Metal ions in the mechanism of enzyme-catalysed phosphate monoester hydrolyses. In: Hill HAO, Sadler PJ, Thomson AJ, editors. Metal Sites in Proteins and Models. Berlin: Springer Berlin Heidelberg; 1997. pp. 133-75. [DOI: 10.1007/3-540-62874-6_11]

Evidence That Speciation of Oxovanadium Complexes Does Not Solely Account for Inhibition of Leishmania Acid Phosphatases

Leishmaniasis is an endemic disease affecting a diverse spectra of populations, with 1.6 million new cases reported each year. Current treatment options are costly and have harsh side effects. New therapeutic options that have been previously identified, but still underappreciated as potential pharmaceutical targets, are Leishmania secreted acid phosphatases (SAP). These acid phosphatases, which are reported to play a role in the survival of the parasite in the sand fly vector, and in homing to the host macrophage, are inhibited by orthovanadate and decavanadate. Here, we use L. tarentolae to further evaluate these inhibitors. Using enzyme assays, and UV-visible spectroscopy, we investigate which oxovanadium starting material (orthovanadate or decavanadate) is a better inhibitor of L. tarentolae secreted acid phosphatase activity in vitro at the same total moles of vanadium. Considering speciation and total vanadium concentration, decavanadate is a consistently better inhibitor of SAP in our conditions, especially at low substrate:inhibitor ratios.

Kinetic Analysis of a Globin-Coupled Histidine Kinase, AfGcHK: Effects of the Heme Iron Complex, Response Regulator, and Metal Cations on Autophosphorylation Activity

The globin-coupled histidine kinase, AfGcHK, is a part of the two-component signal transduction system from the soil bacterium Anaeromyxobacter sp. Fw109-5. Activation of its sensor domain significantly increases its autophosphorylation activity, which targets the His183 residue of its functional domain. The phosphate group of phosphorylated AfGcHK is then transferred to the cognate response regulator. We investigated the effects of selected variables on the autophosphorylation reaction's kinetics. The kcat values of the heme Fe(III)-OH(-), Fe(III)-cyanide, Fe(III)-imidazole, and Fe(II)-O2 bound active AfGcHK forms were 1.1-1.2 min(-1), and their Km(ATP) values were 18.9-35.4 μM. However, the active form bearing a CO-bound Fe(II) heme had a kcat of 1.0 min(-1) but a very high Km(ATP) value of 357 μM, suggesting that its active site structure differs strongly from the other active forms. The Fe(II) heme-bound inactive form had kcat and Km(ATP) values of 0.4 min(-1) and 78 μM, respectively, suggesting that its low activity reflects a low affinity for ATP relative to that of the Fe(III) form. The heme-free form exhibited low activity, with kcat and Km(ATP) values of 0.3 min(-1) and 33.6 μM, respectively, suggesting that the heme iron complex is essential for high catalytic activity. Overall, our results indicate that the coordination and oxidation state of the sensor domain heme iron profoundly affect the enzyme's catalytic activity because they modulate its ATP binding affinity and thus change its kcat/Km(ATP) value. The effects of the response regulator and different divalent metal cations on the autophosphorylation reaction are also discussed.

On the interpretation of the observed linear free energy relationship in phosphate hydrolysis: a thorough computational study of phosphate diester hydrolysis in solution

The hydrolysis of phosphate esters is crucially important to biological systems, being involved in, among other things, signaling, energy transduction, biosynthesis, and the regulation of protein function. Despite this, there are many questions that remain unanswered in this important field, particularly with regard to the preferred mechanism of hydrolysis of phosphate esters, which can proceed through any of multiple pathways that are either associative or dissociative in nature. Previous comparisons of calculated and observed linear free energy relationships (LFERs) for phosphate monoester dianions with different leaving groups showed that the TS character gradually changes from associative to dissociative with the increasing acidity of the leaving group, while reproducing the experimental LFER. Here, we have generated ab initio potential energy surfaces for the hydrolysis of phosphate diesters in solution, with a variety of leaving groups. Once again, the reaction changes from a compact concerted pathway to one that is more expansive in character when the acidity of the leaving group increases. When such systems are examined in solution, it is essential to take into consideration the contribution of solute to the overall activation entropy, which remains a major computational challenge. The popular method of calculating the entropy using a quasi-harmonic approximation appears to markedly overestimate the configurational entropy for systems with multiple occupied energy wells. We introduce an improved restraint release approach for evaluating configurational entropies and apply this approach to our systems. We demonstrate that when this factor is taken into account, it is possible to reproduce the experimental LFER for this system with reasonable accuracy.

