Metal hydroxides phosphate ester hydrolysis

There have been a few reports of first generation coordination complex structural models for the phosphatase enzyme active sites (81,82), whereas there are some examples of ester hydrolysis reactions involving dinuclear metal complexes (83-85). Kim and Wycoff (74) as well as Beese and Steitz (80) have both published somewhat detailed discussions of two-metal ion mechanisms, in connection with enzymes involved in phosphate ester hydrolysis. Compared to fairly simple chemical model systems, the protein active site mechanistic situation is rather more complex, because side-chain residues near the active site are undoubtedly involved in the catalysis, i.e, via acid-base or hydrogenbonding interactions that either facilitate substrate binding, hydroxide nucleophilic attack, or stabilization of transition state(s). Nevertheless, a simple and very likely role of the Lewis-acidic metal ion center is to... 

Figure 17-11 Two-step mechanism for phosphate ester hydrolysis. In the first step, substrate binds to the Fe ion, thereby displacing the bond from the -hydroxo ligand to this metal ion. The resulting Fe -coordinated hydroxide serves as the nucleophile in the hydrolysis reaction. The resulting species is a phosphate bridge, the same species observed in x-rays studies of product-inhibited enzyme.

Alkyl phosphates containing electronegative cyano groups at the -position of one of the alkyl groups readily undergo hydrolysis in alkaline media by P-elim-ination (Scheme 8.5.9). Another important neighboring effect concerns ribonucleotides. Ribonucleoside 3 - or 2 -phosphates are, for example, quantitatively hydrolyzed by lanthanum hydroxide at pH 6, whereas 2-deoxyribonucleotides are not dephosphorylated under these conditions. The role of the hydroxyl group is not known in this case. Heavy metal catalysis of phosphate ester hydrolysis is probably caused by complexation of the metal ions, which renders the phosphorus atom more electrophilic. 

The use of a mixed-valent, dinuclear iron site, similar to those in hemerythrin and ribonucleotide reductase,to catalyze a nonredox reaction such as phosphate ester hydrolysis is novel and unexpected for a variant of the familiar oxo(hydroxo)-bridged diiron center. In contrast to the general agreement that exists regarding the spectroscopic and physical properties of the PAPs, their kinetics properties and especially their mechanism of action remain controversial. Much of the disagreement stems from the different pH dependences of the catalytic activity of BSPAP and Uf, which is due to the fact that the former is isolated in a proteolytically activated form while the latter is not. Proteolysis results in a substantial increase in optimal pH in addition to an increase in catalytic activity at the optimal pH. "" Current data suggest that many of the spectroscopic studies described in the literature were performed on a catalytically inactive form of the enzyme. As a result, the roles of the trivalent and divalent metal ions in catalysis and in particular the identity of the nucleophilic hydroxide that directly attacks the phosphate ester remain unresolved. 

Perhaps the most extensively studied catalytic reaction in acpreous solutions is the metal-ion catalysed hydrolysis of carboxylate esters, phosphate esters , phosphate diesters, amides and nittiles". Inspired by hydrolytic metalloenzymes, a multitude of different metal-ion complexes have been prepared and analysed with respect to their hydrolytic activity. Unfortunately, the exact mechanism by which these complexes operate is not completely clarified. The most important role of the catalyst is coordination of a hydroxide ion that is acting as a nucleophile. The extent of activation of tire substrate througji coordination to the Lewis-acidic metal centre is still unclear and probably varies from one substrate to another. For monodentate substrates this interaction is not very efficient. Only a few quantitative studies have been published. Chan et al. reported an equilibrium constant for coordination of the amide carbonyl group of... 

Inspired by the many hydrolytically-active metallo enzymes encountered in nature, extensive studies have been performed on so-called metallo micelles. These investigations usually focus on mixed micelles of a common surfactant together with a special chelating surfactant that exhibits a high affinity for transition-metal ions. These aggregates can have remarkable catalytic effects on the hydrolysis of activated carboxylic acid esters, phosphate esters and amides. In these reactions the exact role of the metal ion is not clear and may vary from one system to another. However, there are strong indications that the major function of the metal ion is the coordination of hydroxide anion in the Stem region of the micelle where it is in the proximity of the micelle-bound substrate. The first report of catalysis of a hydrolysis reaction by me tall omi cell es stems from 1978. In the years that... 

Phosphate ester crystal structures have been determined of zinc 1,5,9-triazacyclononane including an interesting structure containing an oligophosphate bridged zinc unit.450 The zinc complex of 1,5,9-triazacyclododecane was studied as a hydrolysis catalyst for substituted phenyl acetates.451 Kinetic analysis suggested that hydrolysis occurs by a mechanism involving hydroxide attack of a metal-bound carbonyl. 

