Elimination reaction biological examples

All three elimination reactions--E2, El, and ElcB—occur in biological pathways, but the ElcB mechanism is particularly common. The substrate is usually an alcohol, and the H atom removed is usually adjacent to a carbonyl group, just as in laboratory reactions. Thus, 3-hydroxy carbonyl compounds are frequently converted to unsaturated carbonyl compounds by elimination reactions. A typical example occurs during the biosynthesis of fats when a 3-hydroxybutyryl thioester is dehydrated to the corresponding unsaturated (crotonyl) thioester. The base in this reaction is a histidine amino acid in the enzyme, and loss of the OH group is assisted by simultaneous protonation. 

Elimination reaction, 138. 383-393 biological examples of, 393 summary of, 393-394 Embden-Meyerhof pathway, 1143-1150 see also Glycolysis... 

In the previous chapter we discussed zero order kinetics however, many reactions in nature follow a different reaction scheme, namely a first order reaction. For example the elimination of a certain drug from the metabolism might follow a first order reaction. The decay of a radioactive substance, often used as a tracer in biological experiments is always described by a first order reaction. Another example for a first order kinetics is the growth of a bacterial culture, at least during certain phases of the growth. 

Substitution and elimination reactions are so useful that it is not surprising that they occur in nature. Alkyl halides, however, are not compatible with cytoplasm because they are hydrocarbon-like and therefore insoluble in water. In the cell, alkyl phosphates play the role that alkyl halides do In the laboratory. Adenosine triphosphate (ATP) is an example of a biologically important alkyl phosphate. We will abbreviate its structure here as follows (the full structure is given in Sec. 1 8.12). 

In the presence of a biological unit that can react as a base (such as water or a nitrogen atom from a different molecule), the hydrogen atom from the C=NH unit in 69 is transferring electrons to carbon (forming a CN unit, a nitrile), and this electron transfer process leads to an elimination reaction that forms ethylene, and Fe(II),with transfer of the OH unit to a suitable acid. The products are ethylene and cyanoformate (which then decomposes to HCN + CO2). Ascorbic acid is utilized in this transformation. This sequence is one example in which an elimination process plays a key role in a biosynthetic transformation. 

Crozet and co-workers have used S l reactions for synthesis of new heterocycles, which are expected to be biologically active (see also Section 7.3, which discusses synthesis of alkenes). For example, 2-chloromethyl-5-nitroimidazole reacts with the anion of 2-nitropropane to give 2-isopropylidene-5-nitroimidazole. It is formed via C-alkylation of the nitronate ion followed by elimination of HN02 (Eq. 5.33).51a Other derivatives of nitroimidazoles are also good substrates for SrnI reactions.5113 0... 

With the above-mentioned variety of addition reactions based on the addition-elimination mechanism almost any functional group or molecule can be attached to CgQ. Some examples are acetylenes [43, 52], peptides [53], DNA-fragments [53], polymers [54], macrocycles [55, 56], porphyrins [56, 57], dendrimers [58-60] or ligands for complex formation [56], Cjq can be turned into hybrids that are biologically active, water soluble, amphiphilic or mixable with polymers [53-55, 58, 61-69],... 

To make use of the Mannich reaction it is possible to methylate the N-atom of the new side chain and eliminate trimethylamine. The product, a 3-methyleneindoleninium salt, can then be trapped with suitable nucleophiles. In the example shown in Scheme 7.7b, cyanide ion is used, and reduction of the resultant nitrile yields the important amine trypta-mine. Indol-3-ylacetonitrile is also the source of indol-3-ylacetic acid and other biologically useful compounds (see Section 7.1.7). 

A biological matrix such as serum provides a good example. Nature generally tends to make metabolites more polar than the original compound. These polars exist to aid in elimination and excretion as well as serving as building blocks and reaction components. At the same time, nonpolar molecules are present in transport and structural roles and end up in the circulating blood. 

Waste-treatment processes commonly result in the production of solid wastes that must be disposed of safely. Enzymatic treatment is no exception. For example, although enzymatic treatment may not produce as large a quantity of solid products as does biological treatment, some solid residues may be formed, e.g., the polymer precipitates formed during the treatment of phenols with peroxidases, spent adsorbents such as talc, chitin, or activated carbon that are used to eliminate the soluble products of enzymatic reactions, or residues of plant materials such as raw soybean hulls when they are used in place of purified enzymes during treatment. Perhaps, the polymers and adsorbents could be incinerated to recover some energy if the emission of dangerous combustion by-products can be controlled or prevented. The residues of plant materials could potentially be composted and used as soil conditioners, provided that pollutants do not leach from them at substantial rates. To date, none of these disposal problems have been addressed adequately. 

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