Glycolysis 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. 

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] ...

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. 

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). 

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... 

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 ... 

Step 3 is the second control point of glycolysis and involves the conversion of fructose 6-phosphaie into fructose 1,6-bisphosphate, catalyzed by phosphofructokinase. 

These considerations do not take into account the major glycolytic controls exerted by the hexokinase-phosphofructokinase system (230) but they do show that G-3PDH could act as a regulatory enzyme in response to the NAD+ NADH and ATP ADP X Pi ratios in the cell. These ratios in conjunction with appropriate substrate (i.e., G-3P, DPGA, and Pi) concentrations prime the enzyme for glycolysis or gluconeogenesis in accord with the particular environment and needs of the cell. 

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. 

Phosphofructokinase, the major flux-controlling enzyme of glycolysis is... 

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