Diphosphopyridine nucleotide structure

The nicotinamide coenzymes are involved as proton and electron carriers in a wide variety of oxidation and reduction reactions. Before their chemical structures were known, NAD and NADP were known as coenzymes I and II. Later, when the chemical nature of the pyridine ring of nicotinamide was discovered, they were called diphosphopyridine nucleotide (DPN = NAD) and triphospho-pyridine nucleotide (TPN = NADP). The nicotinamide nucleotide coenzymes are sometimes referred to as the pyridine nucleotide coenzymes. 

Aldol dehydrogenases, from diverse species including yeast, horses and humans, catalyze the oxidation of ethanol or the reduction of acetaldehyde, using diphosphopyridine nucleotide (DPN) as a co-factor. Crystalline yeast alcohol dehydrogenase has a molecular weight of 150,000 and contains four Zn2+ ions and binds four DPN molecules per mole. Its structure and chemistry are not yet known in detail. 

In 1935, von Euler, Albers, and Schlenck studied the preparation of cozymeise, the coenzyme which is necessary for the alcoholic fermentation of glucose by apozymase, shown later to be diphosphopyridine nucleotide (DPN). On hydrolysis, cozymeise yielded nictotinic acid. This was the first evidence that nicotinic acid (in the form of its amide) formed a part of the structure of an enzyme, and placed it among the organic compounds of great importance in biological chemistry. 

Diphosphopyridine nucleotide and analogous quaternary pyridinium salts form complexes with bisulphite and with thiols 2-5, xhe orientation of these additions is usually uncertain and may vary with the reaction solvent used " 4, The dithionite addition products formed by quaternary pyridinium salts have attracted interest because of their connection with the dithionite reduction of diphosphopyridine nucleotide (p. 259). Thus, the formation of a 1,4-dihydropyridine (122) by reduction of (120) has been represented 6 as proceeding through a 1,2-addition product (121). Regardless of the correctness or otherwise of structure (121), the relevance of the intermediate for diphosphopyridine nucleotide reduction by dithionite has been questioned, and the yellow intermediate formed in this reaction has been formulated 8 as a charge-transfer complex between the pyridinium nucleus and 820 (see p. 261). 

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