Forming Phosphoric Ester Bonds

NUCLEOSIDASES AND RELATED ENZYMES - LIGASES FORMING PHOSPHORIC ESTER BONDS (RESTORING BROKEN PHOSPHODIESTER BONDS IN NUCLEIC ACIDS) 

034336 Polyribonucleotide synthetase(ATP) [Poly(ribonucleotide) poly(ribonucleotide) ligase (AMP- 

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Substrate-ligase EC6.5 Forming Phosphoric Ester Bonds (DNA ligase (ATP), RNA ligase (ATP), etc.) ... 

E Polynucleotide synthetases Responsible for the linkage of two poly- or oligo-nudeotide moieties to form polynucleotide chains 6.5.1. Ligases forming phosphoric ester bonds... 

All the phosphates can be viewed as sources of phosphoric acid or poly-phosphoric acid and most of the development work in this area has concentrated on identifying the best co-additives to employ. In simple systems such as APP blended with polyhydric compounds like penta-erythritol it has been established that, above 200°C, the components react to form phosphoric ester bonds. Further elimination of water and ammonia leads to a carbon-phosphorus char. Without the pentaerythritol no char is formed. The char structure can be further improved by the addition of melamine which in this case can be viewed as a blowing agent, increasing the char volume. The formation of the char can therefore be seen as a sequence of overlapping reactions which must occur in a particular order for optimum performance [2]. 

Lipids build up the cell membrane. Chemically, lipids are esters between fatty acids and the threefold alcohol glycerine. The latter can form three ester bonds. The third is an ester with phosphoric acid (Figure 11.5). In the membrane, the hydrophobic parts are parallel. The polar head groups are hydrophilic and point outward. 

The orthophosphoric acid is a three-basic acid that can form three kinds of esters by reaction with alcohols. If all three OH groups of the phosphoric acid undergo an ester bond a neutral trialkyl ester will be the result, as Eq. (4) shows. 

When two phosphate residues bond, they do not form an ester, but an energy-rich phosphoric acid anhydride bond, as... 

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). 

Activated ester method The reactivity of carboxylates and phosphates can be increased by electron drawn and polarizing compounds, l-chloro-4-nitrobenzene or chloroacetonitril are added to form a reactive ester that polarizes the 5 -phosphate group to a partially positive charge. As noted above, the positivated phosphor atom is susceptible to a nucleophilic attack forming a covalent bond. 

Some food proteins are rich in phosphoric acid residues. The acid may either form ester bonds with Ser residues, as in caseins and egg proteins, or stabilize the native conformation of protein micelles by electrostatic interactions with negatively charged groups and calcium ions, as in caseins. In soy proteins Ser and Thr residues can be esterihed and Lys amidated with cyclic sodium trimetaphosphate at pH 11.5 and 35°C (Sung et al., 1983) ... 

As with peptide hydrolysis, several enzyme systems exist that catalyze carboxylic and phosphoric ester hydrolysis without the need for a metal ion. They generally involve a serine residue as the nucleophile in turn, serine may be activated by hydrogen-bond formation—or even proton abstraction—by other acid-base groups in the active site. The reaction proceeds to form an acyl- or phosphory 1-enzyme intermediate, which is then hydrolyzed with readdition of a proton to the serine oxygen. Mechanisms of this type have been proposed for chymotrypsin. In glucose-6-phosphatase the nucleophile has been proposed to be a histidine residue. ... 

How do nucleotides combine to give nucleic Acids Nucleosides are linked hy ester bonds to phosphoric acid to form the phosphodiester backbone. 

In a sugar phosphate, an ester bond is formed between one of the sugar hydroxyls and phosphoric acid. A glycosidic bond is an acetal, which can be hydrolyzed to regenerate the two original sugar hydroxyls. 

The dimer formed by means of hydrogen bonds does not react with free radicals. When the concentration of the ester is increased, the number of dimer molecules increases, and the rate constant of the reaction of di-tert-phenoxyl with the phosphorous ester decreases. 

Phosphoric acid has three —OH groups and forms mono-, di-, and triesters, which are named by giving the name(s) of the alkyl or aryl group (s) bonded to oxygen followed by the word phosphate, as, for example, dimethyl phosphate. In more complex phosphoric esters, it is common to name the organic molecule and then indicate the presence of the phosphoric ester using either the word phosphate or the prefix pfiospfio-. 

In contrast to the high resihence of phosphites toward oxygen and peroxides, they react easily with water [128]. As a result, the corresponding pentavalent species are formed (Scheme 2.106). Stepwise substitution of all aryloxy/alkoxy groups eventually produces phosphorous acid, H3PO3. Because of tautomeric equUibrium, acids are continuously generated, which support the hydrolysis of the ester bonds by autocatalysis. 

The most common phospholipids are phosphatidyl esters in which phosphatidic acid is esterified with another hydroxy compound to form a diester of phosphoric acid. Incorporation of choline and ethanolamine as the hydroxy compound would result in the formation of phosphatidylcholine (lecithin) and phosphatidyl ethanolamine (cephaline), respectively. There are enzymes specifically hydrolyze one or more of the four ester bonds in phospholipids and/or galactolipid. Each of the enzymes, phospholipase Ai (PL-Ai), phospholipase A2 (PL-A2), phospholipase C (PL-C), and phospholipase D (PL-D), cleaves a phosphatidyl ester into two products as shown in Figure 1. 

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