Pep—Pho

Abbreviations Pj, phosphate pyr, pyruvate acylcam, acylcamitine cam, carnitine mal, malate cit, citrate isoc, isocitrate PEP, pho hoenolpyravate o-OG, a-oxoglutarate glu, glutamate asp, aspartate gin, glutamine orn, ornithine citrull, citrulline. 

Abbreviations ABA = abscisic acid 2,4-D = 2,4-dichlorophenoxyacetic acid DNP = 2,4-dinitrophenol GA3 = gibberellic acid IAA = indole-3-acetic acid IB A = indole-3-butyric acid NAA = naphthalene-1-acetic acid NPA = naphthylphthalamic acid PEP = phos-phoenolpyruvate 2,3,5-T = 2,3,5-trichlorophenoxyacetic acid TIBA = 2,3,5-triiodoben-zoic acid. 

Figure 17.1 Anaerobic metabolic pathways involved in SA production in wild-type Actinobacillus succinogenes. PEP, phos-phoenolpyruvate OAA, oxaloacetate MAL, malate FUM, fumarate ICT, IsocItrate CIT, citrate acetyl-P, acetyl-phosphate pck, PEP carboxykinase QOH, menaquinol Idh, lactate dehydrogenase pfi, pyruvate formatelyase ...

X indicates knocked-out gene. PEP, phos-phoenolpyruvate OAA, oxaloacetate MAE, malate FUM, fumarate Suc-CoA, succinyl-CoA a-KG, a-ketoglutarate ICT, isocitrate CIT, citrate PYR, pyruvate AcCoA, acetyl-CoA GOX, glyoxylate IdhA, lactate dehydrogenase pfIB, pyruvate formatelyase pta,... 

Pyruvate kinase possesses allosteric sites for numerous effectors. It is activated by AMP and fructose-1,6-bisphosphate and inhibited by ATP, acetyl-CoA, and alanine. (Note that alanine is the a-amino acid counterpart of the a-keto acid, pyruvate.) Furthermore, liver pyruvate kinase is regulated by covalent modification. Flormones such as glucagon activate a cAMP-dependent protein kinase, which transfers a phosphoryl group from ATP to the enzyme. The phos-phorylated form of pyruvate kinase is more strongly inhibited by ATP and alanine and has a higher for PEP, so that, in the presence of physiological levels of PEP, the enzyme is inactive. Then PEP is used as a substrate for glucose synthesis in the pathway (to be described in Chapter 23), instead... 

The bacterial phosphoenolpyruvate (PEP)-dependent carbohydrate phosphotransferase systems (PTS) are characterised by their unique mechanism of group translocation. The transported solute is chemically modified (i.e. phos-phorylated) during the process (for comprehensive reviews see [151,152] and... 

The first steps of actual gluconeogenesis take place in the mitochondria. The reason for this detour is the equilibrium state of the pyruvate kinase reaction (see p. 150). Even coupling to ATP hydrolysis would not be sufficient to convert pyruvate directly into phos-phoenol pyruvate (PEP). Pyruvate derived... 

Glycerate-3-phosphate, glycerate-2-phosphate, and phos-phoenolpyruvate (PEP) make up another equilibrium group of metabolites that, like the hexose phosphates or the triose phosphates, function as a single metabolic pool (see fig. 12.13). 

The enzyme recognizes modified donors such as uridine diphosphate 2-deoxygalactose (UDP-2-d-Gal). The starting material 2-deoxyglucose-6-phos-phate, is formed with hexokinase from 2-d-Glc and ATP, which in turn can be regenerated from PEP with pyruvate kinase, and this is transformed into 2-deoxyglucose-1 -phosphate. 

Figure 5-25. The conversion of pyruvate to phosphoenolpyruvate (PEP). Follow the diagram by starting with the precursors alanine and lactate. OAA = oxaloacetate FA = fatty acid TG = triacylglycerol PDH = pyruvate dehydrogenase PC = pyruvate carboxylase PEPCK = phosphoenolpyruvate PK = pyruvate kinase PK-P = phos- phorylated pyruvate kinase.

