Fructose 1,6-bisphosphate pyruvate kinase activation

Pyruvate kinase, in general, is inhibited by high concentrations of ATP, alanine, acetyl-CoA and long-chain fatty acids. Thus the affinity of the enzyme for its substrate is lowered when energy requirements can be satisfied by other means. Conversely at low [ATP], the affinity of pyruvate kinase for phosphoenolpyruvate (PEP) increases to support the substrate-level phosphorylation of ADP even at low [PEP] because of its inherent instability. The liver isoenzyme is activated by fructose 1,6-bisphosphate so that pyruvate kinase activity is coordinated with variations in... 

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... 

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. 

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). 

A regulatory effect observed in many biochemical pathways an enzyme, which catalyzes a step (typically a late step) in a pathway, is activated by elevated levels of a precursor of a substrate for that enzyme. A possible example of feed-forward activation may be the action of elevated levels of fructose 1,6-bisphosphate on pyruvate kinase. 

Feed-forward regulation In liver, pyruvate kinase is activated by fructose 1,6-bisphosphate, the product of the phosphofructo-kinase reaction. This feed-forward (instead of the more usual feedback) regulation has the effect of linking the two kinase activities increased phosphofructokinase activity results in elevated levels of fructose 1,6-bisphosphate, which activates pyruvate kinase. 

Pyruvate kinase catalyzes the third irreversible step in glycolysis. It is activated by fructose 1,6-bisphosphate. ATP and the amino acid alanine allosterically inhibit the enzyme so that glycolysis slows when supplies of ATP and biosynthetic precursors (indicated by the levels of Ala) are already sufficiently high. In addition, in a control similar to that for PFK (see above), when the blood glucose concentration is low, glucagon is released and stimulates phosphorylation of the enzyme via a cAMP cascade (see Topic J7). This covalent modification inhibits the enzyme so that glycolysis slows down in times of low blood glucose levels. 

Pyruvate kinase is also stimulated by fructose 1,6-bisphosphate (see Topic J3 feedforward activation) so that its activity rises when needed, as glycolysis speeds up. 

A. Insulin stimulates activation of pyruvate kinase, pyruvate dehydrogenase, and phospho-fructokinase 2 (PFK2). PFK2 then catalyzes formation of fructose 2,6-bisphosphate, which is an activator of PFK1 and an inhibitor of fructose 1,6-bisphosphatase, a gluconeogenic enzyme. 

C. The rates of both glucose utilization and ATP generation by glycolysis increase to compensate for the absence of sufficient ATP production from oxidative phosphorylation as a result of oxygen deprivation. Under these circumstances, ATP is used as rapidly as it is made so it is not present in sufficient concentration to inhibit pyruvate kinase. Furthermore, fructose 1,6-bisphosphate tends to stimulate pyruvate kiuase activity. 

A few inhibitors of these several enzymes are provided in Voet and Voet (1990). These include glucose-6-phosphate as an inhibitor for hexokinase, as it is a reaction product. Also, ATP and citrate are listed as inhibitors of PFK, and ATP is an inhibitor for pyruvate kinase, as it is a conversion product. Other inhibitors are tabulated in Appendix A of Hoffman s Cancer and the Search for Selective Biochemical Inhibitors (1999). On the other hand, Voet and Voet (1990) list a few activators or promoters for PFK, including ADP, fructose 6-phosphate (which is a reactant), fructose 1,6-bisphosphate (which is a product), the ammonium ion, and the orthophosphate ion Pj. The situation can get complicated. 

There are also two activators of liver pyruvate kinase, fructose-1,6-bisphosphate and 6-phosphogluconate, and one inhibitor, alanine. 

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... 

Pyruvate kinase is inhibited by ATP, activated by fructose-1,6-bisphosphate (feedforward activation), and inhibited by acetyl-CoA. It is also responsive to hormonally-regulated phosphorylation in the liver -the phosphorylated form of the enzyme is less active. Alanine is also an inhibitor of the enzyme. 

For lipogenesis, glucose 6-phosphate is converted through glycolysis to pyruvate. Key enzymes that regulate this pathway in the liver are phosphofructokinase-1 (PFK-1) and pyruvate kinase. PFK-1 is aliosterically activated in the fed state by fructose 2,6-bisphosphate and adenosine monophosphate (AMP) (see Fig. 36.1). Phosphofructokinase-2, the enzyme that produces the activator fructose 2,6-bisphosphate, is dephosphorylated and active after a meal (see Chapter 22). Pyruvate kinase is also activated by dephosphorylation, which is stimulated by the increase of the insulin/glucagon ratio in the fed state (see Fig. 36.1). 

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