In oxidation-reduction cofactor

IV. ENAMINES IN OXIDATION-REDUCTION COFACTORS A. Flavin Cofactors... 

In some cases, enzymes require the assistance of coenzymes (cofactors) to ensure the reactions proceed. Coenzymes include vitamins, metal ions, acids, and bases. They can act as transporters or electron acceptors or be involved in oxidation-reduction reactions. At the completion of the reaction, coenzymes are released, and they do not form part of the products. For some reactions that are energetically unfavorable, an energy source provided by the compound adenosine triphosphate (ATP) is needed to ensure the reactions proceed, as shown in the following reactions ... 

Distinct coenzymes are required in biological systems because both catabolic and anabolic pathways may exist within a single compartment of a cell. The nicotinamide coenzymes catalyze direct hydride transfer (from NAD(P)H or to NAD(P)+) to or from a substrate or other cofactors active in oxidation-reduction pathways, thus acting as two-electron carriers. Chemical models have provided... 

Other biomimetic reactions are based on the catalytic properties of metal ions. Many enzymes require metal ions that function, in one way or another, in oxidation-reduction processes. The wide range of such metal-ion reactions precludes mentioning more than a few in addition to the iron-porphyrin class, and in addition to chlorophyll, a number of enzymes require cobalamin as cofactor ferridoxin and high-potential iron proteins require iron-sulfur clusters, and nitrog-... 

Steenkamp, D. J., and Singer, T. P, 1976, On the presence of a novel covalently bound oxidation-reduction cofactor, iron and labile sulfur in trimethylamine dehydrogenase, Biochem. Biophys. Res. Commun. 71 1289nl295. 

The hydrogen atoms and their accompanying electrons generated in the TCA oxidation steps captured by the cofactors NAD and FAD now enter a third pathway called the electron transport pathway. Electrons are passed from protein to protein in this pathway in oxidation-reduction steps, and finally are combined with oxygen to form water. Compare these two separate processes for the production of CO2 and H2O with the simultaneous production of the same two substances from the combustion of glucose. The fate of H atoms and their electrons will be discussed in Sec. 22.6. 

Lead appears to be able to interact with complex small biomolecules as well, such as flavins for example, bis(lO-methylisoalloxazine) perchlorate tetrahy-drate (223). IsoaUoxazine is a planar three-ringed heterocychc amino cofactor associated with riboflavin and is active in oxidation-reduction reactions with metals such as Mo and Fe. Lead binds to bis(lO-methylisoalloxazine) in a 1 1 metal-ligand complex, with two additional waters bound resulting in a four coordinate molecule with a total of four oxygen donors. An active lone pair results in a distorted square-pyramidal structure. As is the case for citrate, extensive hydrogen bonding was observed in the crystal lattice. 

Another potential puzzle lies in the difference between the modes of operation of a coenzyme like NAD+ and a prosthetic group like FAD, both of them oxidation-reduction cofactors. The fundamental difference is that a prosthetic group forms a permanent part of the equipment of the enzyme, whereas a coenzyme like NAD+ arrives and departs just like any other substrate. In an oxidation carried out with the help of FAD, the reduced FAD would just have to wait for another substrate molecule to arrive to put it out of its misery and reoxidise it. In a similar reaction with NAD+, the NADH, once produced, would simply leave and And its own way to be reoxidised with the help of another protein parmer. 

Metallo-Flavoproteins. As was mentioned in the case of cytochrome reductase, enzymes are known that contain metal cofactors in addition to flavin. These are called metallo-flavoproteins. The presence of metals introduces complexity into the reaction, since the metals involved, iron, molybdenum, copper, and manganese, all exist in at least two valence states and can participate in oxidation-reduction reactions. The enzymes known to be metallo-flavoproteins include xanthine oxidase, aldehyde oxidase, nitrate reductase, succinic dehydrogenase, fatty acyl CoA dehydrogenases, hydrogenase, and cytochrome reductases. Before these are discussed in detail some physical properties of flavin will be presented. 

