NADP+-NADPH redox couple

Another class of energy storage compounds consists of redox couples such as NADP+-NADPH (Table 6-1). The reduced form, NADPH, is produced by noncyclic electron flow in chloroplasts (Chapter 5, Section 5.5C). Photosynthesis in bacteria makes use of a different redox couple, NAD+-NADH. The reduced member of this latter couple also causes an... 

A particular half-cell reaction, such as Equation 6.15, can accept or donate electrons. We quantitatively describe this by the redox potential for that reaction, as expressed by Equation 6.9 [Ej = E u — (RT/qF) In (reduced))/(oxidized))]. We will use (NADPH) to represent the activity of all of the various ionization states and complexed forms of the reduced nicotinamide adenine dinucleotide phosphate, and (NADP+) has an analogous meaning for the oxidized component of the NADP+-NADPH couple. For redox reactions of biological interest, the midpoint (standard) redox potential is usually determined at pH 7. By using Equation 6.9, in which the number q of electrons transferred per molecule reduced is 2, we can... 

To determine whether electrons spontaneously flow toward or away from the NADP+-NADPH couple, we must compare its redox potential with that of some other redox couple. As indicated previously, electrons spontaneously flow toward more positive redox potentials, whereas energy must be supplied to move electrons in the energetically uphill direction of algebraically decreasing redox potentials. We will next examine the redox potentials of the various redox couples involved in electron flow in chloro-plasts and then in mitochondria. 

C. Using Equation 6.16b, we can calculate the redox potential of the NADP+-NADPH couple ... 

Therefore, the difference in redox potential between the NADP+-NADPH couple and ferredoxin is... 

Redox potential is defined by the half cell reduction potential that is created by redox couples that are primarily due to GSH, NAD+ and nicotinamide dinucleotide phosphate. These couples are in ratios of the oxidized to reduced form of the molecules (NAD /NAD, NADP /NADPH, and GSSG/2GSH). The redox couples can be independent, as well linked to each other to form related couples. The redox environment is a reflection of these couples. These ratios can be measured by the Nemst equation, similar to a voltaic cell. 

Each of the cellular redox couples has a characteristic formal potential, the value of which is valid under equimolar composition of the oxidized and reduced form. However, the functions of living cells require a non-equimolar composition. For instance, in the case of the redox couple NADP -NADPH, the ratio [NADP ]/[NADPH] 1, and for NAD -NADH, the ratio [NAD ]/[NADH] can approach 1,000. The small value of [NADP ]/[NADPH] and consequently an excess of NADPH is necessary to maintain the bios5mthetic processes that utilize NADPH. The large value of [NAD ]/[NADH] indicates the presence of an excess of NAD, which is needed to support, for instance, mitochondrial respiration that involves continuous reduction of NAD. The actual potential (E) a redox couple has is dependent on the ratio of the oxidized form to the reduced form according to the Nemst equation (Eq. (2)) ... 

Figure 2. Redox cofactor requirement in L-arjabinose catabolism. L-Arabinose conversion to equimolar amounts of CO2 and ethanol is redox neutral, i- -anaerobic fermentation to ethanol should be possible. However, the conversion of L-arabinose to D-xylulose requires NADPH and NAD and produces NADH and NADP. NADPH is mainly regenerated in the oxidative part of the pentose phosphate pathway, where the reduction ofNADP is coupled to C02 production. The abbreviations are G6p, glucose 6-phosphate F6P, fructose 6-phosphate X5P, D-Xylulose 5-phosphate GAP, D-glyceraldehyde 3-phosphate. (Reproduced from Ref. 165 with permission from Elsevier Science)...

FIGURE 9.9. Structures of the NAD /NADH and NADP /NADPH coenzyme redox couples. 

The redox state of the pyridine nucleotides, expressed as ARC (NADPH/NADP+ + NADPH) and CRC (NADH/NAD + NADH) showed a similar range for each couple during the whole treatment (Table 1) ... 

As a redox couple, proline and pyrroline-5-carboxylate provide a mechanism for the intercompartmental and intercellular transfer of redox potential. The transfer of redox potential alters the ratio of NADP /NADPH thereby activating certain metabolic pathways. Although the reduction of pyrroline-5-carboxylate is the central mechanism in the transfer of redox potential, the metabolic interconversions of proline, ornithine, and glutamate with pyrroline-5-carboxylate as the obligate intermediate also may play a role. The endpoint of this regulation appears to be the formation of purine ribonucleotides by both salvage and de novo mechanisms. Proline and pyrroline-5-carboxylate appear to be metabolic signals which can be fine-tuned by humoral factors to coordinate the metabolism of amino acids and ribonucleotides. When the transfer is from cell to cell, proline and pyrroline-5-carboxyl-ate can function as intercellular communicators. 

The resulting thiol pair of the reduced thioredoxin is the reductant used for ribonucleotide reductase (Chapter 16). The standard redox potential E° of E. coli thioredoxin is -0.27 V, appropriately low for coupling to the NADPH / NADP+ system. 

C. If ferredoxin has a redox potential of —0.580 V and the activity of NADPH is 3% of that of NADP+, what is the difference in redox potential between the two couples ... 

Redox potentials have been determined for each of the steps of reduction of the trypsin-solubilized reductase (403) step 1, one electron consumed, Eo = —109 mV step 2, two electrons consumed. Eg = —276 mV and step 3, one electron consumed. Eg = —371 mV at pH 7.0, 26°. As expected, the redox potential of step 3 is more negative than the potential of the NADPH-NADP+ couple and was determined from the dithio-nite titration. The overall potentiometric—spectrophotometric titration curves could be very closely fitted with a computer-generated curve based on the assumptions of four one-electron reduction steps and octinction coefficients of 4.9 and 4.5 mM cm for the semiquinones, FliH and rijH the Eg values assumed for steps 2 and 3 were —270 and —290 mV. The precise fit was very sensitive to all of the assumptions (40 ) ... 

Photosynthesis comprises a light-induced and a dark reaction. The first, called photophosphorylation, involves the two-electron reduction of nicotinamide adenine dinucleotide phosphate (NADP+) by water, to produce NADPH and oxygen. The redox reaction is coupled to the generation of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) ... 

In the light reactions of photosynthesis, water is converted to oxygen hy oxidation and NADP+ is reduced to NADPH. The series of redox reactions is coupled to the phosphorylation of ADP to ATP in a process called photophosphorylation. 

Nicotinamide adenine dinucleotide (NAD ) and nicotinamide adenine dinucleotide phosphate (NADP+) are structurally dinucleotides (Fig. 29). The two substances differ from each other by one phosphate residue, which is present in NADP" " and is attached to the 2 hydroxyl group of one of the ribose residues. The actual redox system is the nicotinamide which can be reversibly reduced, again by a 2 electron transition. Flavoproteins are very often coupled with NAD+or NADP+. In photosynthesis reduced FAD can transmit 2H (= 2e -)- 2H+) to NADP+, leading to NADPH -I- H+. 

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