Reduction cofactors

Electrochemical cofactor reduction can be achieved by direct reduction of the cofactor at the electrode surface, or indirectly by using a mediator molecule to shuttle electrons between the electrode and the cofactor. For details on the direct approach the reader is referred elsewhere [31, 32], since here no transition-metal complexes are involved. One point to be considered in the direct approach is the issue of selectivity. Whereas direct cofactor oxidation can be successfully achieved, special care must be taken to produce enzyme active reduced cofactors by direct electrolysis. 

Recently, the use of pentamethylcyclopentadienyl(l,10-phenanthrohne-5,6-di-one)chloro rhodium(III) hexafluorophosphate [(Cp )Rhm(phend)Cl]PF6, 11 (Fig. 43.4) has been reported for the electrochemical NAD+ reduction. TONs between 7 and 453 h-1 have been achieved by varying pH, temperature and the complex concentrations [44]. This study reveals only preliminary results, so the mechanism of cofactor reduction is not explained however, due to the structural... 

Scheme 43.3 Cofactor reduction using the pentamethyl cyclopentadienyl rhodium(bipyridine) complex (9/10).

Considering that these two transition-metal complexes are the only ones reported for the electrochemical cofactor reduction, the results are quite promising and show the need for further research in this field to identify new catalysts. In addition to the use of soluble redox mediators in electrochemical cofactor regeneration, modified electrodes have also been used. Details on these systems can also be found in the above-mentioned reviews [31, 32]. 

The first example of a chemical cofactor reduction utilizing hydrogen dates back to the 1980s, and is a hybrid approach [45]. 

These three systems are the only ones reported in the literature for achieving cofactor reduction utilizing molecular hydrogen and transition-metal complexes. 

Scheme 43.6 Cofactor reduction using a Pt-carbonyl-cluster/dye system.

Until now, only a few versatile, selective and effective transition-metal complexes have been applied in nicotinamide cofactor reduction. The TOFs are well within the same order of magnitude for all systems studied, and are within the same range as reported for the hydrogenase enzyme thus, the catalytic efficiency is comparable. The most versatile complex Cp Rh(bpy) (9) stands out due to its acceptance of NAD+ and NADP+, acceptance of various redox equivalents (formate, hydrogen and electrons), and its high selectivity towards enzymatically active 1,4-NAD(P)H. 

With biocatalysis becoming increasingly accepted in synthetic organic chemistry on both the laboratory and industrial scale, there is a huge need for new complexes that can utilize electrons or hydrogen as redox equivalents in cofactor reduction. These redox equivalents are very inexpensive, readily available, and produce no side products, which in turn significantly facilitates the downstream processing of products. 

Upon cofactor reduction (substrate oxidation), the terminal Mo=S is lost and is replaced by a sulfur with a Mo—S distance of 2.38 A. Complementary EPR studies of the Mov state suggest that, upon reduction, a hydrosulfido ligand (SH ) is coordinated to the metal [152-158]. The oxido remains in all oxidation states and likely acts as a spectator ligand. 

The protein environment thus exerts a powerful influence over the cluster reduction potentials. This observation applies to all classes of electron transferases—the factors that are critical determinants of cofactor reduction potentials are poorly understood at present but are thought to include the low dielectric constants of protein interiors ( 4 for proteins vs. —78 for H2O), electrostatic effects due to nearby charged amino-acid residues, hydrogen bonding, and geometric constraints imposed by the protein. 

In view of these observations, why are all of the electron transfers associated with mitochondrial respiration required For example, why is cytochrome c needed to shuttle electrons in Figures 6.11 and 6.12 when the cofactor reduction potentials of Complex III are more negative than those of Complex IV Evidently, factors other than AG° are of importance—these will be discussed in Sections III and IV. 

Figure 1 Enzymes as signal transdncers. As drawn, an enzyme specific for only one substrate is immobilized in close proximity to the electrode surface. The substrate (e.g., glutamate) is oxidized by the enzyme to the corresponding enzyme product (e.g., a-ketoglu-tarate) with concurrent rednction of a cofactor in a 1 1 ratio. (NAD" is rednced to NADH for dehydrogenase enzymes, and molecular oxygen is reduced to peroxide in the case of oxidase enzymes.) Of the fonr species present in solution, only the product of the cofactor reduction (NADH or H2O2) is electrochemically active and produces an analytical signal, so the enzyme substrate (glutamate in this example) is transduced to an electroactive species by the enzyme. Barring introduction of NADH (or peroxide) to the solntion, any increase in faradaic cnrrent may then be attributed to the presence of the enzyme snbstrate alone.

Although FeMo-cofactor is clearly knpHcated in substrate reduction cataly2ed by the Mo-nitrogenase, efforts to reduce substrates using the isolated FeMo-cofactor have been mosdy equivocal. Thus the FeMo-cofactor s polypeptide environment must play a critical role in substrate binding and reduction. Also, the different spectroscopic features of protein-bound vs isolated FeMo-cofactor clearly indicate a role for the polypeptide in electronically fine-tuning the substrate-reduction site. Site-directed amino acid substitution studies have been used to probe the possible effects of FeMo-cofactor s polypeptide environment on substrate reduction (163—169). Catalytic and spectroscopic consequences of such substitutions should provide information concerning the specific functions of individual amino acids located within the FeMo-cofactor environment (95,122,149). 

The conversion of a-ketoisovalerate (32) to ketopantoate (21) is cataly2ed by ketopantonate hydroxymethyltransferase and a cofactor tetrahydrofolate (65). Further reduction of ketopantoate (21) to (R)-pantoate (22) is cataly2ed by ketopantoic acid reductase (66). 

Although the stmctures of ribavirin and selenazofutin are similar, they appear to exert their antiviral action at different enzyme sites along the same biochemical pathway. Selenazofutin forms the nicotinamide adenosiae dinucleotide (NAD) analogue, which inhibits IMP dehydrogenase by binding ia place of the NAD cofactor, and hence this potent reduction of guanylate pools is responsible for the antiviral effect of selenazofutin. 

In oiological systems, the most frequent mechanism of oxidation is the remov of hydrogen, and conversely, the addition of hydrogen is the common method of reduc tion. Nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP) are two coenzymes that assist in oxidation and reduction. These cofactors can shuttle between biochemical reac tions so that one drives another, or their oxidation can be coupled to the formation of ATP. However, stepwise release or consumption of energy requires driving forces and losses at each step such that overall efficiency suffers. 

Heme d,6 another isobacteriochlorin, occurs as one of two cofactors in the reductase cytochrome cdj which mediates the nitrite reduction to nitrogen monoxide (NO) and from there to dinitrogen oxide (N20) in denitrifying bacteria.7... 

The elimination of the amino donor, L-aspartic acid, resulted in an almost complete reduction of activity. Neither cell permeabilisation nor cofactor (pyridoxalphosphate) addition were essential for L-phenylalanine production. Maximum conversion yield occurred (100%, 22 g r) when the amino donor concentration was increased. Aspartic add was a superior amino donor to glutamic add 35 g l 1 was used. 

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