Enzyme-catalyzed reactions oxidation-reduction

Enzymes catalyze many oxidation-reduction reactions in which the equivalent of dihydrogen is added or removed from a substrate molecule. 

The third remarkable aspect of enzyme catalysis is the versatility of these species. They catalyze an extremely wide variety of reactions— oxidation, reduction, polymerization, dehydration, dehydrogenation, etc. Their versatility is a reflection of the range and complexity of the chemical reactions necessary to sustain life in plants and animals. 

Despite the diverse range of documented enzyme-catalyzed reactions, there are only certain types of transformations that have thus far emerged as synthetically useful. These reactions are the hydrolysis of esters, reduction/oxidation reactions, and the formation of carbon-carbon bonds. The first part of this chapter gives a brief overview by describing some examples of various biotransformations that can easily be handled and accessed by synthetic organic chemists. These processes are now attracting more and more attention from nonspecialists of enzymes. 

In MET, a low-molecular-weight, redox-active species, referred to as a mediator, is introduced to shuttle electrons between the enzyme active site and the electrode.In this case, the enzyme catalyzes the oxidation or reduction of the redox mediator. The reverse transformation (regeneration) of the mediator occurs on the electrode surface. The major characteristics of mediator-assisted electron transfer are that (i) the mediator acts as a cosubstrate for the enzymatic reaction and (ii) the electrochemical transformation of the mediator on the electrode has to be reversible. In these systems, the catalytic process involves enzymatic transformations of both the first substrate (fuel or oxidant) and the second substrate (mediator). The mediator is regenerated at the electrode surface, preferably at low overvoltage. The enzymatic reaction and the electrode reaction can be considered as separate yet coupled. 

The majority of the enzyme-catalyzed reactions discussed so far are oxidative ones. However, reductive electron transfer reactions take place as well. Diaphorase, xanteneoxidase, and other enzymes as well as intestinal flora, aquatic, and skin bacteria—all of them can act as electron donors. Another source of an electron is the superoxide ion. It arises after detoxification of xenobiotics, which are involved in the metabolic chain. Under the neutralizing influence of redox proteins, xenobiotics yield anion-radicals. Oxygen, which is inhaled with air, strips unpaired electrons from these anion-radicals and gives the superoxide ions (Mason and Chignell 1982). 

This enzyme [EC 1.1.99.8], also referred to as alcohol dehydrogenase (acceptor) and methanol dehydrogenase, catalyzes the oxidation-reduction reaction of a primary alcohol with an acceptor to generate an aldehyde and the reduced acceptor. The cofactor for this enzyme is pyrroloquinoline qutnone (PQQ). A wide variety of primary alcohols can act as the substrate. See also Alcohol Dehydrogenase... 

This enzyme [EC 1.4.99.1] catalyzes the oxidation-reduction reaction of a D-amino acid with an acceptor and water to generate an a-keto acid, ammonia, and the reduced acceptor. The enzyme utilizes FAD as a cofactor. Most D-amino acids, with the noted exceptions of D-aspartate and D-glutamate, can be used as substrates. K. Tsukada (1971) Meth. Enzymol. 17B, 623. 

Reactions catalyzed by 11 (3-hydroxysteroid and 17(3-hydroxysteroid dehydrogenases, (a) 11 (3-hydroxysteroid dehydrogenase type 1, an NADPH-dependent enzyme, catalyzes the conversion of the inactive steroid, cortisone, to cortisol, which is the biologically active glucocorticoid. 11 (3-hydroxysteroid dehydrogenase type 2, an NAD+-dependent enzyme, catalyzes the reverse direction, (b) 17(3-hydroxysteroid dehy-drogenase type 1, an NADPH-dependent enzyme, catalyzes the reduction of estrone to estradiol. Type 2, an NAD+-dependent enzyme, catalyzes the oxidation of estradiol to estrone. Type 3, an NADPH-dependent enzyme, catalyzes the reduction of androstene dione to testosterone. Type 4, an NAD+-dependent enzyme, catalyzes the oxidation of estradiol to estrone, and androstenediol to dehydroepiandrosterone. 

The transfer of phosphoryl groups is a central feature of metabolism. Equally important is another kind of transfer, electron transfer in oxidation-reduction reactions. These reactions involve the loss of electrons by one chemical species, which is thereby oxidized, and the gain of electrons by another, which is reduced. The flow of electrons in oxidation-reduction reactions is responsible, directly or indirectly, for all work done by living organisms. In nonphotosynthetic organisms, the sources of electrons are reduced compounds (foods) in photosynthetic organisms, the initial electron donor is a chemical species excited by the absorption of light. The path of electron flow in metabolism is complex. Electrons move from various metabolic intermediates to specialized electron carriers in enzyme-catalyzed reactions. 

When oxidation and reduction are involved in an enzyme-catalyzed reaction, the standard apparent reduction potential for a half reaction can be calculated by typing the half reaction in calcappredpot and specifying the pHs and ionic strengths. 

Oxidoreductases comprise a large class of enzymes that catalyze biological oxidation/reduction reactions. Because so many chemical transformation processes involve oxidation/reduction processes, the idea of developing practical applications of oxidoreductase enzymes has been a very attractive, but quite elusive, goal for many years [83], Applications have been sought for the production of pharmaceuticals, synthesis and modification of polymers, and the development of biosensors for a variety of clinical and analytical applications [83], In recent years, the use of oxido-reductive enzymes to catalyze the removal of aromatic compounds from... 

Low-potential electron-transfer mediators such as viologens can substitute natural cofactors (particularly NADH) in some enzymatic reactions [184], The electrochemical reduction of viologens has been studied extensively [185] and they and other reductive electron mediators have been utilized to drive enzyme-catalyzed reactions [186], For instance, the electrochemical reduction of NAD(P)+ to NAD(P)H with a current efficiency of more than 97 % was achieved using alcohol dehydrogenase in the presence of acetophenone as an electron mediator [187], The addition of acetone or acetaldehyde as a substrate to the above bioelectrocatalytic system allowed the reduction of the substrate to the corresponding alcohol at alcohol dehydrogenase accompanied by the oxidation of the resulting NAD(P)H. 

During glucose addition to oxygen saturated solution the reduction current response of the biosensor decreased, which resulted from the electrocatalytic reaction restrained to the enzyme catalyzed reaction between the oxidized form of GOD and glucose. With increasing glucose concentration the catalytic reduction current of oxygen decreased. 

NAD(P)+(Fig. la) and the reduced form NAD(P)H (Fig. lb) are the coenzymes of oxidoreductases, enzymes that catalyze the oxidation/reduction of the substrates. The general reaction scheme is ... 

Mitsunobu reaction [67], enzyme-catalyzed reactions [67], oxidation sec. alcohol —> ketone, reduction ketone —> alcohol [69]. 

Dehydrogenases, reductases and a number of other enzymes, such as UDP-glucose epimerase, utilize NAD or NADP as an enzymatic cofactor and catalyze the oxidation/reduction of various substrates, facilitating the usually reversible stereospecific hydride transfer from the C4 position of the 1,4 dihydronicotinamide ring of NAD(P)H to substrate. The reaction catalyzed by lactate dehydrogenase and a schematic drawing of the putative hydride transfer reaction that takes place are shown in Fig. 15.1. 

Racemases and epimerases have been used for production of various optically active compounds from cheaply-available racemic substrates by combination of enzymes that act specifically on one of the isomers of the racemates to catalyze hydrolysis, oxidation, reduction, elimination, replacement, and other reactions. The racemases and epimerases used act exclusively on the substrates, but not on the products of the... 

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