Phosphoester bonds

ATP is hydrolyzed in at least two consecutive steps on F-actin, viz. cleavage of the y-phosphoester bond, followed by Pj release, according to the following scheme ... 

While hydrolysis is a thermodynamically favored reaction, the amide and phosphoester bonds of polypeptides and ohgonucleotides are stable in the aqueous environment of the cell. This seemingly paradoxic behavior reflects the fact that the thermodynamics governing the equihbrium of a reaction do not determine the rate at which it will take place. In the cell, protein catalysts called enzymes are used to accelerate the rate... 

Our interest in the past few years has been on biodegradable polymers. We have been evaluating the potential of poly(phosphoesters) as degradable biomaterials (4).We were attracted to this class of polymers because the phosphoester bond in the backbone is cleavable under physiological conditions, the presence of the P-O-C group would facilitate fabrication, and the versatile chemical structure affords a wide... 

Biodegradable polyurethanes have been proposed and studied before (9-72). The difference in our study is the inclusion of a phosphoester linkage instead of the commonly used polyester component. This seems to provide more flexibility as the side chain of the phosphate or phosphonate can be varied. For controlled drug delivery applications, drugs can be linked to this site to form a pendant delivery system. Moreover, for certain medical applications, fast degradation rate is obtainable by the introduction of these hydrolyzable phosphoester bonds. With the LDI based polyurethanes, drugs or other compounds of interest can also be coupled to the ester side chain of the lysine portion. 

Fluorapatite is the only significant phosphorus-containing mineral in the Earth s crust and schreibersite has been found in iron meteorites. The only organic species to be found containing phosphorus in meteorites are the alkyl phosphonic acids. These are at least promising even if they do not contain the P-O-P phosphoester bond unit. 

Ito, T. Saito, M. Kobayashi, K. Dissociation of a model DNA compound dApdA by monochromatic soft X-rays in solids and comments on the high selectivity for 3 breakage in the phosphoester bond. Int. J. Radiat. Biol. 1992, 62 (2), 129-136. 

Polysaccharide-1-phosphates were extracted in some cases from bacterial cell walls. For example, TA isolated from Staphylococcus lactis 2102 consists of ca. 23 monomeric fragments each built of a-D-N-acetylglucosamine-1 -phosphate 22). These units are linked by the phosphoester bond at the 6 position in the sugar residue, as represented by 7. 

The EDTA-Tb3+ complex is poorly luminescent, but becomes strongly emissive when it forms a ternary complex with salicylic acid. By using alkaliphosphatase (ALP) as the label of the antibody, a unique immunoassay system specific to Tb3+ was constmcted (fig. 11) by Evangelista et al. (1991) and Christopoulos et al. (1992). The enzyme ALP cleaves the phosphoester bond of the substrate and releases 5-fluorosalicylic acid, which binds to EDTA-Tb3+ and sensitizes luminescence from Tb3+. This method is employed for the determination of semm AFP (cx-fetoprotein) and PSA (prostate-specific antigen). 

For anion-exchangers, activation involves washing on a ground glass filter, first with dilute add (0.5 M HC1), then with water, and finally with dilute alkali (0.5 M NaOH). With cation-exchangers, the order of treatment is reversed first alkali, then water and finally acid. Exposure to acid or alkali should be kept shorter than 30 min, and for phosphocellulose and other pH-sensitive ion-exchangers, 0.1 M rather than 0.5 M solutions should be used, and the period of treatment should be limited in order to avoid degrading the phosphoester bonds which are sensitive to hydrolysis. 

Fig. 1. The phosphorylated forms of adenosine the low-energy phosphoester bond and the high-energy phosphoanhydride bonds.

Nucleotide A nucleoside that has a phosphoester bond to one of the hydroxyl groups of the pentose sugar. 

Hydrolysis. Carboxylesterases are frequently one of the major factors in OP resistance. In some insects, for instance the house fly (28), there are highly substrate specific esterases which attack only one or a very few molecules. "Malathionase", the prominent esterase responsible for many cases of malathion resistance, is highly specific for malathion. It cleaves one or both of the ethyl ester groups leaving malathion mono- or diacid (29). This enzyme is a true serine carboxylesterase that is inhibited by malaoxon (28) and does not hydrolyze any of the phosphoester bonds. In Anopheles stephensi from Pakistan, the malathion resistance decreased with adult age, but there was no concommittant decrease in general esterase activity as measured with 1- and 2-naphthylace-tate as model substrates (301. other mosquitoes have a carboxylesterase with broad substrate specificity that is associated with resistance (31-331. As mentioned above, the green peach aphid has a carboxylesterase, E4, with broad substrate specificity that sequesters toxicants (24). 

A phosphoanhydrlde bond or other high-energy hond (commonly denoted by ) is not Intrinsically different from other covalent bonds. High-energy bonds simply release especially large amounts of energy when broken by addition of water (hydrolyzed). For Instance, the AG" for hydrolysis of a phosphoanhydrlde bond in ATP (—7.3 kcal/mol) is more than three times the AG" for hydrolysis of the phosphoester bond (red) in glycerol 3-phosphate (—2.2 kcal/mol) ... 

Several eukaryotic proteins that function In DNA replication are not depicted In Figure 4-34. A topolsomerase associates with the parental DNA ahead of the hellcase to remove torsional stress Introduced by the unwinding of the parental strands. Ribonuclease H and FEN I remove the ribonucleotides at the 5 ends of Okazaki fragments these are replaced by deoxynucleotides added by DNA polymerase 8 as it extends the upstream Okazaki fragment. Successive Okazaki fragments are coupled by DNA ligase through standard 5 —>3 phosphoester bonds. 

Nature s high energy bonds, including phosphoanhydride and phosphoester bonds, are discussed in Section 15.4. 

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