Phosphofructokinase , glycolytic

For many years hemoglobin was the only allosteric protein whose stereochemical mechanism was understood in detail. However, more recently detailed structural information has been obtained for both the R and the T states of several enzymes as well as one genetic repressor system, the trp-repressor, described in Chapter 8. We will here examine the structural differences between the R and the T states of a key enzyme in the glycolytic pathway, 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 ... 

Genetically-determined deficiency of G6PD is the most common cause of haemolysis arising from enzyme defects. Mutated glycolytic enzymes such as hexokinase, phosphofructokinase, aldolase and pyruvate kinase can also bring about haemolysis but the occurrence of these defects are much rarer than for G6PD deficiency (see Case N otes at the end of this chapter). 

To give rise to oscillatory behavior instead of a biochemical explosion, selfamplification must, however, be coupled to a limiting process. Such a limiting process can be viewed as a form of negative feedback because it occurs as a consequence of the positive feedback that precedes it. Thus, in the case of glycolytic oscillations, the activation of phosphofructokinase by a reaction product is followed by a counteracting fall in the rate of the enzymatic reaction, due to the enhanced substrate consumption associated with enzyme activation. In Ca + pulsatile signaling, the explosive rise in cytosolic Ca + due... 

The pathway for gluconeogenesis is shown in Figures 6.23 and 6.24. Some of the reactions are catalysed by the glycolytic enzymes i.e. they are the near-equilibrium. The non-equilibrium reactions of glycolysis are those catalysed by hexokinase (or glucokinase, in the liver), phosphofructokinase and pyruvate kinase and, in order to reverse these steps, separate and distinct non-equilibrium reactions are required in the gluconeogenic pathway. These reactions are ... 

In a series of experiments we have tested the type and range of entrainment of glycolytic oscillations by a periodic source of substrate realizing domains of entrainment by the fundamental frequency, one-half harmonic and one-third harmonic of a sinusoidal source of substrate. Furthermore, random variation of the substrate input was found to yield sustained oscillations of stable period. The demonstration of the subharmonic entrainment adds to the proof of the nonlinear nature of the glycolytic oscillator, since this behavior is not observed in linear systems. A comparison between the experimental results and computer simulations furthermore showed that the oscillatory dynamics of the glycolytic system can be described by the phosphofructokinase model. 

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).]... 

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. 

The final energy payoff in the glycolytic pathway occurs in the hydrolysis of phosphoenolpyruvate to pyruvate and the concomitant phosphorylation of ADP to ATP. Two molecules of ATP are produced for each molecule of hexose phosphate consumed, bringing the net yield of ATP to two molecules for each molecule of glucose (two molecules of ATP are regenerated in the phosphoglycerate kinase step and two in this step, and two are consumed in the hexoki-nase and phosphofructokinase steps). 

Alternatively, it is possible to write a sequence of reactions, including the action of phosphofructokinase and aldolase on seven-carbon intermediates, in which the carbon of ribulose-5-phosphate is converted mainly to glyceralde-hyde-3-phosphate. Such a pathway, with the triose phosphate entering the glycolytic sequence, amounts to a bypass, or shunt, around the first reactions of glycolysis, and the name hexose monophosphate shunt is sometimes used. Any amount of ribose-5-phosphate or erythrose-4-phosphate that may be needed for biosynthetic sequences can also be ob-... 

There are virtually no data whether neurons use ambient glucose, and/or glial-derived lactate under neoropathological conditions. It has been proposed that glycolytic enzymes such as phosphofructokinase (PFK), and glyceraldehyde-3-... 

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. 

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