Phosphatases magnesium ions

While having three metal ions in an enzyme active site is uncommon, it is not unique to PLCBc. The well-known alkaline phosphatase from E. coli (APase) contains two zinc ions and a magnesium ion [67], whereas the a-toxin from Clostridiumperfringens [68]. and the PI nuclease from Penicillium citrinum [69] each contain three zinc ions. Indeed, the zinc ions and coordinating ligands of PI nuclease bear an uncanny resemblance to those of PLCBc as the only differ-... 

Inositol-3,4-bisphosphate 4-phosphatase [EC 3.1.3.66] catalyzes the hydrolysis of D-myo-inositol 3,4-bisphosphate to produce D-myo-inositol 3-monophosphate and orthophosphate. This enzyme is magnesium-ion-independent and is different from inositol-1,4-bisphosphate 1-phosphatase [EC 3.1.3.57], inositol-1,4,5-trisphosphate 1-phosphatase [EC 3.1.3.61], and inositol-1,3-bisphos-phate 3-phosphatase [EC 3.1.3.65]. 

Alkaline phosphatase cleaves phosphate esters at medium alkaline pH in the presence of magnesium ions. 

D-Glucosamine 6-phosphate is hydrolyzed by the D-glucose-6-phosphatase of rat-liver mitochondria. The rate of this hydrolysis is about 8 % of that of D-glucose 6-phosphate hydrolysis. A phosphatase which preferentially catalyzes the hydrolysis of D-glucosamine 6-phosphate has been prepared from Neurospora crassa. This enzyme is not stimulated by magnesium ions and has an optimum activity between pH 6 and 7.5. It appears to be distinct from acid, alkaline, and other specific phosphatases. 

A less desirable feature, however, also indicated in Table II, is that these three phosphatases contain as impurities small amounts of phosphodiesterase and pyrophosphatase. In contrast to the monoesterases these enzymes require the presence of magnesium ions and act at pH values... 

Lithium selectively interferes with the inositol lipid cycle [42,43] and this is the basis for a proposal of a unifying hypothesis for lithium actions [44,45]. Administration of lithium to rats (10 mmol/kg) resulted in a reduction in brain myoinositol and an increase in the reaction substrate inositol-1 -phosphate [46]. The magnesium-dependent enzyme inositol monophosphate phosphatase was inhibited in rat mammary gland by lithium [47]. Lithium has been shown to inhibit inositol monophosphate phosphatase in bovine brain ( (j = 0.8 mM) by substituting noncompetitively for magnesium ions [48]. Lithium may also affect other enzymes involved in phosphoinositide metabolism [43,49]. Lithium reduces the cell concentrations of myoinositol, which would otherwise be converted to phosphatidylinositol, and this attenuates the response to external stimuli [43,50]. 

As shown in Fig. 3, they studied the formation of cholesterol ester With time in rat liver microsomes in imidazole buffer. As can be seen, in the presence of magnesium there was a considerable decline in the amount of cholesterol ester formed, but that it could be completely prevented or blocked if potassium fluoride and EDTA were included in the incubation mixture. In a sepairate experiment, they studied cholesterol ester formation as a function of magnesium ion concentration in the medium. Again, the ACAT activity declined with increasing magnesium in the medium, whereas inclusion of EDTA together with magnesium, stabilized the enzyme activity. This information is consistent with the data from our laboratory which demonstrates the presence in microsomes of a Mg++ sensitive phosphatase which inactivates 7a-hydroxylase. 

The regeneration of the activity of the enzyme by cobalt differs from the behaviour of an alkaline phosphatase isolated from human placenta.228 This enzyme is reported to be re-activated by replacement of the native zinc ion by either zinc, magnesium, or mercury. No other metal is active. 

The four zinc ions in alkaline phosphatase have also been replaced by other bivalent metal ions to probe the metal-binding site. The apodimer binds four cobalt(ii) ions, two Co binding to each of two different types of binding sites. Magnesium activates the cobalt-substituted enzyme, as it does the native zinc... 

Metalloenzyme-catalyzed phosphoric ester hydrolysis can be illustrated by alkaline phosphatase, by far the most-investigated enzyme of this class. The protein is a dimer of 94 kDa containing two zinc(II) and one magnesium(II) ions per monomer, and catalyzes, rather unspecifically, the hydrolysis of a variety of phosphate monoesters as well as transphosphorylation reactions. The x-ray structure at 2.8 A resolution obtained on a derivative in which all the native metal ions were replaced by cadmium(II) reveals three metals in each subunit. 

In some kinases, such as nucleoside diphosphate kinase, " an intermediate step is the phosphoryl transfer to a group belonging to the enzyme, as happens in ATPase and as was discussed in detail for alkaline phosphatase (Section V.B). In other kinases the phosphoryl transfer occurs directly from the donor to the acceptor in a ternary complex of the enzyme with the two substrates.Often metal ions like magnesium or manganese are needed. These ions interact with the terminal oxygen of the ATP molecule, thus facilitating the nucleophilic attack by the acceptor. The metal ion is often associated with the enzyme. For mechanistic schemes, see the proposed mechanism of action of alkaline phosphatase, especially when a phosphoryl enzyme intermediate is involved. 

Early reports on the isolation of a pyrophosphate lyase and a phosphatase activity from E. coli had no apparent follow up." Later, it was proposed that the release of pyrophosphate could proceed without enzyme catalysis under the influence of magnesium or calcium ions." " ... 

Several enzymes bind both magnesium and zinc ions, using them for different purposes. For example, nucleotidyl transferases can use one metal ion, to which is coordinated a phosphate group and a carboxylate group, the latter serving to polarize the water nucleophile. When there are two metal ions present, the carboxylate group also binds the second metal ion which in turn activates the nucleophile. Therefore it appears that the second metal ion aids in the catalysis, replacing the action of an active-site side chain in those enzymes of that activity that only bind one metal ion. T vo enzymes will be described here—the alkaline phosphatase and the 3 -5 exonuclease from E. coli. 

A number of acid and alkaline phosphatases and specific phytases are activated by divalent metal ions. After its inactivation by o-phenanthrolin and a,a -dipyridil at 50°C, the manganese-containing acid phosphatase from sweet potato was reactivated by the addition of metal ions (Uehara et ah, 1974). The EDTA-inhibited alkaline phosphatase of Lysobacter enzymogenes was reactivated by divalent metal ions at 0.05 mM (von Tigerstrom and Stelmaschuk, 1986) specifically, reactivation was caused by magnesium chloride (4.6%), calcium chloride (7%), manganese chloride (14.5%), cobalt chloride (24%) and zinc chloride (66%). 

---