Enzymes phosphofructokinase

Figure 6.24 The function of the enzyme phosphofructokinase. (a) Phosphofructokinase is a key enzyme in the gycolytic pathway, the breakdown of glucose to pyruvate. One of the end products in this pathway, phosphoenolpyruvate, is an allosteric feedback inhibitor to this enzyme and ADP is an activator, (b) Phosphofructokinase catalyzes the phosphorylation by ATP of fructose-6-phosphate to give fructose-1,6-bisphosphate. (c) Phosphoglycolate, which has a structure similar to phosphoenolpyruvate, is also an inhibitor of the enzyme.

Figure 3.30 The fructose 6-phosphate/fructose 1,6-bisphosphate cycle. The forward reaction is catalysed by the enzyme phosphofructokinase, the reverse reaction by fructose bisphosphatase.

C. The key regulatory enzyme phosphofructokinase-1 (PFK-1) catalyzes the synthesis of fructose 1,6-hisphosphate. 

A typical chemical system is the oxidative decarboxylation of malonic acid catalyzed by cerium ions and bromine, the so-called Zhabotinsky reaction this reaction in a given domain leads to the evolution of sustained oscillations and chemical waves. Furthermore, these states have been observed in a number of enzyme systems. The simplest case is the reaction catalyzed by the enzyme peroxidase. The reaction kinetics display either steady states, bistability, or oscillations. A more complex system is the ubiquitous process of glycolysis catalyzed by a sequence of coordinated enzyme reactions. In a given domain the process readily exhibits continuous oscillations of chemical concentrations and fluxes, which can be recorded by spectroscopic and electrometric techniques. The source of the periodicity is the enzyme phosphofructokinase, which catalyzes the phosphorylation of fructose-6-phosphate by ATP, resulting in the formation of fructose-1,6 biphosphate and ADP. The overall activity of the octameric enzyme is described by an allosteric model with fructose-6-phosphate, ATP, and AMP as controlling ligands. 

EXAMPLE 5.5 Determination of the Effective Diameter of an Enzyme Using Dynamic Light Scattering. DLS analysis of a dilute solution of the enzyme phosphofructokinase in water at T = 293K leads to the following data for the correlation function g2(s,td) ... 

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).]... 

Glycolysis and gluconeogenesis. The highly regulated enzymes phosphofructokinase and fructose... 

ATP is both a substrate and an inhibitor of the enzyme phosphofructokinase (PFK). Although the substrate fructose-6-phosphate binds cooperatively to the active site, ATP does not bind cooperatively. Explain how ATP may be both a substrate and an inhibitor of PFK. 

The same protein kinase that phosphorylates glycogen phosphorylase and glycogen synthase does not phosphorylate the enzymes of pseudocycle II. Rather an enzyme gets phos-phorylated that catalyzes the synthesis of a potent allosteric effector of the two relevant enzymes, phosphofructokinase and fructose bisphosphate phosphatase. In the liver the un-phosphorylated form this enzyme synthesizes fructose-2,6-bisphosphate. Phosphorylation converts it into a degradative enzyme for the same compound. Fructose-2,6-bisphosphate is an activator of phosphofructokinase and an inhibitor of fructose bisphosphate phosphatase. As a result the net effect of glucagon on pseudocycle II is to stimulate fructose bisphosphate phosphatase while inhibiting phosphofructokinase (see table 12.2 and fig. 12.30). 

The common yeast genus Rhodotorula is thought to be missing the enzyme phosphofructokinase yet seems to survive nicely. Can you propose a pathway around phosphofructokinase ... 

In vitro,d an uncoupled phosphorylation of fructose-6-phosphate to form the fructose-1,6-bisphosphate has a positive ArG° but in vivo, it is coupled with the ATP-ADP hydrolysis through the enzyme, phosphofructokinase, to yield an overall negative reaction free energy. 

Hydrophobic and osmophobic effects are important not only in the folding of individual polypeptide chains into compact globular proteins, but also in the assembly of multiprotein complexes. Osmophobic effects are noted, for instance, in the self-assembly of subunits of the glycolytic enzyme phosphofructokinase (PFK). Self-assembly is enhanced by the presence of stabilizing organic cosolvents such as trimethylamine-A-oxide (TMAO) (Hand and Somero, 1982). As discussed later, self-assembly driven by osmophobic effects results from the thermodynamic favorability of minimizing the surface area on the proteins that is in contact with the cosolvent. 

The enzyme phosphofructokinase (PFK) catalyzes a reaction in the breakdown of glucose to pyruvate found in many types of cells ... 

In glycolysis, the enzymes phosphofructokinase (PFK) and fructose 1,6-bisphosphatase (FBP) form a substrate cycle ... 

The enzyme phosphofructokinase is allosteric, that is, it is made up of equivalent units that possess specific reaction sites for the fixation of the substrate and product. Each unit exists in two conformational states one active with more affinity for the substrate, and one inactive. The reaction products of phosphofructokinase (FDP and ADP) displace the conformational equilibrium in favor of the active form of the enzyme. This may create a destabilizing effect on the excess entropy production. In the glycolytic cycle, the allosteric properties of the phosphofructokinase may lead to oscillations. Consider the following simple model... 

Figure 16.19. Domain Structure of the Bifunctional Enzyme Phosphofructokinase 2. The kinase domain (purple) is fused to the phosphatase domain (red). The kinase domain is a P-loop NTP hydrolase domain, as indicated by the purple shading (Section 9.4.4). The bar represents the amino acid sequence of the enzyme.

Fructose 6-phosphate is phosphorylated by the key regulatory enzyme, phosphofructokinase. The product is fructose 1,6-bisphosphate, which is cleaved, forming two triose phosphates. 

Fructose 6-phosphate is phosphorylated by ATP, forming fructose 1,6-bis-phosphate and ADP. This reaction is the first committed step in glycolysis. -Enzyme phosphofructokinase 1 (PFK1)... 

Figure 16.19 Regulation of phosphofructokinase by fructose 2,6 bisphosphate. In high concentrations, fructose 6-phosphate F-6P) activates the enzyme phosphofructokinase (PFK) through an intermediary, fructose 2,6-bisphosphate (F-2.6-BP).

Glucose-6-phosphate is then converted by the enzyme phospho-hexoisomerase to fructose-6-phosphate. A further phosphate group is then transferred from ATP to produce the highly reactive molecule fructose 1, 6, diphosphate, catalysed by the enzyme phosphofructokinase. 

---