Phosphofructokinase, allosteric regulation

This reaction is followed by another phosphorylation with ATP catalyzed by the enzyme phosphofructoki-nase (phosphofructokinase-1), forming fructose 1,6-bisphosphate. The phosphofructokinase reaction may be considered to be functionally irreversible under physiologic conditions it is both inducible and subject to allosteric regulation and has a major role in regulating the rate of glycolysis. Fructose 1,6-bisphosphate is cleaved by aldolase (fructose 1,6-bisphosphate aldolase) into two triose phosphates, glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. Glyceraldehyde 3-phosphate and dihydroxyacetone phosphate are inter-converted by the enzyme phosphotriose isomerase. 

Structural Basis of Allosteric Regulation on the Example of Phosphofructokinase... 

Based on high resolution crystal structures, detailed understanding of the molecular principles of allosteric regulation for several enzyme systems could be gleamed. One of the best studied examples is that of phosphofructokinase from Bac. stearothermophilus (Schirmer and Evans, 1990). 

Three glycolytic enzymes are subject to allosteric regulation hexoldnase IV, phosphofructokinase-1 (PFK-1), and pyruvate kinase. 

Heterotropic effectors The effector may be different from the substrate, in which case the effect is said to be heterotropic. For example, consider the feedback inhibition shown in Figure 5.17. The enzyme that converts A to B has an allosteric site that binds the end-product, E. If the concentration of E increases (for example, because it is not used as rapidly as it is synthesized), the initial enzyme in the pathway is inhibited. Feedback inhibition provides the cell with a product it needs by regulating the flow of substrate molecules through the pathway that synthesizes that product. [Note Heterotropic effectors are commonly encountered, for example, the glycolytic enzyme phosphofructokinases allosterically inhibited by citrate, which is not a substrate for the enzyme (see p. 97).]... 

Figure 16.17. Allosteric Regulation of Phosphofructokinase. A high level of ATP inhibits the enzyme by decreasing its affinity for fructose 6-phosphate. AMP diminishes and citrate enhances the inhibitory effect of ATP. 

Allosteric interactions. The flow of molecules in most metabolic pathways is determined primarily by the activities of certain enzymes rather than by the amount of substrate available. Enzymes that catalyze essentially irreversible reactions are likely control sites, and the first irreversible reaction in a pathway (the committed step) is nearly always tightly controlled. Enzymes catalyzing committed steps are allosterically regulated, as exemplified by phosphofructokinase in glycolysis and acetyl CoA carboxylase in fatty acid synthesis. Allosteric interactions enable such enzymes to rapidly detect diverse signals and to adjust their activity accordingly. 

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... 

Activity of glycolysis depends on adenylate energy charge (Figure 13.8) Allosteric Regulation of Phosphofructokinase... 

Denton, H., Thong, K.-W. and Coombs, G. H. (1994) Eimeria tenella contains a pyrophosphate-dependent phosphofructokinase and a pyruvate kinase with unusual allosteric regulators. FEMS Microbiol. Lett. 115 87-92. 

This reaction is catalyzed by the enzyme fructose-l,6-hisphosphatase, an allosteric enzyme strongly inhibited by adenosine monophosphate (AMP) but stimulated by ATP. Because of allosteric regulation, this reaction is also a control point in the pathway. When the cell has an ample supply of ATP, the formation rather than the breakdown of glucose is favored. This enzyme is inhibited by fructose-2,6- >t5phosphate, a compound we met in Section 17.2 as an extremely potent activator of phosphofructokinase. We shall return to this point in the next section. 

Describe the allosteric regulation of phosphofructokinase. Explain the role of fructose 2,6-bisphosphate in its regulation. Describe the fused-domain structure of phosphofructokinase 2 (PFK2)/fructose bisphosphatase 2 that forms and degrades fructose 2,6-bisphosphate. 

Regulation of fructose-1,6-bisphosphatase is via fructose 2,6-bisphosphate. It is synthesized by the allosterically regulated tandem enzyme that is both a kinase and a phosphatase. Fructose 2,6-bisphosphate is an inhibitor of phosphofructokinase and an activator of fructose-... 

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