Phosphofructokinase, function

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.

Clearly, the activity of phosphofructokinase depends both on ATP and AMP levels and is a function of the cellular energy status. Phosphofructokinase activity is increased when the energy status falls and is decreased when the energy status is high. The rate of glycolysis activity thus decreases when ATP is plentiful and increases when more ATP is needed. 

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. 

R2. Raben, N., Exelbert, R., Spiegel, R., Sherman, J. B Nakajima, H., Plotz, P., and Heinisch, J., Functional expression of human mutant phosphofructokinase in yeast Genetic defects in French Canadian and Swiss patients with phosphofructokinase deficiency. Am. J. Hum. Genet. 56, 131-141 (1995). 

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) ... 

Fructose 2,6-bisphosphate (Fru-2,6-bP) plays an important part in carbohydrate metabolism. This metabolite is formed in small quantities from fructose 6-phosphate and has purely regulatory functions. It stimulates glycolysis by allosteric activation of phosphofructokinase and inhibits gluconeogenesis by inhibition of fructose 1,6-bisphosphatase. 

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) ... 

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. 

Connectivity theorems allow to relate the control coefficients (systemic properties) to the elasticity coefficients (properties of the network s enzymes individually as if in isolation) (Westerhoff and Van Dam 1987 Heinrich and Schuster 1996 Fell 1997). The connectivity theorems have given us a strong insight into the functioning of metabolic pathways. For example, it follows directly from these theorems that enzymes that are very sensitive to the concentrations of metabolites, such as substrates, products and allosteric effectors, tend to have little control over the flux. This is illustrated by overproduction of phosphofructokinase in bakers yeast, an enzyme often referred to textbooks as rate-limiting. Yet, overproduction of phosphofructokinase does not lead to a significant flux increase, since the cell compensates by lowering the level of its allosteric effector fructose 2,6-bisphosphate (Schaaff et al. 1989 Davies and Brindle 1992). 

L Phosphofructokinase 1 (PFK1) is regulated by several factors. It functions at a rapid rate in the liver when blood glucose is high, or in cells such as muscle when there is a need for ATP. 

A divalent metal ion (Mg ) is needed to coordinate the phosphate groups on the ATP molecule in order for phosphofructokinase to successfully catalyze this reaction. Mg, Mn, Ca, and often function as cofactors for metal-activated enzymes. 

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