Pyridine nucleotide chemistry

The most important coenzymes in synthetic organic chemistry [14] and industrially applied biotransformations [15] are the nicotinamide cofactors NAD/ H (3a/8a, Scheme 43.1) and NAD(P)/H (3b/8b, Scheme 43.1). These pyridine nucleotides are essential components of the cell [16]. In all the reactions where they are involved, they serve solely as hydride donors or acceptors. The oxidized and reduced form of the molecules are shown in Scheme 43.1, the redox reaction taking place at the C-4 atom of the nicotinamide moiety. 

Lynen had studied chemistry in Munich under Wieland his skill as a chemist led to the successful synthesis of a number of fatty acyl CoA derivatives which proved to be substrates in the catabolic pathway. Many of these C=0 or C=C compounds had characteristic UV absorption spectra so that enzyme reactions utilizing them could be followed spectrophotometrically. This technique was also used to identify and monitor the flavoprotein and pyridine nucleotide-dependent steps. Independent evidence for the pathway was provided by Barker, Stadtman and their colleagues using Clostridium kluyveri. Once the outline of the degradation had been proposed the individual steps of the reactions were analyzed very rapidly by Lynen, Green, and Ochoa s groups using in the main acetone-dried powders from mitochondria, which, when extracted with dilute salt solutions, contained all the enzymes of the fatty acid oxidation system. 

Adams M (1987) Oxido-reductases - pyridine nucleotide-dependent enzymes. In Page MI, Wiliams A (eds) Enzyme mechanisms. The Royal Society of Chemistry, London, p 477... 

The chemistry of flavins is complex, a fact that is reflected in the uncertainity that has accompanied efforts to understand mechanisms. For flavoproteins at least four mechanistic possibilities must be considered.1533 233 (a) A reasonable hydride-transfer mechanism can be written for flavoprotein dehydrogenases (Eq. 15-23). The hydride ion is donated at N-5 and a proton is accepted at N-l. The oxidation of alcohols, amines, ketones, and reduced pyridine nucleotides can all be visualized in this way. Support for such a mechanism came from study of the nonenzymatic oxidation of NADH by flavins, a reaction that occurs at moderate speed in water at room temperature. A variety of flavins and dihydropyridine derivatives have been studied, and the electronic effects observed for the reaction are compatible with the hydride ion mecha-nism.234 236... 

As is indicated in Table 5-3, P680, P70o> the cytochromes, plastocyanin, and ferredoxin accept or donate only one electron per molecule. These electrons interact with NADP+ and the plastoquinones, both of which transfer two electrons at a time. The two electrons that reduce plastoquinone come sequentially from the same Photosystem II these two electrons can reduce the two >-hemes in the Cyt b(f complex, or a >-heme and the Rieske Fe-S protein, before sequentially going to the /-heme. The enzyme ferre-doxin-NADP+ oxidoreductase matches the one-electron chemistry of ferredoxin to the two-electron chemistry of NADP. Both the pyridine nucleotides and the plastoquinones are considerably more numerous than are other molecules involved with photosynthetic electron flow (Table 5-3), which has important implications for the electron transfer reactions. Moreover, NADP+ is soluble in aqueous solutions and so can diffuse to the ferredoxin-NADP+ oxidoreductase, where two electrons are transferred to it to yield NADPH (besides NADP+ and NADPH, ferredoxin and plastocyanin are also soluble in aqueous solutions). 

DCC is also used in nucleotide chemistry to esterify a sugar hydroxyl group with a phosphate group in another nucleotide or oligonucleotide unit. Also p-styrene based polymers with a pyridyl-2-ethanol end group are reacted in pyridine with 3 -0-acetyl-desoxythimidine-5 -phosphate in the presence of DCC." The reaction of mono esters of phosphoric acid with alcohols or phenols, in the presence of DCC, affords phosphoric acid diesters in high yield." This reaction is widely used in nucleic acid chemistry. 

The sum reaction for the oxidative segment of the PPP is shown by reaction (1). Using insightful planning and remarkable chemistry, Warburg et al. (8) had also isolated and characterized (from 100 L of horse erythrocytes) the new pyridine nucleotide coenzyme NADP+ for the above reactions. He recognized and chemically established that the new coenzyme was functionally... 

The chemistry of cord factor, a toxic lipid of virulent or attenuated Mycobacteria has already been described in detail (see p. 210). It has also been mentioned that at least part of this biological activity can be explained by the action of cord factor on dehydrogenases dependent on diphospho-pyridine nucleotide as described by Kato and coworkers (see p. 231). 

The analogy has been drawn between the chemistry catalyzed by dehydrogenases and the base-catalyzed hydride transfer chemistry of Meerwein-Ponndon-Verley (MPV) reductions or the Cannizzaro reaction (for recent reviews of the chemistry of pyridine nucleotides see Refs. 1,3, and 5). In MPV reductions [reaction (2)] aluminum or lithium salts of the alkoxide activate the C-H bond for transfer of hydrogen to suitable acceptors. Both direct hydride transfer (52) and radical intermediates (55) have been suggested for this reac-... 

How do these oxidation-reduction reactions take place All the chemistry of the pyridine nucleotide coenzymes (NAD, NADP, NADH, and NADPH) takes place at the 4-position of the pyridine ring. The rest of the molecule is important for binding the coenzyme to the proper site on the enzyme. If a substrate is being oxidized, it donates a hydride ion (H ) to the 4-position of the pyridine ring. In the following reaction, the primary alcohol is oxidized to an aldehyde. A basic amino acid side chain of the enzyme can help the reaction by removing a proton from the oxygen in the substrate. 

There has been a continued interest in the chemistry and biochemistry of coenzymes, particularly phosphoenolpyruvate, and a book devoted to pyridine nucleotide coenzymes has been published as a tribute to Professor N. O. Kaplan s work on these compounds. Among new techniques which have been applied recently to biologically interesting compounds is fast atom bombardment mass spectrometry, which has been used in the structural elucidation of underivatized phospholipids. ... 

Singer, T. P. Kearney, E. K. (1954). Chemistry, metabolism and scope of action of the pyridine nucleotide coenzymes. Advances in Enzymology 15, 79-139. 

Figure 1.17. Biosynthesis of pyridine alkaloids from compounds in the pyridine nucleotide cycle (Waller et ai, 1966, 1975). Courtesy of the J. of Biological Chemistry, edited by the American Society of Biological Chemists, Inc.

All the chemistry of the pyridine nucleotide coenzymes takes place at the 4-position of the pyridine ring. The purpose of the rest of the molecule is to recognize the coenzyme and bind it to the proper site on the enzyme. 

---