2- phosphoglycerate, dehydration

Another glycolytic enzyme, enolase, catalyzes the reversible dehydration of 2-phosphoglycerate to phospho-enolpyruvate ... 

Enolose. A key reaction in the metabolism of sugars is the dehydration of 2-phosphoglycerate to form phosphoenolpyruvate (PEP), the phospho derivative of the enolic form of pyruvic acid ... 

Phosphoenolpyruvate, a key metabolic intermediate. A compound of central importance in metabolism is the phosphate ester of the enol form of pyruvate, commonly known simply as phosphoenolpyruvate (PEP).249 It is formed in the glycolysis pathway by dehydration of 2-phosphoglycerate (Eq. 13-15) or by decarboxylation of oxaloacetate. Serving as a preformed enol from which a reactive enolate anion can be released for condensation reactions,250 251 PEP... 

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 ... 

Phosphoglycerate and phosphoenolpyruvate differ only by dehydration between C-2 and C-3, yet the difference in the A G° of hydrolysis is about... 

Enolase catalyzes the dehydration of 2-phosphoglycerate to form phospho-enolpyruvate (PEP). This reaction converts the low-energy phosphate ester bond of 2-phosphoglycerate into the high-energy phosphate bond of PEP. 

Step 9 involves the dehydration of D-2-phosphoglycerate by enolase to produce phosphoenolpyruvate. The reaction also requires Mg2+. 

After this, phosphoglycero mutase converts 3-phosphoglycerate into 2-phospho-glycerate, which is then dehydrated in phosphoenol pyruvate by the enzyme eno-lase. Phosphoenol pyruvate contains an energy-rich bond that is used by the enzyme pyruvate kinase to phosphorylate ADP into ATP. This reaction generates pyruvate, which is the final product of glycolysis. 

In the next reaction, an enol is formed by the dehydration of 2-phosphoglycerate. Enolase catalyzes the formation of phosphoenolpyruvate (PEP). This dehydration markedly elevates the transfer potential of the phosphoryl group. An enol phosphate has a high phosphoryl-transfer potential, whereas the phosphate ester, such as 2-phosphoglycerate, of an ordinary alcohol has a low one. The A G° of the hydrolysis of a phosphate ester of an ordinary alcohol is -3 kcal mofi (-13 kJ mol i), whereas that of phosphoenolpyruvate is -14.8 kcal mofi (- 62 kJ mofi). Why does phosphoenolpyruvate have such a high phosphoryl-transfer potential The phosphoryl group traps the molecule in its unstable enol form. 

STEPS 9-10 Dehydration and dephosphorylation. Like the /J-hydroxy carbonyl compounds produced in aldol reactions, 2-phosphoglycerate undergoes a ready dehydration (Section 23.4). The process is catalyzed by enolase, and the product is phosphoenolpyruvate, abbreviated PEP. 

Phosphoglycerate is dehydrated to phosphoenolpyruvate (PEP), which contains a high-energy enol phosphate. 

This reaction is a critical step in glycolysis (Chapter 16). Dehydrations to form double bonds, such as the formation of phosphoenolpyruvate (see Table 15.1) from 2-phosphoglycerate (reaction 8), are important reactions ot this type. 

The removal of HzO from biomolecules containing alcohol functional groups is a commonly encountered reaction. A prominent example of this reaction is the dehydration of 2-phosphoglycerate, an important step in carbohydrate metabolism (Figure 1.17). Other products of elimination reactions include ammonia (NH3), amines (RNH2), and alcohols (ROH). 

Dehydration of 2-phosphoglycerate. Enolase catalyzes the dehydration of glycerate-2-phosphate to form PEP ... 

Both the 1,1-proton transfer reaction catalyzed by mandelate racemase (MR) and the dehydration catalyzed by enolase require Mg + for activity. The values of the pK s for mandelate and 2-phosphoglycerate, the substrates for the MR- and enolase-catalyzed reactions, are estimated as 29 and 32, respectively [1]. The values of the pKaS of the general basic Lys residues are 6 and 9 in MR [6] and enolase [73], respectively. Thus, formation of a dienolate anion intermediate is extremely endergonic, unless the active site can stabilize the intermediate which is the obvious function of the essential Mg. The rate accelerations for the MR- and enolase-catalyzed reactions are 10 as a direct result of the values of the pKaS of the a-protons (Table 6.1). 

Enolase catalyzes the trans dehydration of 2-phosphoglycerate to yield phosphoenolpyruvate and water only a small free energy change ( 1 kcal/mol) is associated with the reaction. The process is entropically driven and is readily reversible. This dimeric protein requires a divalent cation for activity and is rather promiscuous in that any one of about nine different cations can activate the enzyme (86). Depending upon the cation studied, the apoenzyme has either one or two metal binding sites per subunit. Metal ions such as Mg + and Mn + have one site per monomer, whereas Co " and Zn + will bind at two sites. In the presence of substrate there are two sites per subunit for all of the metal ions and, depending upon the pH, a third site is also induced. As the pH decreases, the third site is lost but not sites I and II. This third site is an inhibitory site, as the loss of this site parallels the loss of metal ion inhibition (87). The nature of this inhibition is not clear but may be due to the binding of the substrate at the phos-... 

The dehydration of 2-phosphoglycerate to phosphoenolpyruvate is a critical step in the metabolism of the sugar glucose. In the following structures the circled P represents a phosphoryl group (P04 ). 

Dehydration of 2-phosphoglycerate generates the energy-rich molecule phosphoenolpyruvate. 

In this step the enzyme enolase catalyzes the dehydration (removal of a water molecule) of 2-phosphoglycerate. The energy-rich product is phosphoenolpyruvate, the highest energy phosphorylated compound in metabolism. 

In Section 14.4 we learned that aldehydes and ketones exist in an equilibrium mixture of two tautomers called the keto and enol forms. The dehydration of 2-phosphoglycerate produces the molecule phosphoenolpyruvate, which is in the enol form. In this case, the enol is extremely unstable. Because of this instability, the phosphoester bond in the product is a high-energy bond in other words, a great deal of energy is released when this bond is broken. 

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