Metabolism
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copyright © 2008 - 2015 David A Bender
Is fructose fattening?
One hypothesis to explain the increase in overweight and obesity over the last two decades is that as well as increased food consumption and reduced physical activity, increased consumption of fructose may be a factor. Fructose is 1.7-times sweeter than sucrose, and high fructose syrups, made by hydrolysis of starch followed by isomerisation of much of the glucose to fructose, provide a cheap and convenient alternative to sucrose in the manufacture of sweetened beverages and other foods. There is good experimental evidence in experimental animals, and some preliminary evidence in human beings, that consumption of a relatively large amount of fructose leads to increased fatty acid and triacylglycerol synthesis in the liver, and increased secretion of very low density lipoprotein. Consumption of an equivalent amount of glucose is not associated with the same increase in lipogenesis.
Which step provides the main regulation of glycolysis?
The main control over the rate of glycolysis is by regulation of the activity of phosphofructokinase.
What compounds would you expect to inhibit phosphofructokinase?
Phosphofructokinase is inhibited by three compounds:
ATP (although ATP is a substrate, there is a separate inhibitory ATP-binding site on the enzyme)
phosphoenolpyruvate
citrate
The graphs below show the effect of ATP on the substrate dependence of phosphofructokinase, and the inhibition of phosphofructokinase by ATP in the presence and absence of 5'AMP.
What is the metabolic significance of the inhibition of phosphofructokinase by physiological concentrations ATP?
ATP can be considered to be the most important end-product of glycolysis and the citric acid cycle. Inhibition of phosphofructokinase by ATP thus represents feed-back inhibition of the main controlling enzyme of the pathway. When there is enough ATP in the cell there is no need to produce any more (indeed, it would not be possible, since the rate of oxidation of metabolic fuels is controlled by the availability of ADP to be phosphorylated, see the exercise on Overheating after overdosing on E - and slimming by taking dinitrophenol)
What is the effect of 5'AMP on phosphofructokinase, and what is its significance?
At normal intracellular concentrations of ATP and fructose 6-phosphate the enzyme is almost 90% inhibited. However, in the presence of a relatively small amount of 5'AMP this inhibition is more or less completely reversed, so that the rate of glycolysis increases 10-fold, so permitting more phosphorylation of ADP to ATP.
5'AMP is formed by the reaction of adenylate kinase, shown above. There is normally very little ADP present in cells. However, in response to muscle contraction and other uses of ATP, ADP is formed, but not enough to produce a significant change in the ration of ATP : ADP, which is about 500 : 1. The small amount of 5'AMP that is formed by the adenylate kinase reaction acts as a potent signal of the energy state of the cell, increasing the rate of glycolysis (and other energy-yielding reactions) very considerably.
What is the metabolic significance of inhibition of phosphofructokinase by phosphoenolpyruvate?
Phosphoenolpyruvate is an intermediate in glycolysis, but is rapidly converted to pyruvate by pyruvate kinase. However, when gluconeogenesis is stimulated by the hormone glucagon (in response to a low blood concentration of glucose), pyruvate kinase is inactivated by phosphorylation of the enzyme protein.
More importantly, phosphoenolpyruvate is synthesised from oxaloacetate as a key reaction in gluconeogenesis. It is obviously essential that if ATP is being expended to synthesise glucose from non-carbohydrate precursors (e.g. in the fasting state), then the reactions of glycolysis must be inhibited - hence the inhibition of phosphofructokinase by phosphoenolpyruvate.
