Oxidative reactions dehydrogenases

The substrates of catabolism—proteins, carbohydrates, and lipids—are good sources of chemical energy because the carbon atoms in these molecules are in a relatively reduced state (Figure 18.9). In the oxidative reactions of catabolism, reducing equivalents are released from these substrates, often in the form of hydride ions (a proton coupled with two electrons, H ). These hydride ions are transferred in enzymatic dehydrogenase reactions from the substrates... 

FIGURE 24.11 The acyl-CoA dehydrogenase reaction. The two electrons removed in this oxidation reaction are delivered to the electron transport chain in the form of reduced coenzyme Q (UQH9). 

The third reaction of this cycle is the oxidation of the hydroxyl group at the /3-position to produce a /3-ketoacyl-CoA derivative. This second oxidation reaction is catalyzed by L-hydroxyacyl-CoA dehydrogenase, an enzyme that requires NAD as a coenzyme. NADH produced in this reaction represents metabolic energy. Each NADH produced in mitochondria by this reaction drives the synthesis of 2.5 molecules of ATP in the electron transport pathway. L-Hydroxyacyl-... 

L-Amino acid oxidase has been used to measure L-phenylalanine and involves the addition of a sodium arsenate-borate buffer, which promotes the conversion of the oxidation product, phenylpyruvic acid, to its enol form, which then forms a borate complex having an absorption maximum at 308 nm. Tyrosine and tryptophan react similarly but their enol-borate complexes have different absorption maxima at 330 and 350 nm respectively. Thus by taking absorbance readings at these wavelengths the specificity of the assay is improved. The assay for L-alanine may also be made almost completely specific by converting the L-pyruvate formed in the oxidation reaction to L-lactate by the addition of lactate dehydrogenase (EC 1.1.1.27) and monitoring the oxidation of NADH at 340 nm. 

Figure 15.11 The biochemical reactions that result in the conversion of trans-retinal to ds-retinal, to continue the detection of light To continue the process, trans-retinal must be converted back to c/s-retinal. This is achieved in three reactions a dehydrogenase converts trans-retinal to trans-retinol an isomerase converts the trans-retinol to c/s-retinol and another dehydrogenase converts c/s-retinol to c/s-retinal. To ensure the process proceeds in a clockwise direction (i.e. the process does not reverse) the two dehydrogenases are separated. The trans-retinal dehydrogenase is present in the photoreceptor cell where it catalyses the conversion of trans-retinal to trans-retinol which is released into the interstitial space, from where it is taken up by an epithelial cell. Here it is isomerised to c/s-retinol and the same dehydrogenase catalyses its conversion back to c/s-retinal. This is released by the epithelial cell into the interstitial space from where it is taken up by the photoreceptor cell. This c/s-retinal then associates with the protein opsin to produce the light-sensitive rhodopsin to initiate another cycle. The division of labour between the two cells may be necessary to provide different NADH/NAD concentration ratios in the two cells. A high ratio is necessary in the photoreceptor cell to favour reduction of retinal and a low ration in the epithelial cell for the oxidation reaction (Appendix 9.7).

Alternatively, pyruvate and pyruvate dehydrogenase can be used instead of a-ketoglutarate and its corresponding dehydrogenase. This oxidation reaction... 

A peroxisomal dehydrogenase initiates the 3-oxidation reactions that shorten the chain to -18 carbons or less, at which point the fatty acyl CoA is transferred to mitochondria for complete degradation by (3-oxidation. 

As with adults, the primary organ responsible for drug metabolism in children is the liver. Although the cytochrome P450 system is fully developed at birth, it functions more slowly than in adults. Phase I oxidation reactions and demethylation enzyme systems are significantly reduced at birth. However, the reductive enzyme systems approach adult levels and the methylation pathways are enhanced at birth. This often contributes to the production of different metabolites in newborns from those in adults. For example, newborns metabolize approximately 30% of theophylline to caffeine rather than to uric acid derivatives, as occurs in adults. While most phase I enzymes have reached adult levels by 6 months of age, alcohol dehydrogenase activity appears around 2 months of age and approaches adult levels only by age 5 years. 

As noted earlier, peroxynitrite is formed with a diffusion-controlled rate from superoxide and nitric oxide (Reaction 10). As both these radicals are ubiquitous species, which present practically in all cells and tissues, peroxynitrite can be the most important species responsible for free radical-mediated damage in biological systems. Moreover, it is now known that NO synthases are capable of producing superoxide and nitric oxide simultaneously (see Chapter 22), greatly increasing the possible rate of peroxynitrite production. In addition, another enzyme xanthine dehydrogenase is also able to produce peroxynitrite in the presence of nitrite... 

