Phosphofructokinase reaction catalyzed

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. 

Allow for consumption of ATP by reactions catalyzed by hexokinase and phosphofructokinase... 

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 ... 

The second indirect route involves the formation of fructose 6-phosphate from fructose 1,6-bisphosphate without the intervention of phosphofructokinase reaction. This route is catalyzed by fructose bisphosphatase ... 

This enzyme [EC 2.7.1.11], also known as phosphohexo-kinase and phosphofructokinase 1, catalyzes the reaction of ATP with D-fructose 6-phosphate to produce ADP and D-fructose 1,6-bisphosphate. Both D-tagatose 6-phosphate and sedoheptulose 7-phosphate can act as the sugar substrate. UTP, CTP, GTP, and ITP all can act as the nucleotide substrate. This enzyme is distinct from that of 6-phosphofructo-2-kinase. See also ATP GTP Depletion... 

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. 

The reaction catalyzed by phosphofructokinase A. is activated by high concentrations of ATP and citrate. B. uses fructose 1-phosphate as substrate. C. is the regulated reaction of the glycolytic pathway. D. is near equilibrium in most tissues. E. is inhibited by fructose 2,6-bisphosphate. Correct answer = C. Phosphofructokinase is the pace-setting enzyme of glycolysis. It is inhibited by ATP and citrate, uses fructose 6-phosphate as substrate, and catalyzes a reaction that is far from equilibrium. The reaction is activated by fructose 2,6-bisphosphate. 

Kinetics of the reaction catalyzed by phosphofructokinase. In the presence of 1.5 mM ATP, the rate has a sigmoidal dependence on the concentration of the substrate fructose-6-phosphate. Although ATP also is a substrate for the reaction, the sigmoidal kinetics seen under these conditions are associated with the binding of ATP to an inhibitory allosteric site. The kinetics become hyperbolic if a low concentration of AMP is added. 

The reaction catalyzed by phosphofructokinase. The mechanism of this reaction is very similar to the hexokinase reaction shown in figure 12.16. B is a proton acceptor at the active site. 

The most important control step of glycolysis is the irreversible reaction catalyzed by phosphofructokinase (PFK). The enzyme is regulated in several ways ... 

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

Hexokinase (HK EC 2.7.1.1) catalyzes the phosphorylation of glucose to glucose-6-phosphate (G6P) using ATP as a phosphoryl donor. The activity of HK is significantly higher in reticulocytes compared with mature red cells, where it is very low. The HK reaction is one of two rate-limiting steps in this pathway, the other being the phosphofructokinase reaction. 

An open two-substrate reaction catalyzed by phosphofructokinase of E. coli was modelled by Maikova et al. (1980). The parameter regions where oscillations are observed were also determined. 

At three strategic points, glycolytic and gluconeogenic reactions are catalyzed by different enzymes. For example, phos-phofructokinase and fructose-1,6-diphosphatase catalyze opposing reactions. If both reactions occur simultaneously (i.e., in a futile cycle) to a significant extent, ATP hydrolysis in the reaction catalyzed by phosphofructokinase releases large amounts of heat. If the heat is not quickly dissipated, an affected individual could die of hyperthermia. 

Four molecules of ATP are formed from ADP during glycolysis via substrate-level phosphorylation, which is catalyzed by enzymes in the cytosol (reactions 7 and 10). Unlike ATP formation in mitochondria and chloroplasts, a proton-motive force is not involved in substrate-level phosphorylation. Early In the glycolytic pathway, two ATP molecules are consumed one by the addition of a phosphate residue to glucose In the reaction catalyzed by hex okinase (reaction 1), and another by the addition of a second phosphate to fructose 6-phosphate In the reaction catalyzed by phosphofructokinase 1 (reaction 3). Thus glycolysis yields a net of only two ATP molecules per glucose molecule. 

Allosteric effects can also be positive in that a particular molecule can cause the formation of an active conformation. For example, the reaction catalyzed by phosphofructokinase ... 

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... 

The control points in glycolysis are the reactions catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase. 

The glycolytic pathway down to pyruvate involves 10 enzyme-catalyzed steps with three kinases that use ATP (Fig. 11-32). The reaction catalyzed by phosphofructokinase (PFK) plays the major part in controlling the overall flux of the whole pathway. It catalyzes the phosphorylation of fructose 6-phosphate (Fru6P) to fructose 1,6-bisphosphate (Frul6P2). Hexokinase and pyruvate kinase (PK) have relatively higher maximal velocities. PFK is the target for more effector molecules than the other two kinases (see Fig. 11-14). 

It has been demonstrated that muscle extracts contain a specific phosphofructokinase which catalyzes the formation of fructose-1,6-diphosphate from fructose-l-phosphate (reaction 2). 

Phosphofructokinase is an enzyme that catalyzes one of the steps in the degradation of carbohydrates. Initial rates of the reaction that converts fructose-6-phosphate (S) to fructose-1,6-diphosphate (P) as a function of c 0 (the initial fructose-6-diphosphate concentration) are as follows (the concentration of enzyme added, ce0, is the same in each case) ... 

---