Oxidative reactions enzymatic

The substrates of catabolism—proteins, carbohydrates, and lipids—are good sources of chemical energy because the carbon atoms in these molecules are in a relatively reduced state (Figure 18.9). In the oxidative reactions of catabolism, reducing equivalents are released from these substrates, often in the form of hydride ions (a proton coupled with two electrons, H ). These hydride ions are transferred in enzymatic dehydrogenase reactions from the substrates... 

The enzymatic reactions of peroxidases and oxygenases involve a two-electron oxidation of iron(III) and the formation of highly reactive [Fe O] " species with a formal oxidation state of +V. Direct (spectroscopic) evidence of the formation of a genuine iron(V) compound is elusive because of the short life times of the reactive intermediates [173, 174]. These species have been safely inferred from enzymatic considerations as the active oxidants for several oxidation reactions catalyzed by nonheme iron centers with innocent, that is, redox-inactive, ligands [175]. This conclusion is different from those known for heme peroxidases and oxygenases... 

The yeast-mediated enzymatic biodegradation of azo dyes can be accomplished either by reductive reactions or by oxidative reactions. In general, reductive reactions led to cleavage of azo dyes into aromatic amines, which are further mineralized by yeasts. Enzymes putatively involved in this process are NADH-dependent reductases [24] and an azoreductase [16], which is dependent on the extracellular activity of a component of the plasma membrane redox system, identified as a ferric reductase [19]. Recently, significant increase in the activities of NADH-dependent reductase and azoreductase was observed in the cells of Trichosporon beigelii obtained at the end of the decolorization process [25]. 

One of the best known enzymatic reactions involving glucose is the following oxidation reaction catalysed by glucose oxidase (GOD) ... 

Although the coupled oxidation reaction closely resembles the enzymatic reaction, it may not be identical. One clear difference is that heme free in solution is oxidized at all four meso positions 40), heme in myoglobin only at the a-meso position 6, 46) and heme in hemoglobin at the a- and -positions 6, 46). Prior to the observation that hemin in... 

The synthesis pathway of quinolizidine alkaloids is based on lysine conversion by enzymatic activity to cadaverine in exactly the same way as in the case of piperidine alkaloids. Certainly, in the relatively rich literature which attempts to explain quinolizidine alkaloid synthesis °, there are different experimental variants of this conversion. According to new experimental data, the conversion is achieved by coenzyme PLP (pyridoxal phosphate) activity, when the lysine is CO2 reduced. From cadeverine, via the activity of the diamine oxidase, Schiff base formation and four minor reactions (Aldol-type reaction, hydrolysis of imine to aldehyde/amine, oxidative reaction and again Schiff base formation), the pathway is divided into two directions. The subway synthesizes (—)-lupinine by two reductive steps, and the main synthesis stream goes via the Schiff base formation and coupling to the compound substrate, from which again the synthetic pathway divides to form (+)-lupanine synthesis and (—)-sparteine synthesis. From (—)-sparteine, the route by conversion to (+)-cytisine synthesis is open (Figure 51). Cytisine is an alkaloid with the pyridone nucleus. 

Types of Oxidation Reactions invoived in Enzymatic Drug Metaboiism... 

In subsequent research, it turned out that two-state reactivity can also provide a concept for the understanding of oxidation reactions way beyond the scope of gas-phase ion chemistry and can actually resolve a number of existing mechanistic puzzles. In enzymatic oxidations involving cytochrome P450, for example, changes in spin multiplicity appear to act as a kind of mechanistic distributor for product formation [27-29], and in the case of manganese-catalyzed epoxidation reactions, two-state scenarios have been put forward to account for the experimentally observed stereoselectivities [30-32], Two-state reactivity is not restricted to oxidation reactions, and similar scenarios have been proposed for a number of other experimentally studied reactions of 3d metal compounds [33-37]. Moreover, two-state scenarios have recently also been involved in the chemistry of main group elements [38]. The concept of two-state reactivity developed from the four-atomic system FeO /H2... 

Polyunsaturated fatty acids are oxidized by enzymatic and nonenzymatic pathways. Nonenzymatic oxidation is a free-radical mediated peroxidation. It is a chain reaction providing a continuous supply of free radicals that initiate further peroxidation. The whole process can be depicted as follows [21] ... 

The kinetics of the aerobic oxidation of enzymatically reduced nicotinamide adenine dinucleotide (NADH) have been investigated at pH 7.38 at 30°C. The reaction rate was followed spectropho-tometrically by measuring the decrease in absorbance at 340 nm over a period of 30 min. The reaction may be represented as... 

A number of oxidation reactions of mono- and difluorosteroid compounds has been reported. In some reactions, the specific influence of a fluoro substituent on the reactivity has been observed the presence of a 9a-fluorine in a 11 /i-hydroxy-A4-3-oxo steroid causes completely stcreospecific alkaline epoxidation with hydrogen peroxide in a much slower reaction (4d vs 4 h) in comparison with the nonfluorinated analog.322 Most oxidations are accomplished by the highest selective biochemical (that is, by bacterial enzymatic) transformations. As the biochemical oxidation systems are not discussed in this section, only a list of selected transformations of steroids is presented in Table 21. For additional information see ref 323. 

Popular oxidation reactions of peptide alcohols such as the Parikh-Doering or Dess-Martin in addition to older oxidation reactions such as Collins, pyridinium chlorochromate, or Swern oxidation afford racemization free productsJ9121415 37-39 Oxidations using pyridinium dichromate results in racemization and low yields of product.[l3 Oxidation reactions have also been utilized in semisynthetic pathways of peptide aldehydes (1) peptide aldehydes are obtained through the enzymatic acylation of a peptide ester to an amino alcohol with subsequent oxidation of the peptide alcohol to afford the aldehyde, and (2) peptide aldehydes can also be obtained by direct enzymatic oxidation of the peptide alcohol by alcohol de-hydrogenaseJ40 41 ... 

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