Pyruvate/pyruvic acid phosphoenol

A. Phosphoenolpyruvate.—The mechanisms of hydrolysis of phosphate esters of phosphoenol pyruvic acid (33) have been described in detail, and 0 studies confirm an earlier postulate that attack by water on the cyclic acyl phosphate (34) occurs at phosphorus and not at carbon. In the enolase reaction, the reversible interconversion of 2-phosphoglyceric acid(35)... 

The acid cleavage of a-phenylvinyl diethyl phosphate, which most probably proceeds by an ASj 2 mechanism, was found to be catalyzed by micellar sodium dodecyl sulfate (NaLS) and hexadecyl sulfonic acid (Table 8), whereas that of phosphoenol pyruvic acid was slightly inhibited by cationic and anionic surfactants (Bunton and Robinson, 1969c). The considerably greater rate enhancement by sodium dodecyl sulfate in 0-01m than in 0-10 m acid (Table 8) can reasonably be attributed... 

Some subaerial plants use a different pathway, involving carboxylation of phosphoenol pyruvic acid (PEP) instead of ribulose diphosphate (the C02 is actually transformed into bicarbonate before incorporation), which subsequently forms a C4 compound, oxaloacetic acid, instead of PGA (Fig. 1.7). Consequently such plants are termed C4 plants. The C4 path is a relatively recent evolutionary development of particular advantage in hot dry climates (see Box 1.10).The PEP cycle effectively transfers C02 to the Calvin cycle, and each cycle confers an isotopic fractionation. Some plants, the CAM plants (see Box 1.10), can use the combined PEP-Calvin cycle path (with some leakage of C02 out of the cell between the cycles) or just the Calvin cycle. The effects of these pathways on the overall isotopic fraction are reflected in the 813C values in Table 5.8. 

The activated, enolic form of the same acid, phosphoenol pyruvate (Figure 1.2), adds a nucleophile to carbon dioxide, producing oxaloacetic acid, a precursor by transamination of aspartic acid. 

Of special biochemical importance are the energy-carrying compounds acetyl phosphoric acid (5.348a) and phosphoenol pyruvic acid (11.6) which release large amounts of energy on hydrolysis (Chapter 11.3). 

The enzyme hexokinase, in common with other kinases that catalyse phosphorylation reactions, requires adenosine triphosphate (ATP) as the substrate. The latter is converted to adenosine diphosphate (ADP) during the reaction and must be recycled to avoid the consumption of stoichiometric amounts of the ATP. This can be readily achieved by the introduction of a second enzyme reaction which converts ADP back to ATP. Thus, pyruvate kinase phosphorylates ADP using phosphoenol pyruvate as the phosphate donor, yielding ATP and pyruvic acid. In this way, these phosphorylation reactions can be carried out using a catalytic amount of ATP provided that a stoichiometric quantity of phosphoenol pyruvate is used (Scheme 5.27). 

Obviously, CO2 is the ultimate carbon precursor of acetyl-CoA in plant tissues. The principal transport photosynthetate, sucrose, in the plant cell is converted to glucose 1-phosphate (glucose-1-P) by a series of reactions documented by Yamada et al. (1974) and Simcox et al. (1977). Glucose-l-P is degraded in the cytoplasm by glycolytic enzymes to phosphoenol pyruvate (P-enolpyruvate) which is then further converted either to malic acid or to pyruvic acid ... 

Phosphoenol- N-Acetyl-D-pyruvic acid mannosamine-6-phosphate... 

Malic acid is a very active intermediary product of grape metabolism. The vine contains the L-(—) malic isomer. The vine assimilates carbon dioxide in the air by a C3 mechanism (Ruflher et al., 1983). In this manner, during the dark phase of photosynthesis, the leaves and young green grapes fix CO2 on ribulose 1,5-diphosphate to produce phospho-glyceric acid, which condenses to form hexoses and may also become dehydrated into phosphoenol pyruvic acid. CO2, catalyzed by PEP carboxylase, is fixed on this acid to form oxaloacetic acid, which is, in turn, reduced into malic acid. 

Phenolic compounds derived from a simple unit to a single benzene ring are created from the condensation of erythrose 4-phosphate, an intermediary product of the pentose phosphate cycle, with phosphoenol-pyruvic acid. This biosynthetic pathway, known as the shikimic acid pathway (Figure 10.11), leads to the production of benzoic... 

The enzyme, obtained in cr3rstalline form from human and rat muscle, catalyzes a rapid transfer of high-energy phosphate from phosphoenol-pyruvic acid to ADP as the specific acceptor ... 

A -acetyl-D-neuraminic acid is biosynthesized from A -acetyl-D-mannosamine and phosphoenol pyruvate, catalyzed by N-acetyl-D-neuraminic acid synthase. The first step involves the addition of an electron pair from the double bond of the phosphoenol pyruvate to the aldehyde group to give an aldol-type condensation (see Fig. 10.8A). The product is the nine-carbon sugar acid, A -acetyl-D-neuraminic acid [23]. In some instances the enzyme requires A-acetyl-D-mannosamine-6-phosphate as the substrate and forms A-acetyl-D-neuraminic acid-9-phosphate. Various hydroxyl groups on C-4, -7, -8, and -9 can be acetylated by specific acetyl transferases using acetyl CoAas the donor. KDO (2-keto-3-deoxy-D-mannooctulosonic acid) is biosynthesized by a very similar condensation between D-arabinose-5-phosphate and pyruvic acid, catalyzed by KDO synthase (see Fig. 10.8B) [24]. 

