Orthophosphate ions, reactions

The first example of a tetrakisimido analogue of the orthophosphate ion, PO, the solvent-separated ion pair [(THF)4Li][(THF)4Li2P(Nnaph)4] (16), was reported by Russell et al. [21]. This complex was isolated in low yield from the reaction of P2I4 with a-naphthylamine in THF/NEt3, followed by the addition of "BuLi. The mechanism of this remarkable redox process is not understood. 

A few inhibitors of these several enzymes are provided in Voet and Voet (1990). These include glucose-6-phosphate as an inhibitor for hexokinase, as it is a reaction product. Also, ATP and citrate are listed as inhibitors of PFK, and ATP is an inhibitor for pyruvate kinase, as it is a conversion product. Other inhibitors are tabulated in Appendix A of Hoffman s Cancer and the Search for Selective Biochemical Inhibitors (1999). On the other hand, Voet and Voet (1990) list a few activators or promoters for PFK, including ADP, fructose 6-phosphate (which is a reactant), fructose 1,6-bisphosphate (which is a product), the ammonium ion, and the orthophosphate ion Pj. The situation can get complicated. 

The rate of hydrolysis is influenced by pH, temperature and calcium concentration. Calcium shifts the chemical equilibrium of this reaction to the right as it bonds the orthophosphate ions formed. In addition to chemical hydrolysis, biochemical hydrolysis also takes place in waters, particularly in sewage with a dense biological population. A considerable part of polyphosphates in sewage fed into wastewater treatment plants is hydrolysed. The half-time of polyphosphates in surface waters is given in days and tens of days. 

Hydrolysis. All of the polyphosphates and metaphosphates undergo reaction with water. Such reactions have commonly been termed reversion reactions, and, if carried out under appropriate conditions, will cause depolymerization of poly- and metaphosphates to simple orthophosphate ions. In these reactions, water appears to act as a base with respect to the higher poly- and metaphosphates. A summary of the principal hydrolytic or reversion reactions of the various poly- and metaphosphates is presented in Table... 

Spectrophotometric methods are usually preferred for routine analysis of this parameter, most of them relying on the reaction between orthophosphate ions and molybdate in acidic medium in order to form a heteropoly acid. Color formation can be enhanced by adding vanadate to obtain the yellow vanadomolybdate complex (vanadomolybdophosphoric acid method) or by reducing the molybdo-phosphoric acid to yield strongly colored phosphomolybdenum blue species. 

Crisp et al. (1978) were able to follow the course of the cement-forming reaction using infrared spectroscopy and to confirm previous observations. They found that the technique could be used to distinguish between crystalline and amorphous phases of the cement. Hopeite shows a number of bands between 1105 and 1000 cm this multiplicity has been explained by postulating a distortion of the tetrahedral orthophosphate anion. (Two-thirds of the zinc ions are tetrahedrally coordinated to four phosphate ions, and the remainder are octahedrally coordinated to two phosphate and four water ligands.)... 

This enzyme [EC 2.5.1.17], also referred to as cob(I)-alamin adenosyltransferase or aquacob(I)alamin adeno-syltransferase, catalyzes the reaction of cob(I)alamin with ATP and water to produce adenosylcobalamin, orthophosphate, and pyrophosphate (or, diphosphate). A cofactor for this enzyme is manganese ion. 

This enzyme catalyzes the reaction of glycine with orthophosphate to produce acetyl phosphate and ammonium ion. The oxidized form of the enzyme is subsequently reduced to its dithiol form in a NADH-lmked step. 

In both schemes, the specificities of the pump for catalysis change in the two enzyme states. Jencks points out that coupling is determined (a) by the chemical specificity achieved in catalyzing phosphoryl transfer to and from the enzyme (wherein E-Ca2 reversibly binds ATP, and E reacts reversibly with orthophosphate), and (b) by the vectorial specificity for ion binding and dissociation (wherein E reversibly binds/dissociates cytoplasmic calcium ion, and E—P reversibly binds/dissociates luminal calcium). There must be a single conformation change during the reaction cycle between Ei and E2 in the free enzyme and from Ei P-Ca2 to E2-P-Ca2 after enzyme phosphorylation. 

