Phosphoenolpyruvate formation

Transfer of citrate through the inner membrane of MCh is provided by a tricarboxylate transporter (m.w. 32.5 kD), which also catalyzes transport of treo-Ds-isocitrate, cis-aconitate and other tricarboxylates (LaNoue and School-werth, 1979 Kaplan et al, 1990). This is electroneutral exchange for either another tricarboxylate or dicarboxylate (e.g. malate or succinate), or for phosphoenolpyruvate. Formation of glutathione-citryl thioester is irreversibly inhibited by (-)erythrofluorocitrate (IC50 = 25 pmol FC/mg protein), which makes a stable adduct with the synthase (Kun et al, 1977). However, the block of citrate transport... 

Garber, A. J., Ballard, F. J., and Hanson, R. W., 1972, Significance of mitochondrial phosphoenolpyruvate formation in the regulation of gluconeogenesis in guinea pig liver, in Energy Metabolism and the Regulation of Metabolic Processes (R. W. Hanson and M. A. Mehlman, eds.), p. 109, Academic Press, New York. 

The transport of each COg requires the expenditure of two high-energy phosphate bonds. The energy of these bonds is expended in the phosphorylation of pyruvate to PEP (phosphoenolpyruvate) by the plant enzyme pyruvate-Pj dikinase the products are PEP, AMP, and pyrophosphate (PPi). This represents a unique phosphotransferase reaction in that both the /3- and y-phosphates of a single ATP are used to phosphorylate the two substrates, pyruvate and Pj. The reaction mechanism involves an enzyme phosphohistidine intermediate. The y-phosphate of ATP is transferred to Pj, whereas formation of E-His-P occurs by addition of the /3-phosphate from ATP ... 

As these freely reversible aldol additions often have less favorable equilibrium constants [30,34], synthetic reactions usually have to be driven by an excess of pyruvate to achieve satisfactory conversions. A few related enzymes have been identified that utilize phosphoenolpyruvate instead of pyruvate, which upon C—C bond formation releases inorganic phosphate, and thus renders the aldol addition essentially irreversible (Figure 10.4) [16]. Although attractive from a synthetic point ofview, the latter enzymes have been less studied as yet for preparative applications [35]. 

The Jirst indirect route in glucose synthesis involves the formation of phosphoenolpyruvate from pyruvate without the intervention of pyruvate kinase. This route is catalyzed by two enzymes. At first, pyruvate is converted into oxaloacetate. This reaction occurs in the mitochondria as the pyruvate molecules enter them, and is catalyzed by pyruvate carboxylase according to the scheme... 

Aldolases catalyze asymmetric aldol reactions via either Schiff base formation (type I aldolase) or activation by Zn2+ (type II aldolase) (Figure 1.16). The most common natural donors of aldoalses are dihydroxyacetone phosphate (DHAP), pyruvate/phosphoenolpyruvate (PEP), acetaldehyde and glycine (Figure 1.17) [71], When acetaldehyde is used as the donor, 2-deoxyribose-5-phosphate aldolases (DERAs) are able to catalyze a sequential aldol reaction to form 2,4-didexoyhexoses [72,73]. Aldolases have been used to synthesize a variety of carbohydrates and derivatives, such as azasugars, cyclitols and densely functionalized chiral linear or cyclic molecules [74,75]. 

The oxidative decarboxylation reaction above is part of the TCA cycle and leads to the formation of oxaloacetate, which maybe used to synthesize citrate (with acetyl-CoA) or may be used as a substrate by phosphoenol pyruvate carboxykinase, PEPCK. It should be noted that the phosphoenolpyruvate generated by PEPCK reaction shown above is... 

Pyruvate is derived from phosphoenolpyruvate may be inter-converted into lactate, alanine, oxaloacetate. Formation ofacetyl-CoA from pyruvate is essentially irreversible... 

Once the phosphate ester is hydrolysed, there is an immediate rapid tautomerism to the keto form, which becomes the driving force for the metabolic transformation of phosphoenolpyruvic acid into pyruvic acid, and explains the large negative free energy change in the transformation. This energy release is coupled to ATP formation (see Box 7.25). 

Fig. 5 Speculative metabolic schemes of the main pathways in carbohydrate metabolism in Trimyema compressum (after Goosen et al. 1990). End products are in boxes. Abbreviations AcCoA, acetyl-Co A, Hyd, hydrogenase, PEP, phosphoenolpyruvate carboxykinase, PFL, pyruvate formate lyase, PFO, pyruvate ferredoxin oxidoreductase, PYR, pyruvate, Xox, red> unknown electron carrier...