Niacin refers to a group of compoimds also known as vitamin B3, presenting similar biological activity, including nicotinamide, nicotinic acid, as well as other pyridine nucleotide structures. In the body, these compounds act as cofactors in oxidation—reduction reactions. To determine the total vitamin B3 content, either an acid or alkaline hydrolysis is necessary. The separation is normally performed by RPLC with fluorescence (322 nm ex., 380 nm em.) or UV detection (254 nm). 

In certain bacteria there is a specific nutritional requirement for D-amino acids which are found as components of cell structures or antimetabolites. Bacteria normally meet this need by the conversion of L-amino acids to D-amino acids and in the case of alanine, methionine and tryptophan the evidence suggests that these reactions are directly catalysed by amino acid racemases which have a cofactor requirement for pyridoxal phosphate . An oxidation-reduction cofactor may also be a general feature of racemases of this class. However, the mode of epimerisation of L-phenylalanine to D-phenylalanine necessary for the synthesis of some peptide antibiotics, proceeds in an entirely different way, which as yet has only been partially resolved. 

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. 

Many dehydrogenase enzymes catalyze oxidation/reduction reactions with the aid of nicotinamide cofactors. The electrochemical oxidation of nicotinamide adeniiw dinucleotide, NADH, has been studied in depthThe direct oxidation of NADH has been used to determine concentration of ethanol i s-isv, i62) lactate 157,160,162,163) pyTuvate 1 ), glucose-6-phosphate lactate dehydrogenase 159,161) alanine The direct oxidation often entails such complications as electrode surface pretreatment, interferences due to electrode operation at very positive potentials, and electrode fouling due to adsorption. Subsequent reaction of the NADH with peroxidase allows quantitation via the well established Clark electrode. 

The most important product of the hexose monophosphate pathway is reduced nicotinamide-adenine dinucleotide phosphate (NADPH). Another important function of this pathway is to provide ribose for nucleic acid synthesis. In the red blood cell, NADPH is a major reducing agent and serves as a cofactor in the reduction of oxidized glutathione, thereby protecting the cell against oxidative attack. In the syndromes associated with dysfunction of the hexose monophosphate pathway and glutathione metabolism and synthesis, oxidative denaturation of hemoglobin is the major contributor to the hemolytic process. 

In respect of designing an economic production process, the stoichiometric cofactor required in carbonyl reductions or the respective oxidation reactions needs to be minimized that is, enabled by recycling of the cofactor. The measure for the efficiency of the recycling process is the total turnover number (TTN), which describes the moles of product synthesized in relation to the moles of cofactor needed. The different approaches in cofactor recycling were recently reviewed by Goldberg et at. [12]. 

Sulfite oxidase is a dimetallic enzyme that mediates the two-electron oxidation of sulfite by the one-electron reduction of cytochrome c. This reaction is physiologically essential as the terminal step in oxidative degradation of sulfur compounds. The enzyme contains a heme cofactor in the 10 kDa N-terminal domain and a molybdenum center in the 42 kDa C-terminal domain. The catalytic cycle is depicted in Fig. 9. 

Indicine IV-oxide (169) (Scheme 36) is a clinically important pyrrolizidine alkaloid being used in the treatment of neoplasms. The compound is an attractive drug candidate because it does not have the acute toxicity observed in other pyrrolizidine alkaloids. Indicine IV-oxide apparently demonstrates increased biological activity and toxicity after reduction to the tertiary amine. Duffel and Gillespie (90) demonstrated that horseradish peroxidase catalyzes the reduction of indicine IV-oxide to indicine in an anaerobic reaction requiring a reduced pyridine nucleotide (either NADH or NADPH) and a flavin coenzyme (FMN or FAD). Rat liver microsomes and the 100,000 x g supernatant fraction also catalyze the reduction of the IV-oxide, and cofactor requirements and inhibition characteristics with these enzyme systems are similar to those exhibited by horseradish peroxidase. Sodium azide inhibited the TV-oxide reduction reaction, while aminotriazole did not. With rat liver microsomes, IV-octylamine decreased... 

