Ester in biological systems

Ethers are less significant than esters in biological systems, like esters with organic adds ethers are much less polar than the parent alcohol, and ether formation may be used as a metabolic deactivation reaction such as in the metabolism of adrenaline to metanephrine (Fig. 5.20) which removes its biological activity. 

SCHEME 10.2 Common pathways of QM formation in biological systems, (a) Stepwise two-electron oxidation by cytochrome P450 or a peroxidase, (b) Enzymatic oxidation of a catechol followed by spontaneous isomerization of the resulting n-quinone. (c) Enzymatic hydrolysis of a phosphate ester followed by base-catalyzed elimination of a leaving group from the benzylic position. 

As in the case of esters, hydrolysis of amides is also a fundamental reaction in organic chemistry and plays a key role in biological systems. The reaction has been covered extensively in organic chemistry and biochemistry textbooks. 

Liquid crystal display technology, 15 113 Liquid crystalline cellulose, 5 384-386 cellulose esters, 5 418 Liquid crystalline conducting polymers (LCCPs), 7 523-524 Liquid crystalline compounds, 15 118 central linkages found in, 15 103 Liquid crystalline materials, 15 81-120 applications of, 15 113-117 availability and safety of, 15 118 in biological systems, 15 111-113 blue phases of, 15 96 bond orientational order of, 15 85 columnar phase of, 15 96 lyotropic liquid crystals, 15 98-101 orientational distribution function and order parameter of, 15 82-85 polymer liquid crystals, 15 107-111 polymorphism in, 15 101-102 positional distribution function and order parameter of, 15 85 structure-property relations in,... 

C. J. Anderson, L. J. H. Lucas, T. S. Widlanski, Molecular Recognition in Biological Systems Phosphate Esters vs. Sulfate Esters and the Mechanism of Action of Steroid Sulfatases , J. Am. Chem. Soc. 1995,117, 3889-3890. 

Lipases are enzymes that catalyze the in vivo hydrolysis of lipids such as triacylglycerols. Lipases are not used in biological systems for ester synthesis, presumably because the large amounts of water present preclude ester formation due to the law of mass action which favors hydrolysis. A different pathway (using the coenzyme A thioester of a carboxylic acid and the enzyme synthase [Blei and Odian, 2000]) is present in biological systems for ester formation. However, lipases do catalyze the in vitro esterification reaction and have been used to synthesize polyesters. The reaction between alcohols and carboxylic acids occurs in organic solvents where the absence of water favors esterification. However, water is a by-product and must be removed efficiently to maximize conversions and molecular weights. 

Monosaccharides also form phosphate esters with phosphoric acid. Monosaccharide phosphate esters are important molecules in biological system. For example, in the DNA and RNA nucleotides, phosphate esters of 2-deoxyribose and ribose are present, respectively. Adenosine triphosphate (ATP), the triphosphate ester at C-5 of ribose in adenosine, is found extensively in living systems. 

Hay, R. W., and P. J. Morris, Metal ion-promoted hydrolysis of amino acid esters and peptides . In Metal Ions in Biological Systems, Vol. 5, H. Siegel, Ed., Marcel Dekker, New York, 1976, pp. 173-243. 

PHOSPHORUS (In Biological Systems). Phosphorus is required by every living plant and animal cell. Deficiencies of available phosphorus in soils are a major cause of limited crop production, Phosphorus deficiency is probably the most critical mineral deficiency in grazing livestock. Phosphorus, as orthophosphate or as the phosphoric acid ester of organic compounds, has many functions in the animal body. As such, phosphorus is an essential dietary nutrient. 

Sulfate and Organic Sulfates. Inorganic sulfate ion (SO L-) occurs widely in nature. Thus, it is not surprising that this ion can be used in a number of ways in biological systems. These uses can be divided primarily into two categories (1) formation of sulfate esters and the reduction of sulfate to a form that will serve as a precursor of the amino acids cysteine and methionine and (2) certain specialized bacteria use sulfate to oxidize carbon compounds and thus reduce sulfate to sulfide, while other specialized bacterial species derive energy from the oxidation of inorganic sullur compounds to sulfate. 

However, phosphate esters are slow to hydrolyze in water (unless a catalyst is present). The difference in kinetic and thermodynamic stability of phosphate esters toward hydrolysis is used to great effect in biological systems. 

Deposition of liquid crystalline material containing cholesterol esters on the inner surfaces of human arteries is another nonequilibrium process in which much interest exists. No doubt there are other examples as well where dynamic phenomena involving liquid crystals are important in biological systems. 

Reactions involving the formation and hydrolysis of phosphate and polyphosphate esters are of vital importance in biological systems in which it is found that magnesium ions are almost invariably implicated. The formation and decomposition of adenosine triphosphate are the fundamental reactions involved in energy storage in living systems. In this context, it is perhaps relevant to note that the hydrolysis of ATP is enhanced, albeit in a very modest manner, by some cobalt(m) complexes. 

Derivatives of phosphoric acid, pyrophosphoric acid, and related compounds are very important in biological systems. Pyrophosphoric acid is an anhydride of phosphoric acid. Adenosine triphosphate, an energy carrier that is universally found in living organisms, has a phosphorus dianhydride connected to an adenosine group by a phosphate ester linkage. Phosphorus ester bonds are used to form the polymeric backbone of DNA (see Chapter 27). 

Because of the relative simplicity of carboxylic ester hydrolysis, in general, and that of base catalyzed ester hydrolysis, in particular, these reactions have served well as model systems in investigations of micellar effects on reaction rates and activation parameters. In addition, the prevalence in biological systems of carboxylic ester hydrolyses catalyzed by nucleophiles and by enzymes renders the investigation of micelle-catalyzed ester hydrolyses of obvious importance. 

Photodeprotection of phenacyl and substituted phenacyl esters promises to be an efficient and mild method for the deprotection of carboxylic adds in biological systems.171- 72... 

When an animal eats more carbohydrate than it uses up, it stores the excess some as the polysaccharide glycogen (Sec. 35.9), but most of it as fats. Fats, we know (Sec. 33.2), are triacylglycerols, esters derived (in most cases) from long straight-chain carboxylic acids containing an even number of carbon atoms. These even numbers, we said, are a natural consequence of the way fats are synthesized in biological systems. 

Naphthalene, anthracene, and similar hydrocarbons are termed polycyclic aromatic hydrocarbons (PAHs) because they are composed of multiple aromatic rings. PAHs have been found in meteorites and identified in the material surrounding dying stars. Scientists have mixed PAHs with water ice in a vacuum at -260°C to simulate the conditions found in interstellar clouds. To simulate radiation emitted by nearby stars, they shined ultraviolet light on the mixture. About ten percent of the PAHs were converted to alcohols, ketones, and esters— molecules that can be used to form compounds that are important in biological systems. 

This leads to a consideration of reactions in which oi anosilicates are synthesized and degraded. Equations (3)—(5) describe theoretical reactions by which organosilicon esters or ethers may be formed in biological systems. 

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