Phosphoric monoesters

The pronounced proclivity of phosphoric monoester monoanions to eliminate POf is not always recognizable from the characteristic pH profile of Fig. 1. The hydrolysis rate maximum at pH w 4 may be masked by a faster reaction of the neutral phosphoric ester, as in the case of a-D-glucose 1-phosphate63) or on hydrolysis of monobenzyl phosphate 64). In the latter case, the known ability of benzyl esters to undergo SN1 and SN2 reactions permits fast hydrolysis of the neutral ester with C/O bond breakage. The fact that the monoanion 107 of the monobenzyl ester is hydrolyzed some 40 times faster than the monoanion 108 of the dibenzyl ester at the same pH again evidences the special hydrolysis pathway of 107, rationalized by means of the metaphosphate hypothesis. 

The possible mechanisms for solvolysis of phosphoric monoesters show that the pathway followed depends upon a variety of factors, such as substituents, solvent, pH value, presence of nucleophiles, etc. The possible occurrence of monomeric metaphosphate ion cannot therefore be generalized and frequently cannot be predicted. It must be established in each individual case by a sum of kinetic and thermodynamic arguments since the product pattern frequently fails to provide unequivocal evidence for its intermediacy. The question of how free the PO ion actually exists in solution generally remains unanswered. There are no hard boundaries between solvation by solvent, complex formation with very weak nucleophiles such as dioxane or possibly acetonitrile, existence in a transition state of a reaction, such as in 129, or SN2(P) or oxyphosphorane mechanisms with suitable nucleophiles. 

Phosphatases of documented or potential interest in the context of this book include phosphoric monoester hydrolases (EC 3.1.3), phosphoric diester hydrolases (EC 3.1.4), and phosphoric triester hydrolases (EC 3.1.8). 

Phosphates of pharmaceutical interest are often monoesters (Sect. 9.3), and the enzymes that are able to hydrolyze them include alkaline and acid phosphatases. Alkaline phosphatase (alkaline phosphomonoesterase, EC 3.1.3.1) is a nonspecific esterase of phosphoric monoesters with an optimal pH for catalysis of ca. 8 [140], In the presence of a phosphate acceptor such as 2-aminoethanol, the enzyme also catalyzes a transphosphorylation reaction involving transfer of the phosphoryl group to the alcohol. Alkaline phosphatase is bound extracellularly to membranes and is widely distributed, in particular in the pancreas, liver, bile, placenta, and osteoplasts. Its specific functions in mammals remain poorly understood, but it seems to play an important role in modulation by osteoplasts of bone mineralization. 

Phosphatases are numerous and important enzymes (see also Chapt. 2). They are classified as phosphoric monoester hydrolases (phosphatases, EC 3.1.3), phosphoric diester hydrolases (phosphodiesterases, EC 3.1.4), triphosphoric monoester hydrolases (EC 3.1.5), diphosphoric monoester hydrolases (pyrophosphatases, EC 3.1.7), and phosphoric triester hydrolases (EC 3.1.8) [21] [63]. Most of these enzymes have a narrow substrate specificity restricted to endogenous compounds. However, some of these enzymes are active toward xenobiotic organophosphorus compounds, e.g., alkaline phosphatase (EC 3.1.3.1), acid phosphatase (EC 3.1.3.2), aryldialkylphosphatase (para-oxonase (PON1), EC 3.1.8.1) and diisopropyl-fluorophosphatase (tabunase, somanase, EC 3.1.8.2) [64 - 70]. However, such a classification is far from definitive and will evolve with further biochemical findings. Thus, a good correlation has been found in human blood samples between somanase and sarinase activities on the one hand, and paraoxonase (PON1) type Q isozyme concentrations on the other [71]. 

This enzyme [EC 3.1.3.1], also known as alkaline phos-phomonoesterase, phosphomonoesterase, and glycero-phosphatase, catalyzes the hydrolysis of many ortho-phosphoric monoesters (the substrate specificity is quite wide) to generate an alcohol and orthophosphate. The... 

Symbolized by Q, the quality index is a parameter used in studying the stereochemical nature of a particular enzyme-catalyzed reaction using chiral [ O, 0, 0] phosphoric monoesters. This parameter, which is dependent on the fractional content, allows the investigator a means to assess the reliability of the study(ies). 

An unstable derivative of this type is believed to result when phosphoric monoesters are treated with carbodiimides. The interaction of... 

Firestone and Heath442 found that the alkaline phosphatase (ortho-phosphoric monoester phosphate hydrolase, EC 3.1.3.1) that is induced in cultured cells by dibutanoy 1-cAMP is not detectable in the presence of inhibitors of glycosylation, although messenger RNA is found in proportions that are comparable to those of noninhibited... 

Buchwald SL, Pliura DH, Knowla JR (1980) Stereochemical evidence for pseudorotation in the reaction of a phosphoric monoester. J Am Chem Soc 106 4916-4922... 

In a classification from 1992 (International Union of Biochemistry, 1992) hydrolases of OP were described as a special entity as phosphoric triester hydrolases which comprise three groups of enzymes phosphoric monoester hydrolases (EC 3.1.3), phosphoric diester hydrolases (EC 3.1.4), and phosphoric triester hydrolases (phosphotriesterases) (EC 3.1.8). Phosphoric triester hydrolases are further divided in two similar subgroups aryldialkylphos-phatases (EC 3.1.8.1) and diisopropylfluorophosphatases (EC 3.1.8.2). 

Figure 13-1. Classes of enzymes involved in reaction at phosphorus. A and B represent enzyme types that handle phosphoric monoesters and related compounds ( 0 may be an oxygen of a hydroxyl, carboxyl, or phosphoryl group, or the nitrogen of a guanidine group. For simplicity, displacements at the y-phosphoryl groups of nucleosides triphosphates were classified with these reaction). C, D and E represent the enzymes that catalyze transformations of phosphoric diesters (displacements at a or (5 phosphorous groups of nucleoside triphosphates and transfer of pyrophosphates were classified with the reactions of phosphoric diesters).

Table 13-la. Enzymes accepting phosphoric monoesters as substrates. 

A/B Phosphatases Phosphoryl group transfer from a phosphoric monoester to water as an acceptor molecule. (Phosphoric monoesters are cleaved hydrolytically). 3.1.3. 3.6.1. Phosphoric ester hydrolases Hydrolases acting on acid anhydrides in phosphorous-containing anhydrides... 

NUCLEOTIDASES AND RELATED ENZYMES - PHOSPHORIC MONOESTER HYDROLASES... 

Enzyme assays based on inhibition effects are not as commonly employed as substrate determinations. but one or two are very important. Preeminent is the determination of organophosphorus compounds by using their inhibitory effect on cholinesterase enzymes (E.C. 3.1.1.8—the first digit signifies a hydrolase enzyme, the second that the compounds hydrolyzed are esters, and the third that they are phosphoric monoesters). The latter catalyze the conversion of acylcholines to choline and the corresponding acid ... 

A nucleotide is a nucleoside in which a molecule of phosphoric acid is esterified with a free hydroxyl of the monosaccharide, most commonly either the 3 -hydroxyl or the 5 -hydroxyl. A nucleotide is named by giving the name of the parent nucleoside followed by the word monophosphate. The position of the phosphoric ester is specified by the number of the carbon to which it is bonded. Figure 28.3 shows a structural formula of adenosine 5 -monophosphate, AMP. Monophosphoric esters are diprotic acids with values of approximately 1 and 6. Therefore, at pH 7, the two hydrogens of a phosphoric monoester are fully ionized, giving a nucleotide a charge of -2. 

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