Nicotinamide adenine dinucleotide alcohol substrate

The leading substrate (A) is nicotinamide adenine dinucleotide (NAD ), and NAD and NADH (product Q) compete for a common site on E. A specific example is offered by alcohol dehydrogenase (ADH) ... 

Horse liver alcohol dehydrogenase is a well-documented enzyme capable of operating with high stereoselectivity on a broad structural range of alcohol and carbonyl substrates. The present reaction proceeds via the pathway shown below, where NAD and NADH represent the oxidized and reduced forms, respectively, of the nicotinamide adenine dinucleotide coenzyme. 

The vast majority of alcohol dehydrogenases require nicotanimide cofactors, such as nicotinamide adenine dinucleotide (NADH) and its respective phosphate NADPH. The structure of NAD/NADP is shown in Fig. 3.39. Hydrogen and two electrons are transferred from the reduced nicotinamide to the carbonyl group to effect a reduction of the substrate (see Fig. 3.39). 

The 3-carbamidopyridinium ring is the chemically active portion of the enzymatic cofactors, NAD and NADP (nicotinamide adenine dinucleotide and its phosphate). A typical reaction involving NAD is the stereospecific (with respect to both cofactor and substrate) oxidation of ethanol to acetaldehyde catalyzed by the enzyme, alcohol dehydrogenase (Eq. 33). 

Enzyme Cofactors. In many enzymatic reactions, and in particular biological reactions, a second substrate (i.e., species) must be introduced to activate the enzyme. This substrate, which is referred to as a cofactor or coenzyme even though it is not an enzyme as such, attaches to the enzyme and is most (often either reduced or oxidized during the course of the reaction. The enzyme-cofac-tor complex is referred to as a holoenzyme. The inactive form of the enzyime-cofactor complex for a specific reaction and reaction direction is called an apoenzyme. An example of the type of system in which a cofactor is used is the formation of ethanol from acetaldehyde in the presence of the enzyme alcohol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinucleotide (NAD) ... 

The substrate models concerned were fitted into the model of the active site of alcohol dehydrogenase-nicotinamide adenine dinucleotide (ADH-NAD) with VDW contacts, etc. not considered explicitly. 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/donors. Two of the most important are NAD and NADP (Figure 5.2). Nicotinamide adenine dinucleotide (NAD ) can accept what is essentially two electrons and a proton (a hydride ion) from a substrate like ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidised product, acetaldehyde, and the reduced cofactor NADH plus a proton ... 

ADH is widely used for serum ethanol assay. ADH kinetics is sophisticated due to the reversibility of reaction and the inhibition by both acetaldehyde and NADH as products. To simplify ADH kinetics, some special approaches are employed to make ADH reaction apparently irreversible on single substrate (alcohol). Thus, reaction pH is optimized to 9.2 to scavenge hydrogen ion semicabarzide at final 75 mmol/L is used to remove acetaldehyde as completely as possible final nicotinamide adenine dinucleotide (NA1>) is 3.0 mmol/L final ADH is about 50 U/L (Liao, et al., 2007a). By assigning the maximal absorbance at 340 nm for reduced nicotinamide adenine dinucleotide (NADH) by the equilibrium method to Ame and that by kinetic analysis of reaction curve to Amk, kinetic analysis of ADH reaction curve should predict Amk consistent with Am but requires some special efforts. 

As another example, studies with deuterium-labeled substrates have shown that the reaction of ethanol with the coenzyme nicotinamide adenine dinucleotide (NAD+) catalyzed by yeast alcohol dehydrogenase occurs with exclusive removal of the pro-R hydrogen from ethanol and with addition only to the Re face of NAD" ". 

Horse liver alcohol dehydrogenase (HLADH) catalyzes the oxidoreduction of a variety of compounds [61,62]. It has been demonstrated that HLADH catalyzes the stereospecific oxidation of only one of the enantiopic hydroxyl groups of acyclic and monocyclic me o-diols [63,64]. The oxidation of meso-exo- and enfto-7-oxabicyclo[2.2.1]heptane dimethanol to the corresponding enantiomerically pure y-lactones by HLADH has been demonstrated. Nicotinamide adenine dinucleotide (NAD" ) and flavin adenine dinucleotide (FAD) were required for the stereoselective oxidation of substrate. Due to the high cost of enzyme and required cofactors, this process for preparing chiral lactones was econom-... 

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