Biological systems oxidation-reduction reactions

Most reactions that occur in living cells are some form of oxidation-reduction reactions. Oxidation-reduction reactions must occur together, since no substances can lose electrons without another substance gaining electrons. In biological systems, oxidation-reduction reactions involve not only transfer of electrons but also transfer of hydrogen that has both one proton (H+) and one electron. Oxidation of H2 gas will result in the release of H+ and one electron ... 

Oxidation and reduction reactions involve the transfer of electrons from one compound to another and play a major role in regulating many reactions in biological systems. Oxidation-reduction reactions are two coupled half reactions involving (i) oxidation and (ii) reduction. 

Formally, in redox reactions there is transfer of electrons from a donor (the reductant) to the acceptor (the oxidant), forming a redox couple or pair. Oxidations in biological systems are often reactions in which hydrogen is removed from a compound or in which oxygen is added to a compound. An example is the oxidation of ethanol to acetaldehyde and then to acetic acid where the oxidant is NAD. catalyzed by alcohol dehydrogenase and acetaldehyde dehydrogenase, respectively. 

Harry B. Gray and Walther Ellis,13 writing in Chapter 6 of reference 13, describe three types of oxidation-reduction centers found in biological systems. The first of these, protein side chains, may undergo oxidation-reduction reactions such as the transformation of two cysteine residues to form the cystine dimer as shown in equation 1.28 ... 

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... 

The reduction-oxidation potential (typically expressed in volts) of a compound or molecular entity measured with an inert metallic electrode under standard conditions against a standard reference half-cell. Any oxidation-reduction reaction, or redox reaction, can be divided into two half-reactions, one in which a chemical species undergoes oxidation and one in which another chemical species undergoes reduction. In biological systems the standard redox potential is defined at pH 7.0 versus the hydrogen electrode and partial pressure of dihydrogen of 1 bar. 

We live under a blanket of the powerful oxidant 02. By cell respiration oxygen is reduced to H20, which is a very poor reductant. Toward the other end of the scale of oxidizing strength lies the very weak oxidant H+, which some bacteria are able to convert to the strong reductant H2. The 02 -H20 and H+ - H2 couples define two biologically important oxidation-reduction (redox) systems. Lying between these two systems are a host of other pairs of metabolically important substances engaged in oxidation-reduction reactions within cells. 

COPPER (In Biological Systems), The activity of copper in plant metabolism manifests itself in two forms 11) synthesis of chlorophyll, and 12) activity of enzymes. In leaves, most of the copper occurs in close association with chlorophyll, but little is known of ns rale in chlorophyll synthesis, other than the presence of cupper is required. Copper is a definite constituent of several enzymes catalyzing oxidation-reduction reactions (oxidases), in which the activity is believed to be due to the shuttling of copper between the +1 and +2 oxidalicm states,... 

Actually, such an overall reaction occurs in living systems by a complicated series of individual steps. Some of these steps involve oxidation. The enzymes that bring about oxidation in the presence of free 02 are called oxidases. In general, biological oxidation-reduction reactions are catalyzed by oxidoreductase enzymes. 

For biological systems, the standard redox potential for a substance (E0 ) is measured under standard conditions, at pH 7, and is expressed in volts. In an oxidation-reduction reaction, where electron transfer is occurring, the total... 

NADP Nicotinamide adenine dinucleotide phosphate NADP and the related NAD (the reduced forms are NADPH2 and NADH2) are coenzymes that are involved in diverse oxidation-reduction reactions in biologic systems among the processes that require NADPH2 is the metabolism of many toxicants by microsomal enzymes in the mammalian liver. 

Group 11 (or IB) contains copper, which is the third most common transition metal found in biological systems. Copper in solution has two stable oxidation states, cuprous (Cu1+) and cupric (Cu2+) ion. The ability of copper to easily accept and donate electrons explains its important role in oxidation-reduction reactions and... 

Carbon isotopes are fractionated in biological cycles and in inorganic oxidation-reduction reactions. In equilibrium systems of various oxidation states, 13C enrichment up to 500 °K occurs in the following sequence ... 

Thus, by virtue of its relatively low potential for multiredox states, Mo is capable of mediating many oxidation-reduction reactions in biological systems. 

This shift in mechanism is due to the fact that reaction (79) is considerably slower than reaction (80) (115,141). These reactions have naturally to be considered also in the Cu(I)-catalyzed Fenton-like reactions where R radicals are formed. These processes are of special importance in biological systems in which copper complexes are known to induce oxidative stress (142-147). The Cu(I) species are formed in biological systems by reduction of Cu(II) by ascorbate, thiols, etc. 

There are overwhelming reports on superoxide anion generation in defined biological systems including redox reactions of small molecules, autoxidable proteins and oxidative enzymes The most prominent enzyme is xanthine oxidase. However, some precautions are neccessary, because the single electron reduction of dioxygen is considered to be an unnatural reactivity of this enzyme, which, under in vivo conditions acts like a dehydrogenase... 

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