NAD-dependent dehydrogenation

Degradation of amino acids most often begins with conversion, either by transamination3563 or by NAD+-dependent dehydrogenation,357 to the corresponding 2-oxoacid and oxidative decarboxylation of the latter (Fig. 15-16). Alanine, valine, leucine, and isoleucine are all treated this way in the animal body. Alanine gives pyruvate and acetyl-CoA directly, but the others yield CoA derivatives that undergo... 

Dimethylhydrazine is metabolized by a sequence of oxidation steps, first dehydrogenation to azomethane, A -oxidation of this to azoxymethane and finally a C-oxidation to methylazoxymethanol (Fiala, 1975, 1977). This last metabolite decomposes to give the highly reactive methyldiazonium ion to which the carcinogenicity of the compound has been attributed. The sequential nature of these oxidation steps has been shown in the isolated perfused rat liver (Wolter Frank, 1982). Fiala (1977) showed that the C-oxidation of azoxymethane to methylazoxymethanol is catalysed by hepatic microsomes, while Schoental (1973) found that methylazoxymethanol was converted to the corresponding aldehyde by an NAD-dependent dehydrogenase. 

P-Decarboxylating dehydrogenases are a family of bifunctional enzymes that catalyze the Mg2+- and NAD(P)+-dependent dehydrogenation at C2, followed by their Mg2+-dependent decarboxylation at C3 of P-substituted malate ... 

But useful NAD(P)H dependent enzymes are also present in Clostridia. Some of them contain high enzyme activities by which NAD or NADP can be reduced with the above mentioned electron donors and mediators to NAD(P)H. Because these enzymes are reversible, selective NAD(P) dependent dehydrogenations are also possible. 

The first step is catalysed by the pyridine nucleotide dependent alcohol dehydrogenase (NAD -dependent in C. kluyveri and NADP -dependent in C. tyrobutyricum) leading to the 2-enal which in turn is reduced by enoate reductase to the saturated aldehyde (Reactions [10a] and [10b]). The saturated aldehyde is further reduced to the alcohol. The rate of the reduction depends not only on the activity of the involved en2ymes but also on the concentration and on the ratio of NAD(P) /NAD(P)H. In the presence of MV which is formed by the reduction of MV by the system H2/hydrogenase (Reaction [5a]), the ratio NAD(P) /NAD(P)H is too small for the fast and complete dehydrogenation of an allyl alcohol since the first step of the reaction sequence [16], which needs NAD(P), is too slow. It turned out that ethanol is a better electron donor than hydrogen gas in this case. For the reduction of 50-70 mM ( )-2-methyl-2-butenol to (R) -2-methyl-... 

A second dehydrogenation then takes place with the formation of the )J-keto-acyl-CoA derivative. This reaction is catalysed by a NAD-dependent dehydrogenase ... 

Methyl-3-hydroxybutyryl-CoA is oxidized to 2-methylacetoacetyl-CoA, which is then metabolized by a )8-ketothiolase to acetyl-CoA and propionyl-CoA. 3-Hydroxyisobutyryl-CoA is deacylated and the 3-hydroxy-isobutyric acid is dehydrogenated to methylmalonic semialdehyde by a NAD-dependent reaction (Robinson et al., 1957 Robinson and Coon, 1957 Tanaka, 1975). The methylmalonic semialdehyde is believed to be decarboxylated to form propionaldehyde prior to oxidation to propionyl-CoA via an aldehyde oxidase (Tanaka etal, 1975 Baretz and Tanaka, 1978). 

A flavin-dependent formate dehydrogenase system found in Methanobacterium passes electrons from dehydrogenation of formate to FAD and then to the deazaflavin coenzyme F q.673 In contrast to these Mo-containing enzymes, the formate dehydrogenase from Pseudomonas oxalaticus, which oxidizes formate with NAD+ (Eq. 16-66), contains neither Mo or Se.674... 

Enoate reductase exclusively splits off the (4S)-hydrogen atom from NADH. There is no direct hydrogen transfer from NADH to the products. If (45)-[ H]-labelled NADH with a total tritium activity of 4.67.10 decays per minute was dehydrogenated with an excess of enoate, the isolated product showed a tritium content of less than 0.005 % of that of the NADH. Almost all the tritium was in the water. In the absence of an enoate as acceptor, the tritium exchange from (45)-[4- H]-NADH catalysed by enoate reductase is very slow. Depending on the substrate concentration, the isotope effect of the reduction of ( )-2-methyl-2-butenoate with (4S)-[4- H]-NADH varies from 6.8 to 1.3. The presence of NAD" decreases the isotope effect (17). 

More than 10 experiments. Values of enantiomeric excess depend on temperature, reaction time etc.. The ee values decrease with increase of temperature and reaction time. A total volume of 50 ml contained 50 mM substrate, 1 mg ( 12 U) enoate reductase. After 17 h at 18 "C > 95 % product. A total volume of 4 ml contained 70 pmol ethanol, 1.4 pmol NADH, 100 mg C. klityveri or 1 U enoate reductase. After 22 h at 25 °C > 95 % product. The NADH contained about 5 % NAD, which is necessary for the dehydrogenation of the substrate to 2-methyl-3-phenyl-enal which is then reduced to the (R)-2-methyl-3-phenyl-propanol. Horse liver alcohol dehydrogenase, 2.5 U. After 22 h at 25 °C > 95 % product. 

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