Redox coenzyme

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

This particular reaction has been chosen for the reason of its high value of standard chemical affinity for this reaction (j / = — 7.1 kcal/mole). As we noted above, due to this circumstance the system behavior can reveal the deviation from that prescribed by the classical Arrhenius mechanism. The conformational changes in malatdehydrogenase were tested by measuring the average life-time of intrinsic tryptophane fluorescence (ff). This parameter is known to be sensitive to the immediate surrounding of tryptophane residues. The chemical transformation of the substrate was detected from changes in the coenzyme redox state measured in terms of the sample optical density at 340 nm (NADH absorption maximum). 

NAD and NADP are required as redox coen2ymes by a large number of enzymes and ia particular dehydrogenases (Fig. 6). NAD" is utilized ia the catabohe oxidations of carbohydrates, proteins, and fats, whereas NADPH2 is the coenzyme for anaboHc reactions and is used ia fats and steroid biosynthesis. NADP+ is also used ia the cataboHsm of carbohydrates (2). 

Figure 1.9 Examples of functionally important intrinsic metal atoms in proteins, (a) The di-iron center of the enzyme ribonucleotide reductase. Two iron atoms form a redox center that produces a free radical in a nearby tyrosine side chain. The iron atoms are bridged by a glutamic acid residue and a negatively charged oxygen atom called a p-oxo bridge. The coordination of the iron atoms is completed by histidine, aspartic acid, and glutamic acid side chains as well as water molecules, (b) The catalytically active zinc atom in the enzyme alcohol dehydrogenase. The zinc atom is coordinated to the protein by one histidine and two cysteine side chains. During catalysis zinc binds an alcohol molecule in a suitable position for hydride transfer to the coenzyme moiety, a nicotinamide, [(a) Adapted from P. Nordlund et al., Nature 345 593-598, 1990.)...

FIGURE 18.19 The structures and redox states of the nicotinamide coenzymes. Hydride ion (H , a proton with two electrons) transfers to NAD to produce NADH. 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

Access to three different redox states allows flavin coenzymes to participate in one-electron transfer and two-electron transfer reactions. Partly because of this, flavoproteins catalyze many different reactions in biological systems and work together with many different electron acceptors and donors. These include two-electron acceptor/donors, such as NAD and NADP, one- or two-elec-... 

The final step of the reaction involves the transfer of two electrons from iron-sulfur clusters to coenzyme Q. Coenzyme Q is a mobile electron carrier. Its isoprenoid tail makes it highly hydrophobic, and it diffuses freely in the hydrophobic core of the inner mitochondrial membrane. As a result, it shuttles electrons from Complexes I and II to Complex III. The redox cycle of UQ is shown in Figure 21.5, and the overall scheme is shown schematically in Figure 21.6. 

In the third complex of the electron transport chain, reduced coenzyme Q (UQHg) passes its electrons to cytochrome c via a unique redox pathway known as the Q cycle. UQ cytochrome c reductase (UQ-cyt c reductase), as this complex is known, involves three different cytochromes and an Fe-S protein. In the cytochromes of these and similar complexes, the iron atom at the center of the porphyrin ring cycles between the reduced Fe (ferrous) and oxidized Fe (ferric) states. 

Zincke salts have played an important role in the synthesis of NAD /NADH coenzyme analogs since a 1937 report on the Zincke synthesis of dihydropyridine 7 for use in a redox titration study.The widely utilized nicotinamide-derived Zincke salt 8, first synthesized by Lettre was also used by Shifrin in 1965 for the preparation and study of NAD /NADH analogs. In 1972, Secrist reported using 8 for synthesis of simplified NAD analogs such as 10 for use in spectroscopic studies (Scheme 8.4.4). Subsequent utilization of 8 is discussed later in this article. 

The redox properties of quinones are crucial to the functioning of living cells, where compounds called ubiquinones act as biochemical oxidizing agents to mediate the electron-transfer processes involved in energy production. Ubiquinones, also called coenzymes Q, are components of the cells of all aerobic organisms, from the simplest bacterium to humans. They are so named because of their ubiquitous occurrence in nature. 

As a rule, the anabolic pathway by which a substance is made is not the reverse of the catabolic pathway by which the same substance is degraded. The two paths must differ in some respects for both to be energetically favorable. Thus, the y3-oxidation pathway for converting fatty acids into acetyl CoA and the biosynthesis of fatty acids from acetyl CoA are related but are not exact opposites. Differences include the identity of the acvl-group carrier, the stereochemistry of the / -hydroxyacyl reaction intermediate, and the identity of the redox coenzyme. FAD is used to introduce a double bond in jS-oxidalion, while NADPH is used to reduce the double bond in fatty-acid biosynthesis. 

