Dehydrogenase, catalyzed reaction

Future directions in alcohol dehydrogenase-catalyzed reactions... 

How can dehydrogenases catalyze reactions of very hydrophobic substrates ... 

Both the enzymes were prepared by a special technique from the insoluble portion of guinea pig liver mitochondria, and they are quite specific with respect to the requirement of pyridine nucleotide (H9, Hll). However, dehydrogenases catalyzing reaction (25) with NAD as coenzyme have been reported (Mil, S13, T3), thus confirming the importance of the source of the enzyme and the purification procedure employed. 

For further important work on this and related concepts, see Rucker, J. and Kliman, J.P. (1999). Computational study of tunneling and coupled motion in alcohol dehydrogenase-catalyzed reactions Implication for measured hydrogen and carbon isotope effects. J. Am. Chem. Soc. 121, 1997 -2006, and Kohen, A. and Jensen, J.H. (2002). Boundary conditions for the Swain-Schaad relationship as a criterion for hydrogen tunneling. J. Am. Chem. Soc. April 17, 124(15), 3858-3864. 

Reaction (1) involves a stereospecific net transfer of a hydride ion between a substrate and the C(4) of the pyridine ring of the coenzyme and an exchange of a proton with the medium 14). The generally accepted formal potential, E° of the NAD+/NADH redox couple is -560 mV vs SCE (pH 7, 25°C) 15). This value is obtained from equilibrium constants of dehydrogenase catalyzed reactions and thermal data. For most systems the equilibrium of reaction (1) favors the substrate rather than the product side. The reason for this is the low oxidizing power of NAD+, which is reflected by the low value of E° ... 

The over-all reaction (Eq. 7) is believed to involve ferredoxin in addition to two enzymes pyruvic dehydrogenase, catalyzing reactions (4) and (5) and hydrogenase, catalyzing reaction (6) (Mortenson (70)). 

In the succinate dehydrogenase-catalyzed reaction, why is the appropriate electron acceptor FAD rather than NAD+, which is used in the other redox reactions of the citric acid cycle (Chap. 12) ... 

Another efficient method is the electrochemical oxidation of NADH at 0.585 V vs Ag/AgCl by means of ABTS2- (2,2,-azinobis(3-ethylbenzothiazoline-6-sulfonate)) as an electron transfer mediator [96]. Due to the unusual stability of the radical cation ABTS, the pair ABTS2 /ABTS is a useful mediator for application in large-scale synthesis even under basic conditions. Basic conditions are favorable for dehydrogenase catalyzed reactions. This electrochemical system for the oxidation of NADH using ABTS2 as mediator was successfully coupled with HLADH to catalyze the oxidation of a meso-diol (ws >-3,4-dihydroxymethylcyclohex-l-ene) to a chiral lactone ((3aA, 7aS )-3a,4,7,7a-tetrahydro-3//-isobenzofurane- l-one) with a yield of 93.5% and ee >99.5% (Fig. 18). 

The yeast-induced reduction of (5e) —> (6e), (R = substituted phenyls, R = Me) is one of the very first preparative-scale alcohol dehydrogenase catalyzed reactions to have been reported. While the transformations of (5a-j) demonstrate that broad structural mlerances are possible in the substrate ketones, the enzymes are sometimes very discriminating. For example, while the BY-catalyzed reductions of the 2-, 3- and 4-substituted pyridyl ketones (5g) proceed smoothly with high stereospecificitythe analogous furanyl and thiophenyl ketones give virtually racemic product alcohols, and 2-acetylpyrrole is not a substrate at all. ... 

TPP is necessary as a cofactor for the pyruvate cmd a-ketoglutarate dehydrogenase catalyzed reactions as well as the transketolase catalyzed reactions of the pentose phosphate pathway. 

ALCOHOL DEHYDROGENASE-CATALYZED REACTIONS Stereoselective reduction of aldehydes and ketones... 

Example in most dehydrogenase-catalyzed reactions the coenzyme has to bind first hexokinase)... 

Lactate dehydrogenase Catalyzes reactions in muscle and liver cells... 

Pyruvate dehydrogenase Catalyzes reactions connecting glycolysis to the Krebs cycle... 

Enzymes that use NADPVNADPH Glucose 6-phosphate dehydrogenase Catalyzes reactions in the pentose phosphate pathway... 

Chloroplast glyceraldehyde Phosphate dehydrogenase Catalyzes reactions in the Calvin cycle, glucose synthesis... 

Razumiene, J. Meskys, R. Gureviciene, V. Laurinavicius, V. Reshetova, M. D. Ryabov, A. D. 4-ferrocenylphenol as an electron transfer mediator in pqq-dependent alcohol and glucose dehydrogenase-catalyzed reactions. Electrochem. Commun. 2000, 2, 307—311. 

It does not seem feasible to us that modulation of kinase activity by regulation of proton flow in membrane-associated electric fields can be the sole determinant of the directionality and flux of metabolic pathways. If, as suggested by others, " dehydrogenase-catalyzed reactions were to be poised close to chemical equilibrium, the inevitable consequence would be a pooling of the components of these reactions and proticity would be ineffective as a driving force over the total pathway. It seems reasonable to assume, therefore, that cellular redox reactions are also driven.The electrochemical interpretation of metabolism presented here provides a straightforward mechanism for explaining how this could come about. 

The inherent nature of the redox reaction makes it natural to couple these dehydrogenase-catalyzed reactions to electrochemical methods. The transfer of electron(s) from a substrate to an electrode (or the reverse) may then take place via electrochemical redox reactions of the coenzymes, as depicted in Fig. 2. The utility of combining enzymes and electrochemical methods for electroanalytical applications was predicted by Clark and Lyons by an enzyme electrode [28] and by Shaw in energy production by a biofuel cell anode [29] in the early 1960s. 

Fig. 2 Electron-transfer pathway in dehydrogenase-catalyzed reactions coupled to electrodes via electrochemical redox reactions of the coenzymes. Electron fluxes represent the oxidation of the substrate (e.g. Pyruvate/Lactate ° = -435 mV vs. SCE, pH 7.0).

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