Pyruvate kinase glycolysis

Synthesis of pyruvate. In the final reaction of glycolysis, pyruvate kinase catalyzes the transfer of a phosphoryl group from PEP to ADP. Two molecules of ATP are formed for each molecule of glucose. 

The last enzyme in glycolysis, pyruvate kinase, is also subject to allosteric regulation. In this case, fructose-l,6-bisphosphate is the allosteric activator. It is interesting that fructose-l,6-bisphosphate is the product of the reaction catalyzed by phosphofructokinase. Thus, activation of phosphofructokinase results in the activation of pyruvate kinase. This is an example of feedforward activation because the product of an earlier reaction causes activation of an enzyme later in the pathway... 

See also Enzymes of Glycolysis, Molecules of Glycolysis, Glycolysis, Pyruvate Kinase Regulation, Pyruvate Kinase Isozymes, Substrate Level Phosphorylation, Phosphoglycerate Kinase, Action of Glucagon... 

FIGURE 3.13 Phosphoenolpyruvate (PEP) is produced by the euolase reaction (hi glycolysis see Chapter 19) and hi turn drives the phosphorylation of ADP to form ATP in the pyruvate kinase reaction. 

The second ATP-synthesizing reaction of glycolysis is catalyzed by pyruvate kinase, which brings the pathway at last to its pyruvate branch point. Pyruvate kinase mediates the transfer of a phosphoryl group from phosphoenolpyru-vate to ADP to make ATP and pyruvate (Figure 19.27). The reaction requires... 

Acetyl-CoA is a potent allosteric effector of glycolysis and gluconeogenesis. It allosterically inhibits pyruvate kinase (as noted in Chapter 19) and activates pyruvate carboxylase. Because it also allosterically inhibits pyruvate dehydrogenase (the enzymatic link between glycolysis and the TCA cycle), the cellular fate of pyruvate is strongly dependent on acetyl-CoA levels. A rise in... 

Figure 5.3 Major control points of glycolysis and the TCA cycle. Enzymes I, hexokinase II, phosphofructokinase III, pyruvate kinase IV, pyruvate dehydrogenase V, citrate synthase VI, aconitase VII, isocitrate dehydrogenase VIII, a-oxoglutarate dehydrogenase.

Let us consider Figure 5.3 again. Both pyruvate kinase and dtrate synthase (enzymes III and V) are inhibited by elevated ATP concentrations. During citric acid production ATP concentrations are likely to arise (ATP produced in glycolysis) and either of these enzymes could, if inhibited, slow down the process. In fact all of the evidence suggests that both enzymes are modified or controlled in some way such that they are insensitive to other cellular metabolites during citric add production. 

Glycolysis A net formation of two results from the formation of lactate from one molecule of glucose, generated in two reactions catalyzed by phospho-glycerate kinase and pyruvate kinase, respectively (Figure 17-2). 

Glycolysis Glyceraldehyde-3-phosphate dehydrogenase Phosphoglycerate kinase Pyruvate kinase... 

Glycolysis is regulated by three enzymes catalyzing nonequilibrium reactions hexokinase, phosphoffuc-tokinase, and pyruvate kinase. 

Three nonequilibrium reactions catalyzed by hexoki-nase, phosphofructokinase, and pyruvate kinase prevent simple reversal of glycolysis for glucose synthesis (Chapter 17). They are circumvented as follows ... 

Pyruvate kinase (PK) is one of the three postulated rate-controlling enzymes of glycolysis. The high-energy phosphate of phosphoenolpyruvate is transferred to ADP by this enzyme, which requires for its activity both monovalent and divalent cations. Enolpyruvate formed in this reaction is converted spontaneously to the keto form of pyruvate with the synthesis of one ATP molecule. PK has four isozymes in mammals M, M2, L, and R. The M2 type, which is considered to be the prototype, is the only form detected in early fetal tissues and is expressed in many adult tissues. This form is progressively replaced by the M( type in the skeletal muscle, heart, and brain by the L type in the liver and by the R type in red blood cells during development or differentiation (M26). The M, and M2 isozymes display Michaelis-Menten kinetics with respect to phosphoenolpyruvate. The Mj isozyme is not affected by fructose-1,6-diphosphate (F-1,6-DP) and the M2 is al-losterically activated by this compound. Type L and R exhibit cooperatively in... 

Mechanism for Gluconeogenesis. Since the glycolysis involves three energetically irreversible steps at the pyruvate kinase, phosphofructokinase, and hexokinase levels, the production of glucose from simple noncarbohydrate materials, for example, pyruvate or lactate, by a reversal of glycolysis ( from bottom upwards ) is impossible. Therefore, indirect reaction routes are to be sought for. 

H. G. Holzhtitter, G. Jacobasch, and A. Bisdorff, Mathematical modeling of metabolic pathways affected by an enzyme deficiency. A mathematical model of glycolysis in normal and pyruvate kinase deficient red blood cells. Eur. J. Biochem. 149(1), 101 111 (1985). 

Examples of substrate level phosphorylation are to be found in glycolysis. Phos-phoglycerate kinase (PGK) and pyruvate kinase (PK) catalyse the following reactions ... 

Feed-forward control is more likely to be focused on a reaction occurring at or near the end of a pathway. Compounds produced early in the pathway act to enhance the activity of the control enzyme and so prevent a back log of accumulated intermediates just before the control point. An example of feed-forward control is the action of glucose-6-phosphate, fructose-1,6-bisphosphate (F-l,6bisP) and phosphoenol pyruvate (PEP), all of which activate the enzyme pyruvate kinase in glycolysis in the liver. 

GNG exploits the fact that most of the reactions of glycolysis are reversible so the enzymes are shared between the two pathways. There are three kinase reactions (glucokinase/hexokinase, PFK and pyruvate kinase), which are not physiologically reversible are therefore the problem steps in the synthesis of glucose these three steps are overcome using alternative enzymes (Table 6.5, see also Section 1.7.1). 

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. 

Pyruvate kinase the last enzyme in aerobic glycolysis, it catalyzes a substrate-level phosphorylation of ADP using the high-energy substrate phosphoenolpyruvate (PEP). Pyruvate kinase is activated by fructose 1,6-bisphosphate from the PFK-1 reaction (feedforward activation). 

The red blood cell has no mitochondria and is totally dependent on anaerobic glycolysis for ATP. In pyruvate kinase deficiency, the decrease in ATP causes the erythrocyte to lose its characteristic biconcave shape and signals its destruction in the spleen. In addition, decreased ion pumping by Na /K -ATPase results in loss of ion balance and causes osmotic fragility, leading to swelling and lysis. 

Answer E, A right-shift in the Oj binding curve is indicative of abnormally elevated 2,3-BPG secondary to a defect in red cell anaerobic glycolysis. Only pyruvate kinase participates in this pathway. 

Figure 2.6 The process of glycolysis illustrating the three non-eguilibrium reactions. The reactions are catalysed by hexokinase, phosphofructokinase and pyruvate kinase which are indicated by the heavy unidirectional arrows. The reactions in which ATP is utilised and those in which it is produced are indicated (see Appendix 2.7).

The pathway for gluconeogenesis is shown in Figures 6.23 and 6.24. Some of the reactions are catalysed by the glycolytic enzymes i.e. they are the near-equilibrium. The non-equilibrium reactions of glycolysis are those catalysed by hexokinase (or glucokinase, in the liver), phosphofructokinase and pyruvate kinase and, in order to reverse these steps, separate and distinct non-equilibrium reactions are required in the gluconeogenic pathway. These reactions are ... 

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