Phosphorylation of acetate

For the hydrolysis of acetyl phosphate under the conditions prevailing in the body, AG = —41 kj-mol. If the phosphorylation of acetic acid (the reverse of the hydrolysis of acetyl phosphate) was driven by coupling to the hydrolysis of ATP at pH = 7, what is the minimum amount of ATP molecules (in moles) that would have to be hydrolyzed to form... 

Enzymatic catalysis lowers AG to accelerate the reaction rate, but does not affect the AG° that controls the position of equilibrium. However, cells also have an elegant ability to overcome an unfavorable equilibrium by coupling an endothermic reaction with exothermic ATP hydrolysis. For example, the reaction of acetate with coenzyme A (CoASH) giving acetyl CoA is uphill by about 7 kcal/mol. The hydrolysis of ATP to ADP and phosphate is downhill by about the same amount. By coupling these two reactions shown below for acyl-CoA synthetase, the cell achieves a total AG° of almost 0 for a ATgq of about 1. The phosphorylation of acetate by ATP improves the leaving group for the second reaction with CoASH. There are many enzymes whose catalytic process remains a mystery enzymes still have much to teach us about reaction mechanisms. 

Acetate kinase of E. coli catalyzes reaction (16), the reversible phosphorylation of acetate by ATP to form acetyl-P and ADP. 

Reaction (16) is written in the thermodynamically favored direction, which is the reverse of the phosphorylation of acetate. However, the enzyme is known to be required for the activation of acetate in intermediary metabolism as a means of generating acetyl-P, which then reacts as a substrate for phosphotransacetylase (phosphate acetyltransferase) and is thereby converted to acetyl-CoA and utilized as a source of carbon and energy. 

Another example is provided by the phosphorylation of acetic acid (substrate) to form acetyl phosphate, which is catalysed by the phosphokinase enzyme known as acetate kinase (11.38). This reaction can occur in reverse in which case the acetyl phosphate is said to phosphorylate the ADP to ATP. Both diphosphate and triphosphate esters can act as phosphorylating agents. 

Another reaction of this type is the phosphorylation of acetate and of butyrate, yielding the respective acyl phosphates. ... 

The hydrolysis of acetyl phosphate has AG=-42 kJ mol" under typical biological conditions. If the phosphorylation of acetic acid were to be coupled to the hydrolysis of ATP. what is the minimum number of ATP molecules that would need to be involved ... 

In keeping with its biogenetic origin m three molecules of acetic acid mevalonic acid has six carbon atoms The conversion of mevalonate to isopentenyl pyrophosphate involves loss of the extra carbon as carbon dioxide First the alcohol hydroxyl groups of mevalonate are converted to phosphate ester functions—they are enzymatically phosphorylated with introduction of a simple phosphate at the tertiary site and a pyrophosphate at the primary site Decarboxylation m concert with loss of the terti ary phosphate introduces a carbon-carbon double bond and gives isopentenyl pyrophos phate the fundamental building block for formation of isoprenoid natural products... 

As summarized in Figure 27.7, the mevalonate pathway begins with the conversion of acetate to acetyl CoA, followed by Claisen condensation to yield acetoacety) CoA. A second carbonyl condensation reaction with a third molecule of acetyl CoA, this one an aldol-like process, then yields the six-carbon compound 3-hydroxy-3-methylglutaryl CoA, which is reduced to give mevalonate. Phosphorylation, followed by loss of C02 and phosphate ion, completes the process. 

Fischer projections and, 977-978 glycosides of, 989-990 hemiacetals of, 984-986 osazones from, 1013 oxidation of, 992-994 phosphorylation of, 991 reaction with acetic anhydride, 988... 

The reaction of acetic anhydride with glycerol in the presence of phosphoryl trichloride as a catalyst was carried out the following way ... 

