NADPH/NADP" ratio

Because an alteration of this mass action ratio profoundly influences every reaction that involves ATP, organisms have been under strong evolutionary pressure to develop regulatory mechanisms that respond to the [ATP]/[ADP] ratio. Similar arguments show the importance of maintaining appropriate [NADH]/[NAD+] and [NADPH]/[NADP+] ratios. 

Function of Cyclic Photophosphorylation When the [NADPH]/[NADP+] ratio in chloroplasts is high, photophosphorylation is predominantly cyclic (see Fig. 19 49). Is 02 evolved during cyclic photophosphorylation Is NADPH produced Explain. What is the main function of cyclic photophosphorylation ... 

Again, the resulting high [NADPH]/[NADP+] ratio provides the reducing environment that favors reductive anabolic processes such as fatty acid synthesis. 

NADPH/ NADP+ ratio 776 Naphthalene, resonance energy 299 Nathans, Daniel 84 Natural killer cells 185 Near attack conformation 485 Near-field scanning microscope 130 Neighboring group assistance in displacement reactions 601, 602 Nekoflavin 783 Nematocyst 427 Nematodes 24, 25 Neocarzinostatin 218,224s Neopterin 803... 

How is the cytoplasmic [NADPH]/[NADP+] ratio maintained at a value higher than that of [NADH]/ [NAD+] Part of the answer is from operation of the pentose phosphate pathway (Section E,3). The reactions of Eq. 17-12, if they attained equilibrium, would give a ratio of cytosolic [NADPH]/[NADP+] > 2000 at 0.05 atm C02. Compare this with the ratio 1 /634 for [NADH/[NAD+] deduced from the observation on the reactions of Eq. 17-42. 

When the NADPH/NADP+ ratio is high and little NADP+ is available to accept electrons, an alternative electron transport pathway is used that involves only PSI and a few electron carriers (Fig. 3). Here the high-energy electron is passed by ferredoxin to the cytochrome bf complex instead of to NADP+. It then flows... 

Answer Neither 02 nor NADPH is produced. At high [NADPH]/[NADP + ] ratios, electron transfer from reduced ferredoxin to NADP+ is inhibited and the electrons are diverted into the cytochrome b6f complex. These electrons return to P700 and ATP is synthesized by photophosphorylation. Because electrons are not lost from P700, none are needed from PSII. 

Answer Within organelles, reaction intermediates and enzymes can be maintained at different levels from those in the cytosol and in other organelles. For example, the ATP/ADP ratio is lower in mitochondria than in the cytosol because the role of adenine nucleotides in the mitochondrial matrix is to accept a phosphoryl group, whereas the role in the cytosol is to donate a phosphoryl group. Similarly, different NADH/NAD+ and NADPH/NADP+ ratios reflect the reductive (biosynthetic) functions of the cytosol and the oxidative (catabolic) functions of the mitochondrial matrix. By segregating reaction sequences that share intermediates, the cell can regulate catabolic and anabolic processes separately. 

Niacin ia a nutritional term applied to both nicotinic acid and nicotinamide and to a mixture of the two. Their structures and those of their coenzymes are given in Table 6.1. Numerous redox reactions use NAD+ and NADP+ or NADH and NADPH. The latter are used largely in reactions designed to reductively synthesize various substances, mostly in the extramitochondrial areas of the cell. NAD+, on the other hand, is used largely in its oxidized form in catabolic redox reactions. The rat liver cytosol NADPH/NADP+ ratio is about 80, whereas its NADH/NAD+ ratio is only 8 x 10 4. Table 6.3 lists some biochemical reactions in which these cofactors participate. It shows that they are of crucial importance in the metabolism of carbohydrates, fats, and amino acids. 

Interconversion of cortisone and cortisol catalyzed by llfSHSDHl is fully reversible in vitro. The physiologic direction in vivo is cortisone reduction, which is driven by a high [NADPH]/[NADP+] ratio in the ER lumen. The high ratio can be maintained only by local NADP+ reduction catalyzed by certain luminal dehydrogenases, such as the H6PDH (15, 16, 24), because the permeability of the ER membrane to pyridine nucleotides is negligible (17, 18). 

Here, special attention should be paid to the NADPH/NADP+ ratio with regard to the fact that this ratio, in contrast to the NADH/NAD+ ratio, is not directly linked to ATP production. It has been demonstrated that not only the NADH/NAD+, but also the NADPH/NADP+, ratio depends on... 

If we accept that the redox state of nicotinamide nucleotides is a modulating system, we need to know how this works. An attractive idea involves the fact that the NADPH/NADP+ ratio is in equilibrium with the GSH/GSSG ratio which itself is of importance for the redox state and therefore the activity of functional proteins (Barron, 1951). 

---