 proteins,6 11 and phosphodiesters.12 The reaction of nucleophiles with ortho- and /iara-quinone methides is pH dependent and can occur via either acid-catalyzed or uncatalyzed pathways.13-17 The electron transfer chemistry that is typical of the related quinones does not appear to play a role in the nucleophilic reactivity of QMs.18... 

Bolton, J. L. Le Blanc, J. C. Y. Siu, K. W. M. Reaction of quinone methides with proteins analysis of myoglobin adduct formation by electrospray mass spectrometry. Biol. Mass... 

Schnurr et al. [22] showed that rabbit 15-LOX oxidized beef heart submitochondrial particles to form phospholipid-bound hydroperoxy- and keto-polyenoic fatty acids and induced the oxidative modification of membrane proteins. It was also found that the total oxygen uptake significantly exceeded the formation of oxygenated polyenoic acids supposedly due to the formation of hydroxyl radicals by the reaction of ubiquinone with lipid 15-LOX-derived hydroperoxides. However, it is impossible to agree with this proposal because it is known for a long time [23] that quinones cannot catalyze the formation of hydroxyl radicals by the Fenton reaction. Oxidation of intracellular unsaturated acids (for example, linoleic and arachidonic acids) by lipoxygenases can be suppressed by fatty acid binding proteins [24]. 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

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