D-Xylulose 5-phosphate (ii-threo-2-pentulose 5-phosphate, XP) stands as an important metabolite of the pentose phosphate pathway, which plays a key fimction in the cell and provides intermediates for biosynthetic pathways. The starting compound of the pathway is glucose 6-phosphate, but XP can also be formed by direct phosphorylation of D-xylulose with li-xylulokinase. Tritsch et al. [114] developed a radiometric test system for the measurement of D-xylulose kinase (XK) activity in crude cell extracts. Aliquots were spotted onto silica plates and developed in n-propyl alcohol-ethyl acetate-water (6 1 3 (v/v) to separate o-xylose/o-xylulose from XP. Silica was scraped off and determined by liquid scintillation. The conversion rate of [ " C]o-xylose into [ " C]o-xylulose 5-phosphate was calculated. Some of the works devoted to the separation of components necessary while analyzing enzyme activity are presented in Table 9.8. 

Nakagome and co-workers effected the successful cyclization of N-ethyl-N-arylaminomethylenemalonates (749) in poly phosphoric acid, prepared from orthophosphoric acid and phosphorus pentoxide in polyphosphate ester (PPE), prepared from phosphorus pentoxide and anhydrous diethyl ether in chloroform in phosphoryl chloride on the action of boron trifluoride etherate on the action of acetic anhydride and concentrated sulfuric acid or on the action of phosphorus pentoxide in benzene [71GEP2033971, 71JHC357 76JAP(K) 18440]. Depending on the work-up process, l-ethyl-4-oxoquinoline-3-carboxylates (750, R1 = Et), l-ethyl-4-oxoquinoline-3-carboxylic acids (750, R2 = H) and 3-ethoxycarbonyl-4-chloroquinolinium iodides (751) were obtained. Only the cyclization of... 

Phosphorylation of H3 is not limited to mitosis and also occurs in G1 phase of the cell cycle. Activation of the Ras-Raf-MEK-ERK signal transduction pathway and/or of the p38 stress kinase pathway when cells are treated with epidermal growth factor (EGF), 12-G-tetradecanoylphorbol-13-acetate (TPA), anesomycin, okadiac acid, and stresses such as UV irradiation induces the rapid phosphorylation of H3 at Ser-10 and/or Ser-28 [68-72] (Figs. 5 and 6). Inhibition of the MEK1,2 activity with PD98059 prevents the activation of ERK and TPA-induced H3... 

This phosphotransferase [EC 2.7.2.1] catalyzes the thermodynamically favored phosphorylation of ADP to form ATP Aeq = [ATP][acetate]/ [acetyl phosphate] [ADP] = 3000). GDP is also an effective phosphoryl group acceptor. This enzyme is easily cold-denatured, and one must use glycerol to maintain full catalytic activity. Initial kinetic evidence, as well as borohydride reduction experiments, suggested the formation of an enzyme-bound acyl-phosphate intermediate, but later kinetic and stereochemicaT data indicate that the kinetic mechanism is sequential and that there is direct in-line phosphoryl transfer. Incidental generation of a metaphosphate anion during catalysis may explain the formation of an enzyme-bound acyl-phosphate. Acetate kinase is ideally suited for the regeneration of ATP or GTP from ADP or GDP, respectively. 

Acetate Acetyl-P Pyr PEP poly P)n-i poly(P) Scheme 4.7 General scheme of the kinase-dependent phosphorylation of DHA with in situ regeneration of ATP. 

A corresponding reaction of acetate ion with AJP is also catalyzed by a bivalent metal ion. The reaction probably results in the formation of an acyl phosphate, which has not been identified as such but has been identified by trapping of the product with hydroxylamine. The best catalyst is beryllium ion, which catalyzes optimally at molar ratios of 1 to 1 or less. Acetate ion is presumably the reactive species, since the pH optimum of the reaction is 5. It is concluded from the pH effects in this study and in the transphosphorylation reaction that a complex of the metal ion and nucleophile must occur. Since acetate ion is a monodentate ligand, the mechanism postulated for the phosphorylation reaction above cannot be completely applicable to this case (36). 

Steroids are members of a large class of lipid compounds called terpenes. Using acetate as a starting material, a variety of organisms produce terpenes by essentially Lire same biosynlheLic scheme (Fig. 3). The sell-condensation of two molecules of acetyl coenzyme A (CoA) forms acetoacetyl CoA. Condensation of acetoacetyl CoA with a third molecule of acetyl CoA, then followed by an NADPH-mediated reduction of the thioester moiety produces mevalonic acid (22). Phosphorylation of (22) followed by concomitant decarboxylation and dehydration processes... 

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