Nitrogenase (ferredoxin) [EC 1.18.6.1] catalyzes the reaction of three reduced ferredoxin molecules with protons, N2, and n ATP molecules to produce three oxidized ferredoxin molecules, two ammonia molecules, n ADP molecules, and n orthophosphate molecules where n is between 12 and 18. This iron-sulfur system also uses either molybdenum or vanadium ions. (2) Nitrogenase (flavodoxin) [EC 1.19.6.1] catalyzes the reaction of six reduced flavodoxin molecules with protons, N2, and n ATP molecules to produce six oxidized flavodoxin molecules, two ammonia molecules, n ADP molecules, and n orthophosphate molecules. This system uses iron-sulfur and molybdenum ions. 

This enzyme [EC 3.4.21.53], also known as endopepti-dase La, ATP-dependent serine proteinase, and ATP-dependent protease La, catalyzes the hydrolysis of peptide bonds in large proteins (for example, globin, casein, and denaturated serum albumin) in the presence of ATP (which is hydrolyzed to ADP and orthophosphate). Vanadate ion inhibits both reactions. A similar enzyme occurs in animal mitochondria. Protease La belongs to the peptidase family S16. 

Under acid conditions, molybdate reacts with orthophosphate, P04 to form a blue heteropoly acid, molybdophosphoric acid. A similar reaction occurs with arsenate ion, As04. In the presence of vanadium, the product is yellow vanadomolybdophosphoric acid. These reactions are used for colorimetric analyses of phosphate, arsenate, and many other substances. Colloidal molybdenum blue has limited apphcations such as dyeing silk. It readily absorbs onto surface-active materials. 

Lowenstein reacted ATP with orthophosphate (33) in the presence of metal ions, and obtained ADP and pyrophosphate as products. The most active metals in this reaction were, rather surprisingly for a nonenzymatic reaction, the alkaline earths, Cd+2 and Mn+2 the members of the first transition series exhibited low7 activity. The reactive intermediate was formulated as follows ... 

The metal ion specificity for the reaction with acetate was different from that in the reaction with phosphate in the former beryllium was most active, followed by nickel. The alkaline earths that were so effective with phosphate did not catalyze the reaction with acetate at all. The difference in metal specificity in the two reactions was explained by assuming that complexation with the orthophosphate and acetate constitutes an important function in the reaction. 

Some alkali orthophosphates have been well studied. The most important among these are KH2PO4 and NH4H2PO4. In the KH2PO4 structure, hydrogen bond links the PO4 tetrahedron to four others in a continuous three-dimensional network, while the K ion is coordinated 8-fold by oxygen atoms. In the case of NH4H2PO4, the structure is similar, but a system of N-H-0 bonds exists instead of coordination of the K ion. These bonds are mostly ionic, and hence, acid phosphates are soluble and used as such in the acid-base reaction to form CBPCs. 

As shown in these reactions, the phosphorus must be in the phosphate form. The reaction occurs in water, so the phosphate ion originates a series of equilibrium orthophosphate reactions with the hydrogen ion. This series is shown as follows (Snoeyink and Jenkins) ... 

Let us consider the mechanism of glyceraldehyde 3-phosphate dehydrogenase in detail (Figure 16.8). In step 1, the aldehyde substrate reacts with the sulfhydryl group of cysteine 149 on the enzyme to form a hemithioacetal. Step 2 is the transfer of a hydride ion to a molecule of NAD + that is tightly bound to the enzyme and is adjacent to the cysteine residue. This reaction is favored by the deprotonation of the hemithioacetal by histidine 176. The products of this reaction are the reduced coenzyme NADH and a thioester intermediate. This thioester intermediate has a free energy close to that of the reactants. In step 3, orthophosphate attacks the thioester to form 1,3-BPG and free the cysteine residue. This displacement occurs only after the NADH formed from the aldehyde oxidation has left the enzyme and been replaced by a second NAD+. The positive charge on the NAD+ may help polarize the thioester intermediate to facilitate the attack by orthophosphate. 

Figure 16.8 Catalytic mechanism of glyceraldehyde 3-phosphate dehydrogenase. The reaction proceeds through a thioester intermediate, which allows the oxidation of glyceraldehyde to be coupled to the phosphorylation of 3-phosphogIycerate. (1) Cysteine reacts with the aldehyde group of the substrate, forming a hemithioacetal. (2) An oxidation takes place with the transfer of a hydride ion to NAD. forming a thioester. This reaction is facilitated by the transfer of a proton to histidine. (3) The reduced NADH is exchanged for an NAD" molecule. (4) Orthophosphate attacks the thioester. forming the product 1,3-BPG.

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