Since many of the transformations undergone by metabolites involve changes in oxidation state, it is understandable that cofactors have been developed to act as electron acceptors/ donors. One of the most important is that based on NAD/NADP. NAD+ can accept what is essentially two electrons and a proton (a hydride ion) from a substrate such as ethanol in a reaction catalysed by alcohol dehydrogenase, to give the oxidized product, acetaldehyde and the reduced cofactor NADH plus a proton (Figure 5.2). Whereas redox reactions on metal centres usually involve only electron transfers, many oxidation/reduction reactions in intermediary metabolism, as in the case above, involve not only electron transfer but... 

The oxidation/reduction of redox cofactors in biological systems is often coupled to proton binding/release either at the cofactor itself or at local amino acid residues, which provides the basic mechanochem-ical part of a proton pump such as that foimd in cytochrome c oxidase (95). Despite a thermodynamic cycle that provides that coupling of protonation of amino acids to the reduction process will result in a 60 mV/pH decrease unit in the reduction potential per proton boimd between the pAa values in the Fe(III) and Fe(II) states, the essential pumping of protons in the respiratory complexes has yet to be localized within their three-dimensional structures. 

The FeMo cofactor (or M center) in the MoFe-proteinis in the native paramagnetic M state. Reduction of the MoFe-protein by the Fe-protein results in the reduction of FeMo-co from the M state to the M state at a potential estimated to be less than —0.465 V (NHE). The electron paramagnetic resonance (EPR) silent M state is only transiently produced during catalysis, and relaxes to the M state when catalysis stops. The intimate consequences of the M state reduction are not precisely known. A more oxidized diamagnetic state may also be generated (M ) at —0.042 V but its biological relevance is unclear [9]. 

Dehydrogenases also represent a class of interesting enzymes since enantiose-lective reduction of ketones can lead to the production of enantiomerically pure secondary alcohols for the fine chemicals industry. Compared to liquid systems, in which the cofactor is often eliminated by the circulating phase in continuous systems, solid/gas catalysis can be highly suitable since it has been demonstrated that the cofactor is stable and its regeneration effective by addition of a second substrate. Also, stereoselective oxidation of secondary alcohols by these systems can help in the resolution of racemic mixtures. 

On the other hand, Feigelson and his associates169,170,172-175 have reported that not only heme but also copper is an essential cofactor in both Pseudomonad and hepatic enzymes and that there are three different oxidation-reduction states of the enzyme, namely, a fully reduced form, E(Cu+, Fe2+) a half-reduced form, E(Cu+, Fe3+) or its valence isomer E(Cu2+, Fe2+) and a fully-oxidized form,... 

RGURE 7 An oxidation-reduction reaction. Shown here is the oxidation of lactate to pyruvate. In this dehydrogenation, two electrons and two hydrogen ions (the equivalent of two hydrogen atoms) are removed from C-2 of lactate, an alcohol, to form pyruvate, a ketone. In cells the reaction is catalyzed by lactate dehydrogenase and the electrons are transferred to a cofactor called nicotinamide adenine dinucleotide. This reaction is fully reversible pyruvate can be reduced by electrons from the cofactor. In Chapter 13 we discuss the factors that determine the direction of a reaction. 

The fifth cofactor of the PDH complex, lipoate (Fig. 16-4), has two thiol groups that can undergo reversible oxidation to a disulfide bond (—S—S—), similar to that between two Cys residues in a protein. Because of its capacity to undergo oxidation-reduction reactions, lipoate can serve both as an electron hydrogen carrier and as an acyl carrier, as we shall see. 

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