Artificial enzymes with metal ions can also hydrolyze phosphate esters (alkaline phosphatase is such a natural zinc enzyme). We examined the hydrolysis of p-nitro-phenyfdiphenylphosphate (29) by zinc complex 30, and also saw that in a micelle the related complex 31 was an even more effective catalyst [118]. Again the most likely mechanism is the bifunctional Zn-OH acting as both a Lewis acid and a hydroxide nucleophile, as in many zinc enzymes. By attaching the zinc complex 30 to one or two cyclodextrins, we saw even better catalysis with these full enzyme mimics [119]. A catalyst based on 25 - in which a bound La3+ cooperates with H202, not water - accelerates the cleavage of bis-p-nitrophenyl phosphate by over 108-fold relative to uncatalyzed hydrolysis [120]. This is an enormous acceleration. 

Hendry P, Sargeson AM. Metal ion promoted phosphate ester 43. hydrolysis. Intramolecular attack of coordinated hydroxide ion. 

Lanthanide hydroxide gels have been known for about 50 years to catalyze the hydrolysis of polyphosphates and phosphate esters (20). However, the study of these systems is fraught with similar difficulties to those mentioned previously, that is, the inability to be able to identify the complexes responsible for the activation. Butcher and Westheimer (20) in 1955 investigated the hydrolysis of a variety of phosphate esters by La(OH)3 gels. The gel promotes the hydrolysis of the esters by up to 10 -fold. The reaction is at the P-center as shown by tracer studies. In that communication, the mechanism by which the metal ion promotes hydrolysis could not be specified although a mechanism involving the intermediacy of the metaphosphate anion was suggested as a possibility. 

Many of the enzymes involved in the cleavage of organic phosphorus compounds require metal ions for activation. However many metal ions can also facilitate the hydrolysis of organic phosphorus compounds in the absence of enzymes. As early as 1938 it was shown that lanthanide (lanthanum and cerium) and actinide (thorium) hydroxides could accelerate the hydrolysis of a-glycerol phosphate in alkaline solution (Bamann and Mersenheimer, 1938). A similar study by Butcher and Westheimer (1955) showed that lanthanum hydroxides could accelerate the hydrolysis of three simple phosphate esters (methoxyethyl phosphate, hyrdroxyethyl phosphate and aminoethyl phosphate) by a factor of about 1000. Using... 

A combination of kinetic and labeling studies established a mechanism involving attack by copper-bound hydroxide, followed by PO bond cleavage. Further details of the mechanisms of these reactions has come from a detailed study of the hydrolysis of ci5-[Ir(en)2(0H) 0P(=0)(0R)2 ] (36) complexes (R = ethyl or 4-nitrophenyl). The reaction involves intramolecular attack by coordinated hydroxide, and rate enhancements of 10 are found. The products of the reactions are not the chelated phosphate esters (37) expected from a knowledge of cobalt(III) chemistry, but monodentate phosphate monoesters (38). This is assigned to relative differences in the sizes of the metal ions and the basicity of the coordinated... 

Metal-mediated reactions involving water are essential to life and catalytic industrial processes [1-3]. In biological systems, metalloenzymes containing various divalent metal ions catalyze the hydrolysis of amide, carboxylic ester and phosphate ester bonds using both mono- and multinuclear active-site structural motifs [4—6], Mononuclear metal centers are also found within the active sites of enzymes that catalyze the hydration, or the addition of water, to CO2 [Zn(II)] and nitriles [Co(III)/Fe(III)j [7-10]. In many of these processes, formation of a metal hydroxide moiety via deprotonation of a metal-coordinated water molecule is a key proposed step in the reaction pathway. Thus, a substantial amount of research over the past several years has been directed at dehneating how the structural and electronic environments of biological metal ions influence the pKa of a metal-bound water molecule. In this regard, studies directed at the preparation, characterization and elucidation of the reactivity of discrete metal aqua and hydroxo complexes have been paramoimt [11-13]. 

To understand the role of metal ions in hydrolysis reactions, it is useful to first consider the background hydrolysis reactions. Table 6.1 lists the second-order rate constants for hydroxide-catalyzed hydrolysis of various substrates. The reactivity of methyl acetate (first entry in Table 6.1) [16] is comparable to those of other unactivated esters found in nature (e.g. acetyl choline and carboxyl esters in phospholipids). The reactivity of N-methylacetamide (second entry in Table 6.1) [17] is comparable to those of typical peptides (1.1 x 10 6 m-1 s 1) [18] and that of dimethyl phosphate (P-O bond... 

The hydrolysis of 2-(l,10-phenanthrolyl)phosphate not unexpectedly shows a catalytic effect for a variety of metal ions (59). The metal ions Cu, NP", Co, and Zn " have some effect on the rate of its hydrolysis, but only Cu has a large effect. The ester binds all four metal ions very well, with saturation kinetics observed above 1 mM for all the metal ions involved. The Cu ion was most effective at promoting hydrolysis of the ester. The rate enhancement is not great, however, amounting to 300-fold at pH 8 and 85 °C. The reaction was proposed to proceed via attack of H2O on complex 6 or its kinetic equivalent, for example, attack of hydroxide ion on the protonated form of complex 6. Apparently, the Cu " coordinated OH ion, when formed as expected at pH 7, is sterically restrained from attack at the P center. 

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