Dehydrogenases nicotinamide coenzyme-dependent

Glutamate dehydrogenase, the enzyme responsible for the liberation of ammonia from amino acids, occurs in two forms one (cytosolic) is nicotinamide adenine dinucleotide (NAD+) dependent whilst the other (mitochondrial) requires NADP+ as coenzyme. 

Nicotinamide adenine dinucleotide is a coenzyme which is only loosely bound to the active site of the enzymes with which it interacts and is free therefore, to dissociate from the enzyme during the catalytic cycle. The role of the dehydrogenase enzyme is to bring together the substrate and the NAD+ in the correct orientation for the two to react. These NAD+-dependent enzymes are known as dehydrogenases. They work in conjunction with NAD+ to oxidise substrates by the transfer of 1H+ and 2e from the substrate to the 4-position of the nicotinamide ring of the NAD+ (see Fig. 2.1). The overall reaction is the equivalent of a hydride transfer and is commonly referred to as such. NAD+-dependent enzymes are primarily involved in respiration (NAD+ occurs in significant amounts in mitochondria), whereas, NADP+-dependent coenzymes are primarily involved in the transfer of electrons from intermediates in catabolism. 

Enzyme biosensors are usually constructed with enzymes that are oxido-reductases. The largest known group of oxidoreductases is dehydrogenases, of which more than 250 depend on the soluble coenzyme nicotinamide adenine dinucleotide, NAD "/NADH couple. Excellent review for electrocatalytic... 

Absorption spectra of NAD+ (NADP" ") and NADH (NADPH). At 340 nm, the reduced coenzymes (NADH or NADPH) show significant absorbance, whereas the oxidized forms (NAD or NADP" ") show negligible absorbance. Thus, many enzymatic reactions can be monitored at 340 nm if the reaction is directly or indirectly dependent upon a dehydrogenase reaction involving a nicotinamide adenine dinucleotide as a coenzyme. 

In this chapter we will discuss the mechanisms by which NAD-dependent enzymes catalyze chemistry at two sites in the coenzyme molecule. First we discuss the dehydrogenase-catalyzed redox chemistry that occurs at the 4-position of the nicotinamide ring, and then cleavage of the C-l -N ribosyl linkage cata-... 

A number of oxidations and reductions have been carried out in microemulsions using enzymes such as cholesterol oxidase [64,108,109], bilirubin oxidase [110], horseradish peroxidase [111,112], and horse liver alcohol dehydrogenase (HLADH) [73,112-118]. It is noteworthy that cofactor-dependent enzymes also work well in different types of microemulsions. In fact, kinetic studies on the HLDAH-NADH (NADH stands for the reduced form of nicotinamide adenine dinucleotide) system showed that the presence of coenzyme was essential for the long-term stability of the enzyme in a microemulsion [113,115]. 

Nicotinamide-adenine-dinucleotide, NAD, di-phosphopyrUUne nudeotide, DPN, codehydrogenase I, coenzyme I, cozymase a pyridine nucleotide coenzyme involved in many biochemical redox processes. It is the coenzyme of a large number of oxidoreducta-ses, which are classified as pyridine nucleotide-dependent dehydrogenases. Mechanistically, it serves as the electron acceptor in the enzymatic removal of hydrogen atoms from specific substrates. 

Kojima M, Toda F, Hattori K (1980) The cyclodextrin-nicotinamide compound as a dehydrogenase model simulating apoenzyme-coenzyme-substrate ternary complex system. Tetrahedron Lett 21 2721-2724 Kojima M, Toda F, Hattori K (1981) 3 Cyclodextrin-nicotinamide as a model for NADH dependent enzyme. J Chem Soc Perkin Trans 1 1647-1651... 

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