The role of metal ions in phosphate ester hydrolysis

Many phosphatases make use of metal ions to aid catalysis of phosphate ester hydrolysis. Here, we investigate the impact of metal ions on the potential energy surface (PES), and hence the preferred reaction mechanism, for a simple model for hydrolysis of phosphate ester monoanions. We show that, while both associative (A(N) + D(N)) and dissociative (D(N) + A(N)) mechanisms are represented on the potential energy surfaces both in the presence and absence of metal ions, the D(N) + A(N) process is favoured when there are no metal ions present and the A(N) + D(N) process is favoured in the presence of two metal ions. A concerted (A(N)D(N)) process is also available in the presence of two metal ions, but proceeds via a high-energy transition state. In the presence of only a single metal ion the A(N)D(N) process is the most favoured, but still proceeds via a high-energy transition state. Thus, we conclude that metallo-enzyme phosphatases are likely to utilise an associative process, while those that function without metal ions may well follow a dissociative process.

Macrocyclic copper(II) and zinc(II) complexes incorporating phosphate esters

Slow evaporation of solutions prepared by adding either Cu(ClO(4))(2).6H(2)O or Zn(ClO(4))(2).6H(2)O to solutions containing appropriate proportions of Me(3)tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) and sodium phenyl phosphate (Na(2)PhOPO(3)) gave dark blue crystals of [Cu(3)(Me(3)tacn)(3)(PhOPO(3))(2)](ClO(4))(2).(1)/(2)H(2)O (1) and colorless crystals of [Zn(2)(Me(3)tacn)(2)(H(2)O)(4)(PhOPO(3))](ClO(4))(2).H(2)O (2), respectively. Blue crystals of [Cu(tacn)(2)](BNPP)(2) (3) formed in an aqueous solution of [Cu(tacn)Cl(2)], bis(p-nitrophenyl phosphate) (BNPP), and HEPES buffer (pH 7.4). Compound 1 crystallizes in the triclinic space group P1 (No. 2) with a = 9.8053(2) A, b = 12.9068(2) A, c = 22.1132(2) A, alpha = 98.636(1) degrees, beta = 99.546(1) degrees, gamma = 101.1733(8) degrees, and Z = 2 and exhibits trinuclear Cu(II) clusters in which square pyramidal metal centers are capped by two phosphate esters located above and below the plane of the metal centers. The trinuclear cluster is asymmetric having Cu...Cu distances of 4.14, 4.55, and 5.04 A. Compound 2 crystallizes in the monoclinic space group P2(1)/c (No. 14) with a = 13.6248(2) A, b = 11.6002(2) A, c = 25.9681(4) A, beta = 102.0072(9) degrees, and Z = 4 and contains a dinuclear Zn(II) complex formed by linking two units of [Zn(Me(3)tacn)(OH(2))(2)](2+) by a single phosphate ester. Compound 3 crystallizes in the monoclinic space group C2/c (No. 15) with a = 24.7105(5) A, b = 12.8627(3) A, c = 14.0079(3) A, beta = 106.600(1) degrees, and Z = 4 and consists of mononuclear [Cu(tacn)(2)](2+) cations whose charge is balanced by the BNPP(-) anions.

Protein phosphatase 1 catalyses the direct hydrolytic cleavage of phosphate monoester in a ternary complex mechanism

The catalytic subunit of the Ser/Thr protein phosphatase 1 (PP1cat) hydrolyses N-acetyl Arg-Arg-Ala-phosphoThr-Val-Ala (K(M) = 3.7 mM) in a reaction that is inhibited competitively by inorganic phosphate (Pi, Ki = 1.6 mM) but unaffected by the product peptide alcohol at concentrations up to 3 mM. The enzyme does not catalyse the incorporation of 18O-label from 18O-labelled water into Pi whether, or not, the product alcohol is present. The dephosphorylated product alcohol of phosphorylated histone. an alternative substrate for the enzyme, serves as a competitive inhibitor for phosphopeptide hydrolysis (Ki = 60 microM) and co-mediates 18O-label exchange into Pi in a concentration-dependent manner (K(M) = 64 microM). These results indicate that hydrolysis occurs through the direct attack of an activated water molecule on the phosphate ester moiety of the substrate in a ternary complex mechanism.

Metal ion-dependent hydrolysis of RNA phosphodiester bonds within hairpin loops. A comparative kinetic study on chimeric ribo/2'-O-methylribo oligonucleotides

Several chimeric ribo/2'- O -methylribo oligonucleotides were synthesized and their hydrolytic cleavage studied in the presence of Mg2+, Zn2+, Pb2+and the 1,4,9-triaza-cyclododecane chelate of Zn2+(Zn2+[12]aneN3) to evaluate the importance of RNA secondary structure as a factor determining the reactivity of phosphodiester bonds. In all the cases studied, a phosphodiester bond within a 4-7 nt loop was hydrolytically more stable than a similar bond within a linear single strand, but markedly less stable than that in a double helix. With Zn2+and Zn2+[12]aneN3, the hydrolytic stability of a phosphodiester bond within a hairpin loop gradually decreased on increasing the distance from the stem. A similar but less systematic trend was observed with Pb2+. Zn2+- and Pb2+-promoted cleavage was observed to be considerably more sensitive to the secondary structure of the chain than that induced by Zn2+[12]aneN3. This difference in behaviour may be attributed to bidentate binding of uncomplexed aquo ions to two different phosphodiester bonds. Mg2+was observed to be catalytically virtually inactive compared with the other cleaving agents studied.