Phosphatase-like Activity

The phosphatase-like activity promoted by the complex [Cd2(C02EtH2Ll) (CH3C00)2] in 50 50 acetonitrile/water was measured at pH 10.4 with a BDNPP concentration of 5 mM. The [complex] dependence was linear in the region 0.005-0.05 mM. Dependence of pH on activity was measured from pH 7-10.7 

The initial rate of BDNPP cleavage, determined at pH 10.4 as a function of substrate concentration, reveals typical saturation behavior (Fig. 5.21). The data were fitted by non-linear regression, using the Michaelis-Menten equation (Eq. 2.5, Chap. 2). Here = 9.4 2.1 mM, v ax = 3.76 x 10 M s with 

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A clean conversion of 9.118 to 9.119 was obtained, with moderate stereoselection (15-20% enantiomeric excess e.e.) for the RJi (9.123,9.124) or the SJS enantiomer (9.125). Resynthesis of the unbound analogues of 9.120-9.125 was planned to test their effectiveness in the same reaction and to evaluate the influence of the soM support on the reaction outcome. The deconvolution-prone, pooled format for catalyst discovery has been used by Venton (175) for the synthesis of a >28,000-member p-cyclodextrin library (13 pools) as a source of Zn-based catalytic systems with phosphatase-like activity. 

Phosphatase-like activities of all complexes were measured using the activated substrate BDNPP over a pH range 5-10.5. The data were fitted to equations derived for monoprotic (Eq. 4.1, Fig. 4.19) or diprotic (Eq. 4.2 and Fig. 4.19) systems [67]. Vq is the initial reaction rate, refers to the maximum rate with 5 mM BDNPP as substrate, and Kai and Ka2 are the catalytically relevant protonation equilibrium constants. 

Hydrogen peroxide (H202) exhibits insulin-like activity in isolated cells. Like that of vanadate, this effect is thought to be mediated by inhibition of protein-tyrosine phosphatases. 

Lacks and Greenberg (10) have purified an exonuclease from Diplococcus pneumoniae with properties very similar to E. coli exonuclease III. The pneumococcal enzyme acts preferentially on native DNA producing 5 -mononucleotides and single strands which are not susceptible to further attack. Like exonuclease III it shows an intrinsic 3 -phosphoryl-DNA phosphatase activity. A DNA phosphatase-exonuclease activity has also been reported in B. subtilis (11). 

The influence of the ovarian cycle on protease activity in the vagina has also been demonstrated. For example, the trypsin-like activity in rat vaginal smears was found to be maximal at proestrus. The activity of //-glucuronidase, acid phosphatase, alkaline phosphatase, and esterase all vary in the vaginal tissue of premenopausal and postmenopausal women. 

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... 

Some phosphatases are activated, just like some kinases, by second messengers. 

Oshima states that the activity of his preparations suggests a mode by which phosphatase-like polymers could have formed on the primitive Earth. 

Cazzolli, R, Carpenter, L, Biden, TJ and Schmitz-Peiffer, C (2001) A role for protein phosphatase 2A-like activity, but not atypical protein kinase Czeta, in the inhibition of protein kinase B/Akt and glycogen synthesis by pahnitate. Diabetes, 50, 2210-2218. 

Second, most purple acid phosphatases require activation by reducing agents. Although intracellular metabolites such as NADH and NADPH have redox potentials sufficiently low to reduce the purple phosphatases, it is not yet clear whether they, or other reductants, are present in effective concentration in the lysosome-like organelles to which the enzymes are localized. 

Vanadate (sodium orthovanadate or peroxovanadate) exhibits insulin-like effects in vitro (activation of insulin receptor tyrosine kinase, PI 3-kinase, Akt) and in vivo (diabetic rats, humans). These effects can be explained at least in part by the inhibition of phosphotyrosine phosphatases which deactivate the INSR tyrosine kinase. 

Finally, if the phosphorylation of myosin is the activation mechanism, then dephosphorylation is likely to be the deactivation mechanism, and so it seems. However, there are several myosin phosphatases in smooth muscle cells and they have some range of substrate specificities. Thus, there are several possible candidates for a regulatory role. 

Figure 6. A hypothetical scheme for the control of the number of active crossbridges in smooth muscle. Following the activation of a smooth muscle by an agonist, the concentrations of intermediates along the main route begins to build up transiently. This is shown by the thickened arrows. Also, cAMP is generated which is universally an inhibitor in smooth muscle. Cyclic AMP in turn combines with protein kinase A, which accounts for most of its action. The downstream mechanisms, however, are not well worked out and at least three possibilities are likely in different circumstances. First, protein kinase A is known to catalyze the phosphorylation of MLCK, once phosphorylated MLCK has a relatively lower affinity for Ca-calmodulin so that for a given concentration of Ca-calmodulin, the activation downstream is reduced. The law of mass action predicts that this inhibition should be reversed at high calcium concentrations. Other cAMP inhibitory mechanisms for which there is evidence include interference with the SR Ca storage system, and activation of a MLC phosphatase.

Phosphorylation of HSFl is rapid, and one or more protein kinases are likely to be activated upon HS. An alternative explanation is that heat inactivates a phosphatase which is more active than the HSF kinase at 37 °C. Inactivation of the phosphatase by heat would allow the presumptive heat stable HSF kinase activity to predominate, thus increasing the phosphorylation of HSFl. 

Like Na,K-ATPase, gastric H,K-ATPase also exhibits a p-nitrophenylphosphatase (/>NPPase) activity. This phosphatase activity is dependent on Mg and K, or one of its congeners with the same order of selectivity as for the ATPase activity, yielding a specific activity of 6D84% of the maximal ATPase activity [4,136,137]. Phosphorylation by pNPP has not been demonstrated and transport is also not catalyzed by this substrate. As in the ATPase reaction the effect of on the... 

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