For foreign compounds, the majority of oxidation reactions are catalyzed by monooxygenase enzymes, which are part of the mixed function oxidase (MFO) system and are found in the SER (and also known as microsomal enzymes). Other enzymes involved in the oxidation of xenobiotics are found in other organelles such as the mitochondria and the cytosol. Thus, amine oxidases located in the mitochondria, xanthine oxidase, alcohol dehydrogenase in the cytosol, the prostaglandin synthetase system, and various other peroxidases may all be involved in the oxidation of foreign compounds. 

Another oxidation reaction, which shows variation in human populations, is the oxidation of ethanol. This has been shown to be significantly lower in Canadian Indians compared with Caucasians, and thus the Indians are more susceptible to the effects of alcoholic drinks. The rate of metabolism in vivo in Indians is 0.101 g kg-1 hr-1 compared with 0.145 g kg-1 hr-1 in Caucasians. This seems to be due to variants in alcohol dehydrogenase, although differences in aldehyde dehydrogenase may also be involved. Variants of alcohol dehydrogenase resulting in increased metabolism have also been described within Caucasian and Japanese populations. 

The results obtained with the biosensor for five different honey samples were compared with those obtained with a commercial enzyme kit from R-biopharm using spectrophotometric detection (Table 16.1). This method is based on the phosphorylation of gluconate by gluconate kinase in the presence of ATP, then the 6-phospho-D-gluconate produced was oxidized with NADP+ by 6-phosphogluconate dehydrogenase. The reduced form of NADP, NADPH, formed in this oxidation reaction is measured spectrophotometrically at 340 nm. 

In fact, the a-ketoglutarate/glutamate dehydrogenase is a generally applicable method for the regeneration of NAD and NADP in laboratory scale productions. Both components involved are inexpensive and stable. Quite recently, a method for the oxidation of the reduced nicotinamide coenzymes based on bacterial NAD(P)H oxidase has been described [225], This enzyme oxidizes NADH as well as NADPH with low Km values. The product of this reaction is peroxide, which tends to deactivate enzymes, but it can be destroyed simultaneously by addition of catalase. The irreversible peroxide/catalase reaction favours the ADH catalyzed oxidation reaction, and complete conversions of this reaction type are described. 

Other forms of vanadium have been implicated in the stimulation of the plasma membrane vanadate-dependent NAD(P)H oxidation reaction. Decavanadate has been shown to be a more potent stimulator of the vanadate-dependent NADH oxidation activity than added orthovanadate [30,31], Interestingly, decavanadate reductase activity has been found to be an alternative activity of an NADP-specific isocitrate dehydrogenase [32], Diperoxovanadium derivatives have also been shown to be involved in this type of reaction [33,34], Decavanadate may play a role in the biological role of vanadium, as it is found in yeast cells growing in the presence of orthovanadate [8] and is a potent inhibitor of phosphofructokinase-1, the control step of glycolysis, and other metabolic reactions [35],... 

Answer Oxygen is the terminal electron acceptor in oxidative phosphorylation, and thus is needed to recycle NAD+ from NADH. NADH is produced in greatest quantities by the oxidative reactions of the citric acid cycle. In the absence of 02, the supply of NAD+ is depleted, and the accumulated NADH allosterically inhibits pyruvate dehydrogenase and cc-ketoglutarate... 

The driving force in the coupled enzyme process is independent of the equilibrium of the LeuDH catalyzed reaction because of the irreversibility of the NOX reaction. This is most favorable because the oxidation reaction of many NAD(P)+-dependent dehydrogenases is hampered by their equilibrium, which prefers the reduction reaction. By applying this system D-ferf-leucine was obtained with an excellent ee >99%. 

Non-cytochrome P450 enzymes may also be involved in oxidative reactions. One such enzyme is alcohol dehydrogenase whose substrates include vitamin A, ethanol, and ethylene glycol. Aldehyde dehydrogenase is another enzyme. Most reduction reactions also involve microsomal enzymes, with the exception of ketone reduction. Nitro compounds are reduced to amines and volatile anesthetics undergo dehalo-genation by microsomal enzymes. Hydrolysis reactions are involved in metabolism of compounds with amide bonds or ester linkages, as in the conversion of aspirin to salicylate (Brown, 2001). 

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