Pyruvic acid and its derivative phosphoenol pyruvate have already appeared in Figure 1.1, and pyruvic acid was required in discussion of the action of thiamine diphosphate (Figures 2.12 to 2.14). They are important intermediates and will appear again. It is worth looking briefly at the origins of these compounds now. 

Figure 2.20 A summary of the steps by which pyruvic acid and phosphoenol pyruvate are...

The shikimic acid pathway requires the C4 sugar erythrose-4-phosphate, and phosphoenol pyruvic acid (a derivative of pyruvic acid, locked in its enol form, see Figures 1.1 and 2.19) as starting materials. The route to aromatic compounds has more steps than those met earlier, and, not surprisingly, for a plant process, uses sugar derivatives as starting materials. A total of ten carbon atoms are required, four from erythrose, and six from two molecules of pyruvate one of these is later lost as CO2. The final product therefore is a C9 compound, so that such products are... 

Figure 8.1 The shikimic acid pathway from erythrose-4-phosphate and phosphoenol-pyruvic acid to phenylpyruvic acid...

Pathway V is based on the conversion of pyruvate (or) phosphoenol pyruvate to succinic acid via propionate as the intermediate. The dehydrogenation of propionyl-CoA removes two hydrogens and results in the formation of acryloyl CoA, which is hydrogenated to form 3- hydroxyl propionyl-CoA followed by hydrolysis to yield 3-HP (Jiang et al., 2009). There is no net ATP generation (instead the ATP yield is -1) in this pathway as a result, pathway... 

In contrast to the preceding observations and conclusions, there are reports from one laboratory that the mechanism of peptide formation resembles that of protein synthesis. Cellfree preparations of B. hrevis were observed to synthesize gramicidin and tyrocidine provided that both a 140,000 X g supernatant solution and a ribosomal preparation are present (in addition to Mg+, ATP, phosphoenol-pyruvic acid, pyruvic kinase, glutathione, and amino acid mixture). The supernatant solution presumably supplies s-RNA and amino acid activating enzymes. In this system, peptide synthesis is suppressed by exposure of either the supernatant or ribosomal portions to RNase (Uemura et al., 1963)- Furthermore, neither chloramphenicol nor puromycin are capable of differentiating protein from peptide synthesis in this system the formation of each of the products is inhibited 98% by either 10 (xg/ml of the former or 100 fi.g/ml of the latter antibiotic (Okuda et al., 1964b). 

The 3-phosphoglyceric acid then undergoes an internal chemical change and releases a molecule of water to give phosphoenol pyruvic acid (PEP). The phosphoenol pyruvic acid now acted upon by a second kinase, transfers its phospho-group to a molecule of ADP... 

Step 2 of Figure 29.13 Decarboxylation and Phosphorylation Decarboxylation of oxaloacetate, a jB-keto acid, occurs by the typical retro-aldol mechanism like that in step 3 in the citric acid cycle (Figure 29.12), and phosphorylation of the resultant pyruvate enolate ion by GTP occurs concurrently to give phosphoenol-pyruvate. The reaction is catalyzed by phosphoenolpyruvate carboxykinase. 

An intriguing stress-induced alteration in gene expression occurs in a succulent plant, Mesembryanthemum crystallinum, which switches its primary photosynthetic CO2 fixation pathway from C3 type to CAM (Crassulacean acid metabolism) type upon salt or drought stress (Winter, 1974 Chapter 8). Ostrem et al. (1987) have shown that the pathway switching involves an increase in the level of mRNA encoding phosphoenol-pyruvate carboxylase, a key enzyme in CAM photosynthesis. 

Figure 5 Model of phosphorus (P) deficiency-induced physiological changes associated with the release of P-mobilizing root exudates in cluster roots of white lupin. Solid lines indicate stimulation and dotted lines inhibition of biochemical reaction sequences or mclaholic pathways in response to P deliciency. For a detailed description see Sec. 4.1. Abbreviations SS = sucrose synthase FK = fructokinase PGM = phosphoglueomutase PEP = phosphoenol pyruvate PE PC = PEP-carboxylase MDH = malate dehydrogenase ME = malic enzyme CS = citrate synthase PDC = pyruvate decarboxylase ALDH — alcohol dehydrogenase E-4-P = erythrosc-4-phosphate DAMP = dihydraxyaceConephos-phate APase = acid phosphatase.

It is interesting to note that the dihydroxybenzoyl nucleus arises from chorismic acid which, in turn, is derived from erythrose phosphate and phosphoenol pyruvate, both of these substances being intermediates in the anaerobic metabolism of carbohydrate (74). Accordingly, the biogenesis of the catechol type ligand is independent of the presence of oxygen gas. 

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