Phosphopyruvate

The free glucose produced by this reaction is supplied to the blood from the tissues. As exemplified by gluconeogenesis, one may easily envision the economical organization of these metabolic routes, since, apart from four special gluconeogenesis enzymes-pyruvate carboxylase, phosphopyruvate carboxylase, fructose bisphosphatase, and glucose 6-phosphatase-individual glycolytic enzymes are also used in the gluconeogenesis. 

Sodium fluoride (104) (1-10 mM) inhibits two enzymes of glycolysis the enolase (phosphopyruvate hydratase) and pyruvate kinase. Therefore, aerobic glucose utilization and lactate formation are blocked. 

In the preceding sections the conversion of purines and purine nucleosides to purine nucleoside monophosphates has been discussed. The monophosphates of adenosine and guanosine must be converted to their di- and triphosphates for polymerization to RNA, for reduction to 2 -deoxyribonucleoside diphosphates, and for the many other reactions in which they take part. Adenosine triphosphate is produced by oxidative phosphorylation and by transfer of phosphate from 1,3-diphosphoglycerate and phosphopyruvate to adenosine diphosphate. A series of transphosphorylations distributes phosphate from adenosine triphosphate to all of the other nucleotides. Two classes of enzymes, termed nucleoside mono-phosphokinases and nucleoside diphosphokinases, catalyse the formation of the nucleoside di- and triphosphates by the transfer of the terminal phosphoryl group from adenosine triphosphate. Muscle adenylate kinase (myokinase)... 

Aconitate hydratase [Fe4S4]— aconitase Phosphopyruvate hyd ratase— enolase ... 

Phosphoenolpyruvate carboxykinase (GTP) [EC 4.1.1.32], also known as phosphoenolpyruvate carboxylase and phosphopyruvate carboxylase, catalyzes the reaction of GTP with oxaloacetate to produce GDP, phosphoenolpyruvate, and carbon dioxide. ITP can replace GTP as the phosphorylating substrate. 

This enzyme [EC 4.1.1.38] (also known as phosphoenolpyruvate carboxytransphosphorylase, phosphopyruvate carboxylase, and phosphoenolpyruvate carboxylase) catalyzes the reaction of phosphoenolpyruvate with orthophosphate and carbon dioxide to produce oxaloacetate and pyrophosphate (or diphosphate). The enzyme also catalyzes the reaction of phosphoenolpyruvate with orthophosphate to produce pyruvate and pyrophosphate. 

Phosphopyruvic acid triNa salt [5541-93-5] M 360.0. It is recrystd from MeOH-Et2O the salt (Ig)... 

Phosphopyruvic acid triNa salt [5541-93-5] M 360.0. It is recrystd from MeOH-Et20 the salt (lg) is dissolved in MeOH (40ml) and dry Et20 is added in excess. The white crystals are collected and dried over P205 at 20°. [B 92 952 1959]. 

As discussed briefly in Section I,A, glucose-6-phosphatase is now known to be a multifunctional enzyme capable of catalyzing potent phosphotransferase as well as phosphohydrolase reactions [see Eqs. (1)—(4) ]. Compounds demonstrated to function as effective phosphoryl donors include fructose-6-P (30), mannose-6-P (40), PPi (35-38), a variety of nucleosidetriphosphates and nucleosidediphosphates—most effectively CTP, CDP, deoxy-CTP, ATP, ADP, GTP, GDP, and ITP (41, 45)— carbamyl-P (43), phosphoramidate (44), phosphopyruvate (42, 43) and glucose-6-P itself (30, 31). The various phosphoryl donors are also hydrolyzed by action of the enzyme (see preceding references). Eqqa-tions (1)—(4), which describe these various activities, are given in Section I,A. 

The fixation of CO2 in carbohydrate must be reconciled with two factors (1) the C 02 must in some way enter the carboxyl group of pyruvic acid since the isotope is eventually found exclusively in carbons 3 or 4 of the glycogen (2) the formation of phosphopyruvate prior to the synthesis of D-glucose is an essential step. This conversion of pyruvate to phosphopyruvate was considered to be irreversible until Lardy and Ziegler demonstrated that this reaction can occur in extracts of rat muscle in the presence of potassium ions. 

It is possible, however, that phosphopyruvate is formed in another way. Kalckar has suggested that phosphopyruvate synthesis occurs... 

In this way Solomon and coworkers attempted to explain the conversion of lactate, pyruvate and carbon dioxide to glycogen. According to the proposed mechanism, carboxyl-labelled pyruvate must first undergo the Wood-Werkman reaction to form oxalacetate before carboxyl-labelled phosphopyruvate can be formed by way of fumarate, phosphomalate and phospho-oxalacetate. This hypothesis is no longer necessary in view of Lardy and Ziegler s results described above. It is possible, however, that both pathways are utilized. 

Pyruvate kinase, phosphopyruvate kbtase (EC 2.7.1.40) a widely distributed, metal-ion dependent phosphotransferase, present in yeast, muscle, liver, erythrocytes and other organs and cells. It catalyses the last reaction of glycolysis Phosphoeno/pyruvate (PEP) + ADP -> Pyruvate + ATP (substrate level phosphorylation). Each subunit of P. k. forms an intermediate, cyclic, ternary metal bridge complex ... 

A reversible reaction catalyzes the conversion of pyruvate to phosphopyruvate, and the enzyme involved is pyruvic kinase. The equilibrium of that reaction is on the side of the formation of ATP. Thus, pyruvate kinase is the enzyme responsible for the conversion of phosphoenolpyruvate to pyruvate. The enzyme has been crystallized from muscle it requires ADP, potassium, and magnesium and is noncompetitively inhibited by some estrogenic steroids. Steroids alter the enzyme s viscosity and electrophoretic properties. From this observation, it was assumed that steroids act by modifying the protein molecule. 

In gluconeogenesis, phosphopyruvate carboxylase permits the surmount of a thermodynamic hurdle and determines the conversion from oxaloacetate back to pyruvate, which can now enter the gluconeogenic stream, provided, of course, that the common steps for glycolysis and gluconeogenesis are placed on the gluconeogenic track. 

Scheme 11.25. A representation of the conversion of 2-phosphoglycerate to phosphoenol-pyruvate with catalysis by phosphopyruvate hydratase (EC 4.2.1.11).

Then, in the presence of the enzyme enolase (phosphopyruvate hydratase, EC 4.2.1.11) and as shown in Scheme 11.25, dehydration of the 2-phospho-D-glycerate (2-phosphoglycerate) occurs to produce the phosphate enol of pyruvic acid, phos-phoenolpyruvate (PEP). 

---