Flavin Adenine Dinucleotide (FAD) (C27 H33 N9 O15P2) is a coenzyme that acts as a hydrogen acceptor in dehydrogenation reactions in an oxidized or reduced form. FAD is one of the primary cofactors in biological redox reactions. 

CODH/ACS is an extremely oxygen-sensitive protein that has been found in anaerobic microbes. It also is one of the three known nickel iron-sulfur proteins. Some authors would consider that there are only two, since the CODH and ACS activities are tightly linked in many organisms. However, there is strong evidence that the ACS and CODH activities are associated with different protein subunits and the reactions that the two enzymes catalyze are quite different. CODH catalyzes a redox reaction and ACS catalyzes the nonredox condensation of a methyl group, a carbonyl group, and an organic thiol (coenzyme A). 

The water-soluble B vitamins supply important components of numerous coenzymes. Many coenzymes contain, in addition, the adenine, ribose, and phosphoryl moieties of AMP or ADP (Figure 7-2). Nicotinamide and riboflavin are components of the redox coenzymes... 

Flavins — Riboflavin is first of all essential as a vitamin for humans and animals. FAD and FMN are coenzymes for more than 150 enzymes. Most of them catalyze redox processes involving transfers of one or two electrons. In addition to these well known and documented functions, FAD is a co-factor of photolyases, enzymes that repair UV-induced lesions of DNA, acting as photoreactivating enzymes that use the blue light as an energy source to initiate the reaction. The active form of FAD in photolyases is their two-electron reduced form, and it is essential for binding to DNA and for catalysis. Photolyases contain a second co-factor, either 8-hydroxy-7,8-didemethyl-5-deazariboflavin or methenyltetrahydrofolate. ... 

Although the reduction potentials of DNA bases and UV induced DNA lesions inside a DNA double strand or inside the active site of a DNA photolyase, together with the reduction potential of the photoexcited FADH- in the photolyases, are not known, currently available redox potentials indicate that the single electron reduction of a nucleobase or a UV induced dimer lesion by a reduced and deprotonated flavin coenzyme is a weakly exothermic process. The reduced and deprotonated FADH- in its photoexcited state is... 

A redox reaction is a special case of the equilibrium reaction of A + B in Equation 13.1 B is now a reducible group in a biomolecule with an EPR spectrum either in its oxidized or in its reduced state (or both), and A is now an electron or a pair of electrons, that is, reducing equivalents provided by a natural redox partner (a reductive substrate, a coenzyme such as NADH, a protein partner such as cytochrome c), or by a chemical reductant (dithionite), or even by a solid electrode ... 

The acceleration mechanism of redox mediators are presumed by van der Zee [15]. Redox mediators as reductase or coenzymes catalyze reactions by lowering the activation energy of the total reaction. Redox mediators, for example, artificial redox mediators such as AQDS, can accelerate both direct enzymatic reduction and mediated/indirect biological azo dye reduction (Fig. 3). In the case of direct enzymatic azo dye reduction, the accelerating effect of redox mediator will be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the azo dye. Possibly, both reactions will be catalyzed by the same nonspecific periplasmic enzymes. In the case of azo dye reduction by reduced enzyme cofactors, the accelerating effect of redox mediator will either be due to an electron shuttle between the reduced enzyme cofactor and redox mediator or be due to redox mediator enzymatic reduction in addition to enzymatic reduction of the coenzymes. In the latter case, the addition of redox mediator simply increases the pool of electron carriers. 

Ubiquinones (coenzymes Q) Q9 and Qi0 are essential cofactors (electron carriers) in the mitochondrial electron transport chain. They play a key role shuttling electrons from NADH and succinate dehydrogenases to the cytochrome b-c1 complex in the inner mitochondrial membrane. Ubiquinones are lipid-soluble compounds containing a redox active quinoid ring and a tail of 50 (Qio) or 45 (Q9) carbon atoms (Figure 29.10). The predominant ubiquinone in humans is Qio while in rodents it is Q9. Ubiquinones are especially abundant in the mitochondrial respiratory chain where their concentration is about 100 times higher than that of other electron carriers. Ubihydroquinone Q10 is also found in LDL where it supposedly exhibits the antioxidant activity (see Chapter 23). 

The microbes use two general strategies to synthesize ATP respiration and fermentation. A respiring microbe captures the energy released when electrons are transferred from a reduced species in the environment to an oxidized species (Fig. 18.1). The reduced species, the electron donor, sorbs to a complex of redox enzymes, or a series of such complexes, located in the cell membrane. The complex strips from the donor one or more electrons, which cascade through a series of enzymes and coenzymes that make up the electron transport chain to a terminal enzyme complex, also within the cell membrane. 

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. 

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