Liver pyruvate kinase control

The enzyme that catalyzes the conversion of PEP to pyruvate is pyruvate kinase. Liver pyruvate kinase is stimulated allosterically by fructose-1,6-diphosphate, AMP, ADP, and glyceraldehyde-3-phosphate. It is inhibited by alanine, ATP, NADH, and, more importantly, by cAMP- and Ca2 calmodulin-controlled phosphorylation. High blood glucagon levels thus inhibit the activities of both PFK II and pyruvate kinase in the liver through phosphorylation. Transcription of pyruvate kinase is also decreased by glucagon and increased by insulin. Muscle pyruvate kinase is not subject to cAMP or Ca2+ regulation. The pyruvate kinase reaction is practically irreversible. 

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... 

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. 

Figure 16.30. Reciprocal Regulation of Gluconeogenesis and Glycolysis in the Liver. The level of fructose 2,6-bisphosphate is high in the fed state and low in starvation. Another important control is the inhibition of pyruvate kinase by phosphorylation during starvation.

Pyruvate kinase is the enzyme that catalyzes this reaction. Like phospho-fructokinase, it is an allosteric enzyme consisting of four suhunits of two different types (M and L), as we saw with phosphofructokinase. Pyruvate kinase is inhibited hy ATP. The conversion of phosphoenolpyruvate to pyruvate slows down when the cell has a high concentration of ATP—that is to say, when the cell does not have a great need for energy in the form of ATP. Because of the different isozymes of pyruvate kinase found in liver versus muscle, the control of glycolysis is handled differently in these two tissues, which we will look at in detail in Chapter 18. 

The patient s RBC ADA activity was 43 000 nmol.min . ml RBC " (normal values 495 i 60), There was an about 3-fold increase of red cell pyrimidine 5 -nucleotidase and orotate phosphoribosyl-transferase, whereas other enzymes of purine and pyrimidine metabolism (inosine phosphorylase, adenosine kinase, adenine phosphoribo-syltransferase, hypoxanthine-guanine-phosphoribosyltransferase, phosphoribosylpyrophosphate synthetase) were normal or slightly elevated. There was a 6-fold increase of pyruvate kinase activity relatively to comparably reticulocyte-rich blood, and a 1.5 to 3-fold increase of the other enzymatic activities of glucose and glutathione metabolism. Plasma ADA was much elevated (30.5 pmol.min . ml normal value 5.1 - 2.5), probably reflecting intravascular hemolysis. ADA activity in lymphocytes (2.13 nmol.min 1.10 cells normal 1.93 0.61) and in fibroblasts (26 nmol.min l.mg protein 1 normal range 14-118) was normal, whereas the small increase of activity in platelets (59.5 nmol.min . 10 cells control 26.7) and in the liver (8.4 pmol.min . mg protein" normal ... 

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