PEP Carboxylase catalyzes a reaction homologous with that of pyruvate kinase (52) in which an oxygen of bicarbonate attacks the leaving phos-... 

PEP has a higher phosphoryl group transfer potential than does glycerate-2-phos-phate because it contains an enol-phosphate group instead of a simple phosphate ester. The reason for this difference is made apparent in the next reaction. Aldehydes and ketones have two isomeric forms. The enol form contains a carbon-carbon double bond and a hydroxyl group. Enols exist in equilibrium with the more stable carbonyl-containing keto form. The interconversion of keto and enol forms, also called tautomers, is referred to as tautomerization ... 

The glycolytic enzyme phosphoenolpyruvate (PEP) hydratase (enolase, E.C. 4.2.1.11) catalyzes the addition of water to 2-phospho-D-glycerate (26). The enzyme from E. coli1351 also accepts 3-phospho-D-erythronate (28) and thereby forms phos-phoenol-4-deoxy-3-tetrulosonate (29 in Scheme 11.5-4). Both PEP and phosphoenol-... 

When intermediates are shunted from the citric acid cycle to other pathways, they are replenished by several anaplerotic reactions, which produce four-carbon intermediates by carboxylation of three-carbon compounds these reactions are catalyzed by pyruvate carboxylase, PEP carboxykinase, PEP carboxylase, and malic enzyme. Enzymes that catalyze carboxylations commonly employ biotin to activate CO2 and to carry it to acceptors such as pyruvate or pho spho enolpyruvat e. 

Citrulline as well as organic acids, aspartate, glutamate, alanine, and glutamine were labeled after exposure of nodules of A. glutinosa to C02 for 10 min. Approximately 35% of the total radioactivity was in citrulline. These findings suggest that both PEP carboxylase and carbamyl phos-... 

Fig. 6.1 Reaction scheme of the system under study. Glc, glucose HK, hexokinase PGI, phos-phoglucose isomerase PFK, phosphofructokinase ALD, aldolase TIM, triose phosphate isomerase G3PDH, glycerol 3-phosphate dehydrogenase 1,3BPG 1,3-bisphosphoglycerate PGK, phospho-glycerate kinase G6P, glucose 6-phosphate F6P, fructose 6-phosphate ATP, adenosine triphosphate ADP, adenosine diphosphate PEP, phospho-enolpyruvate F1,6BP, fructose 1,6-bisphosphate DHAP, dihydroxyacetone phosphate G3P, glycerol 3-phosphate GAP, glyceraldehyde...

Modulation of the Mi pyruvate kinase activity also occurs when chick embryo fibroblasts are transformed by Rous sarcoma virus. The pyruvate kinase shows a lower affinity for PEP. The oncoprotein from Rous sarcoma virus pp60 is able to phos-phorylate the Mi pyruvate kinase in vitro (Presek, Remacher Eigenbrodt, 1988). Phosphorylation occurs on a tyrosine residue. Three other glycolytic enzymes have been found to be phosphorylated on tyrosine residues when chick embryo fibroblasts are transformed by Rous sarcoma virus. They are enolase. 

Phosphofrudo-2-kinase adivity is inhibited by dtrate and by PEP. A difference between this bifunctional enzyme in liver and in muscle is that the former is pho horylated by cAMP-depaident protein kinase, causing inadivation of the kinase adivity and adivation of the phosphatase. The pigeon muscle enzyme, however, is not phosphorylated by cAMP-dqrendent protein kinase but, nevertheless, its activities... 

Some of the first successes of cell-free systems have been in the use of purified enzymes. In 1985, Welch enabled cell-free ethanol production by reconstituting the yeast glycolytic system in vitro [41]. In 1992, Fessner and Walter successfully produced dihydroxyacetone phosphate (DHAP), a key metabolic intermediate in glycolysis and the Calvin cycle [63]. Fessner s system required high-energy phos-phoenolpyruvate (PEP) for ATP generation and did not quantify DHAP. That said, similar to Welch s work, it demonstrated the feasibility of a multienzymatic pathway in vitro. 

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