All carbons are derived from either erythrose 4-phosphate (light purple) or phosphoenolpyruvate (pink). Note that the NAD+ required as a cofactor in step (3) is released unchanged it may be transiently reduced to NADH during the reaction, with formation of an oxidized reaction intermediate. Step (6) is competitively inhibited by glyphosate (COO—CH2—NH—CH2—PO ), the active ingredient in the widely used herbicide Roundup. The herbicide is relatively nontoxic to mammals, which lack this biosynthetic pathway. The chemical names quinate, shikimate, and chorismate are derived from the names of plants in which these intermediates have been found to accumulate. 

Recently Benkovic and Schrayl28b and Clark and Kirby,26c have investigated the hydrolysis of dibenzylphosphoenolpyruvic acid and mono-benzylphospho-enolpyruvic acid which proceed via stepwise loss of benzyl alcohol (90%) and the concomitant formation of minor amounts (10%) of dibenzylphosphate and monobenzylphosphate, respectively. The pH-rate profiles for release of benzyl alcohol reveal that the hydrolytically reactive species must involve a protonated carboxyl group or its kinetic equivalent. In the presence of hydroxylamine the course of the reaction for the dibenzyl ester is diverted to the formation of dibenzyl phosphate (98%) and pyruvic acid oxime hydroxamate but remains unchanged for the monobenzyl ester except for production of pyruvic acid oxime hydroxamate. The latter presumably arises from phosphoenolpyruvate hydroxamate. These facts were rationalized according to scheme (44) for the dibenzyl ester, viz. 

The mixed anhydride of phosphoric acid and glyceric acid then is used to convert ADP to ATP and form 3-phosphoglycerate. Thereafter the sequence differs from that in photosynthesis. The next few steps accomplish the formation of pyruvate by transfer of the phosphoryi group from C3 to C2 followed by dehydration to phosphoenolpyruvate. Phosphoenolpyruvate is an effective phosphorylating agent that converts ADP to ATP and forms pyruvate ... 

The first step in the gluconeogenic direction involves the formation of phosphoenolpyruvate from pyruvate. Reversal of the pyruvate kinase reaction requires at least two ATP-to-ADP conversions. One means by which this is done is shown in figure 12.26. 

In the formation of phosphoenolpyruvate from pyruvate in the mesophyll cells (fig. 15.28), notice the energetics of the reaction. How does this process compare energetically with the formation of phosphoenolpyruvate from pyruvate in the start of gluco-neogenesis ... 

Aromatic amino acid biosynthesis proceeds via a long series of reactions, most of them concerned with the formation of the aromatic ring before branching into the specific routes to phenylalanine, tyrosine, and tryptophan. Chorismate, the common intermediate of the three aromatic amino acids, (see fig. 21.1) is derived in eight steps from erythrose-4-phosphate and phosphoenolpyruvate. We focus on the biosynthesis of tryptophan, which has been intensively studied by both geneticists and biochemists. 

Three types of lyases have been identified that catalyze the addition of phosphoenolpyruvate (PEP) to aldoses or to terminally phosphorylated sugar derivatives. With simultaneous release of inorganic phosphate from the preformed enolpyruvate nucleophile during C-C bond formation the additions are essentially irreversible and, therefore, these lyases are often referred to as synthases. The mechanistic details of these reactions, however, have yet to be elucidated but it seems obvious that the chances of variation on the part of the nucleophile will be strictly limited. Although the thermodynamic advantage makes these enzymes highly attractive for synthetic applications, none of them is yet commercially available and only few data have been reported concerning the individual specificities towards aldehydic substrates. 

Inhibits enol-pyruvate transferase, which catalyses the incorporation of phosphoenolpyruvic acid (PEP) into uridine diphospho-N-acetylglucoamine (UDPNAG), a precursor involved in the formation of bacterial cell walls. 

For the aromatic pathway (Figure 30.20), the critical control points are the condensation of phosphoenolpyruvate and erythrose-4-phosphate to 3-deoxy-D-arabinoheptulosonate 7-phosphate, DAHP, by DAHP synthase. For tryptophan, the formation of anthranilic acid from chorismic acid by anthranilate synthase is the second critical control point. The transcriptional regulation was overcome through the use of alternative promoters and allosteric regulation was circumvented by the classical technique of selection for feedback-resistant mutants using toxic analogues of the repressing compounds. 

Formation of Oxalacetate by CO2 Fixation on Phosphoenolpyruvate Merton F. Utter and Harold M. Kolenbrander... 

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