Biological reaction, alcohol oxidation

Section 15 11 Oxidation of alcohols to aldehydes and ketones is a common biological reaction Most require a coenzyme such as the oxidized form of nicotin amide adenine dmucleotide (NAD" )... 

Enzyme Cofactors- In many enzymatic reactions, and in particular biological reactions, a second substrate (i.e., species) must be introduced to activate the enzyme. This substrate, which is referred to as a cofactor or coenzyme even though it is not an enzyme as such, attaches to the enzyme and is most often either reduced or oxidized during the course of die reaction. The enzyme-cofactor complex is referred to as a holoenzyme. The inactive form of the enzyme-cofactor complex for a specific reaction and reaction direction is called an apoenzyme. An example of the type of system in which a cofactor is used is the formation of ethanol from acetaldehyde in the presence of the enzyme alcohol dehydrogenase (ADH) and the cofactor nicotinamide adenine dinuoleotide (NAD) ... 

Since both alcoholic oxidation and O2 reduction are two-electron processes, the catalytic reaction is conceptually equivalent to a transfer of the elements of dihydrogen between the two substrates. Biological hydrogen transfer generally involves specialized organic redox factors (e.g., flavins, nicotinamide, quinones), with well-characterized reaction mechanisms. Galactose oxidase does not contain any of these conventional redox factors and instead utilizes a very different type of active site, a free radical-coupled copper complex, to perform this chemistry. The new type of active site structure implies that the reaction follows a novel biochemical redox mechanisms based on free radicals and the two-electron reactivity of the metalloradical complex. 

Because biological reactions involve chiral enzymes, enantiotopic groups and faces typically show different reactivity. For example, the two methylene hydrogens in ethanol are enantiotopic. Enzymes that oxidize ethanol, called alcohol dehydrogenases, selectively remove the pro-/ hydrogen. This can be demonstrated by using a deuterated analog of ethanol in the reaction. 

Many biological processes involve oxidation of alcohols to carbonyl compounds or the reverse process, reduction of carbonyl compounds to alcohols. Ethanol, for example, is metabolized in the liver to acetaldehyde in a reaction catalyzed by the enzyme alcohol... 

Substitution and Elimination Reactions of Alcohols Oxidation Biological Redox Reactions Oxidation of Phenol Synthesis Strategies... 

Stahl and Sheldon have shown how oxidations can be driven by air as primary oxidant, or source of stochiometric oxidizing power. Like the catalysts in this subsection, biological oxidases are enzymes that use O2 but do not incorporate its O atoms into the substrate. For example, Pd(OAc)2-pyridine is active for alcohol oxidation, intramolecular hetero- and carbocyclization of alkenes, intermolecular O-C and C-C coupling reactions with alkenes, and oxidative C-C bond cleavage of tertiary alcohols. A pathway for alcohol oxidation is shown in Eq. 9.27. Normally a 4e process, reduction of O2 can be hard to couple with oxidation of the catalytic intermediates, processes that often proceed in 2e steps. In this case, intermediate rj -peroxo Pd(II) complexes can be formed from reaction of Pd(0) intermediates with O2, which thus acts as a 2e oxidant. 

Osmium tetroxide, reaction with alkenes, 235-236 toxicity of, 235 Oxalic add, structure of, 753 Oxaloacetic acid, structure of, 753 Oxetane, reaction with Grignard reagents, 680 Oxidation, 233, 348 alcohols, 623-626 aldehydes, 700-701 aldoses, 992-994 alkenes, 233-236 biological, 625-626 phenols, 631 sulfides, 670 thiols, 668... 

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. 

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