Pyridine nucleotide coenzymes

Mechanism of Hydrogen Transfer by Pyridine Nucleotide Coenzymes 239... 

The pyridine nucleotides NAD and NADP always function in unbound form. The oxidized forms contain an aromatic nicotinamide ring in which the positive charge is delocalized. The right-hand example of the two resonance structures shown contains an electron-poor, positively charged C atom at the para position to nitrogen. If a hydride ion is added at this point (see above), the reduced forms NADH or NADPH arise. No radical intermediate steps occur. Because a proton is released at the same time, the reduced pyridine nucleotide coenzymes are correctly expressed as NAD(P)H+HT... 

Dolphin, D., Avramovic, O., Poulson, R (eds) (1987) Pyridine Nucleotide Coenzymes Chemical, Biochemical, and Medical Aspects, John Wiley Sons, Inc., New ork. 

The oxidation-reduction potential of a pyridine nucleotide coenzyme system is determined by the standard redox potential for the free coenzyme (Table 6-8) together with the ratio of concentrations of oxidized to reduced coenzyme ([NAD+] / [NADH], Eq. 6-64). If these concentrations are known, a redox... 

Three facts account for the need of cells for both the flavin and pyridine nucleotide coenzymes (1) Flavins are usually stronger oxidizing agents than is NAD+. This property fits them for a role in the electron transport chains of mitochondria where a sequence of increasingly more powerful oxidants is needed and makes them ideal oxidants in a variety of other dehydrogenations. (2) Flavins can be reduced either by one- or two-electron processes. This enables them to participate in oxidation reactions involving free radicals and in reactions with metal ions. (3) Reduced flavins... 

Eklund, H., and Branden, C.-I. (1987) in Pyridine Nucleotide Coenzyme Chemical, Biochemical and Medical Aspects, Vol. 2,... 

Since NADPH is continuously used in biosynthetic reactions, and is thereby reconverted to NADP+, the cycle of Eq. 17-46 must operate continuously. As in Eq. 17-42, a true equilibrium does not exist but steps b and c are both essentially at equilibrium. These equilibria, together with those of Eq. 17-42 for the NAD system, ensure the correct redox potential of both pyridine nucleotide coenzymes in the cytoplasm. 

Keilin soon realized that three of the absorption bands, those at 604,564, and 550 nm (a, b, and c), represented different pigments, while the one at 521 nm was common to all three. Keilin proposed the names cytochromes a, b, and c. The idea of an electron transport or respiratory chain followed6 quickly as the flavin and pyridine nucleotide coenzymes were recognized to play their role at the dehydrogenase level. Hydrogen removed from substrates by these carriers could be used to oxidize reduced cytochromes. The latter would be oxidized by oxygen under the influence of cytochrome oxidase. 

An alternative pathway for synthesis of quinoli-nate from aspartate and a triose phosphate exists in bacteria and in plants and provides the major route of nicotinic acid synthesis in nature. In E. coli the reaction is catalyzed by two enzymes, one an FAD-containing L-aspartate oxidase which oxidizes aspartate to a-iminoaspartate.228 The latter condenses with dihydroxyacetone-P to form quinolinate (Eq. 25-13).229 There are at least two other pathways for synthesis of quinolinic acid as well as five or more salvage pathways for resynthesis of degraded pyridine nucleotide coenzymes.224/230/231... 

A. Ohno and K. Ushio, in Pyridine nucleotide coenzymes, Part B, D. Dolphin and R. Poulson, eds., John Wiley, 275 (1987). 

Dolphin, D., R. Poulson, and O. Avamovic (eds.), Pyridine Nucleotide Coenzymes, part A part B. New York Wiley-Interscience, 1987. Chemical, biochemical, and medical aspects of pyridine nucleotide coenzymes. 

Frey PA (1987) Complex pyridine-dependent transformations. In Dolphin D, Poulson R, Avamovic O (eds) Pyridine nucleotide coenzymes Chemical biochemical, medical aspects Vol 2B. Wiley, New York, 462... 

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. 

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... 

An important aspect of the function of photosynthetic complexes is their asymmetric arrangement in respect to the membrane and to the external and internal phases of the cellular compartments. This arrangement allows the catalysis of vectorial electron transfer and the performance of electrical work by promoting charge separation across the membrane dielectric barrier. It allows also in some cases the net translocation of protons across the membrane. These two processes are at the basis of the mechanism of energy conservation in photosynthesis coupled to the formation of ATP, which is added, in oxygenic photosynthesis, to the conservation of redox energy in the form of reduced pyridine nucleotide coenzymes. 

The final destination of the electrons transferred to low redox potentials by PSI-RC is NADP. The electron transfer system to this pyridine nucleotide coenzyme is formed by a sequence of two enzymes, ferredoxin and ferredoxin-NADP" reductase. 

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