Phosphofructokinase control

Figure 5.3 Major control points of glycolysis and the TCA cycle. Enzymes I, hexokinase II, phosphofructokinase III, pyruvate kinase IV, pyruvate dehydrogenase V, citrate synthase VI, aconitase VII, isocitrate dehydrogenase VIII, a-oxoglutarate dehydrogenase.

Within glycolysis, the main allosteric control is exercised by phosphofructokinase, a complicated enzyme unusual in that its activity is stimulated by one of its products (ADP) and inhibited by one of its substrates (ATP). One further point about this enzyme which will be important to us later, in Aspergillus spp., elevated levels of ammonium ions relieve phosphofructokinase of inhibition by titrate. 

Nitrogen is normally supplied as an ammonium compound in dtric acid fermentations and suffident has to be supplied to enable the effect of manganese deficiency (increased levels of ammonium in the metabolic pool) to occur. Remember that increased metabolic pool ammonium has the effect of releasing the allosteric controls exerted on phosphofructokinase. 

A decreased glycolytic rate has been proposed as a cause of muscle fatigue and related to pH inhibition of glycolytic enzymes. Decreasing pH inhibits both phosphorylase kinase and phosphofructokinase (PFK) activities. PFK is rate determining for glycolytic flux and therefore must be precisely matched to the rate of ATP expenditure. The essential characteristic of PFK control is allosteric inhibition by ATP. This inhibition is increased by H and PCr (Storey and Hochachka, 1974 ... 

The most potent positive allosteric effector of phospho-ffuctokinase-1 and inhibitor of fructose-1,6-bisphos-phatase in liver is fructose 2,6-bisphosphate. It relieves inhibition of phosphofructokinase-1 by ATP and increases affinity for fructose 6-phosphate. It inhibits fructose-1,6-bisphosphatase by increasing the for fructose 1,6-bisphosphate. Its concentration is under both substrate (allosteric) and hormonal control (covalent modification) (Figure 19-3). 

Fructose 2,6-bisphosphate is formed by phosphorylation of fructose 6-phosphate by phosphofructoki-nase-2. The same enzyme protein is also responsible for its breakdown, since it has fructose-2,6-hisphos-phatase activity. This hifrmctional enzyme is under the allosteric control of fructose 6-phosphate, which stimulates the kinase and inhibits the phosphatase. Hence, when glucose is abundant, the concentration of fructose 2,6-bisphosphate increases, stimulating glycolysis by activating phosphofructokinase-1 and inhibiting... 

Figure 19-3. Control of glycolysis and gluconeoge-nesis in the liver by fructose 2,6-bisphosphate and the bifunctional enzyme PFK-2/F-2,6-Pase (6-phospho-fructo-2-kinase/fructose-2,6-bisphosphatase). (PFK-1, phosphofructokinase-1 [6-phosphofructo-1 -kinase] ...

Where are the control points Reactions catalysed by. Hexokinase/Glucokinase Phosphofructokinase Pyruvate kinase... 

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. 

Figure 11-2 Roles of phosphofructose kinase and fructose 1,6-bisphosphatase in the control of the breakdown and storage (—+) of glycogen in muscle. The uptake of glucose from blood and its release from tissues is also illustrated. The allosteric effector fructose 2,6-bisphosphate (Fru-2,6-P2) regulates both phosphofructokinase and fructose 2,6-bisphosphatase. These enzymes are also regulated by AMP if it accumulates. The activity of phosphofructokinase-2 (which synthesizes Fru-2,6-P2) is controlled by a cyclic AMP-dependent kinase and by dephosphorylation by a phosphatase.

Reaction 2 of Fig. 17-7 is a simple isomerization that moves the carbonyl group to C-2 so that (1 cleavage to two three-carbon fragments can occur. Before cleavage a second phosphorylation (reaction 3) takes place to form fructose 1,6-bisphosphate. This ensures that when fructose bisphosphate is cleaved by aldolase each of the two halves will have a phosphate handle. This second priming reaction (reaction 3) is the first step in the series that is unique to glycolysis. The catalyst for the reaction, phosphofructokinase, is carefully controlled, as discussed in Chapter 11 (see Fig. 11-2). 

An interesting suggestion for the regulation of carbohydrate metabolism by FDPase is that of Newsholme and Crabtree (79), who have proposed that it can act as a modulator of the control of phosphofructokinase by AMP in muscle. A similar suggestion for regulation of carbohydrate metabolism in liver has been advanced by Williamson and co-workers (114). 

Allosteric Enzymes Typically Exhibit a Sigmoidal Dependence on Substrate Concentration The Symmetry Model Provides a Useful Framework for Relating Conformational Transitions to Allosteric Activation or Inhibition Phosphofructokinase Allosteric Control of Glycolysis Is Consistent with the Symmetry Model Aspartate Carbamoyl Transferase Allosteric Control of Pyrimidine Biosynthesis Glycogen Phosphorylase Combined Control by Allosteric Effectors and Phosphorylation... 

Phosphofructokinase Allosteric Control of Glycolysis Is Consistent with the Symmetry Model... 

Fructose-2,6-bisphosphate is a potent activator of the liver phosphofructokinase (PFK-1) and a potent inhibitor of liver fructose-1,6-bisphosphate phosphatase (FBPase-1). Fructose-2,6-bisphosphate is the product of a second phosphofructokinase (PFK-2) and is hydrolyzed to fructose-6-phosphate by FBPase-2. The activities of PKF-2 and FBPase-2 reside on a single, bifunctional protein in liver. The bifunctional protein is under glucagon control imposed via cAMP. 

Other forms of vanadium have been implicated in the stimulation of the plasma membrane vanadate-dependent NAD(P)H oxidation reaction. Decavanadate has been shown to be a more potent stimulator of the vanadate-dependent NADH oxidation activity than added orthovanadate [30,31], Interestingly, decavanadate reductase activity has been found to be an alternative activity of an NADP-specific isocitrate dehydrogenase [32], Diperoxovanadium derivatives have also been shown to be involved in this type of reaction [33,34], Decavanadate may play a role in the biological role of vanadium, as it is found in yeast cells growing in the presence of orthovanadate [8] and is a potent inhibitor of phosphofructokinase-1, the control step of glycolysis, and other metabolic reactions [35],... 

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

Chance, B., Eleff, S., Bank, W Leigh, J.S., Wamell, R. (1982). 3IP NMR studies of control of mitochondrial function in phosphofructokinase-deficient human skeletal muscle. Proc. Natl. Acad. Sci. USA 79, 7714-7718. 

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