Muscle fructokinase

Crude muscle hexokinase phosphorylates both glucose and fructose. The two activities may be separated with ammonium sulfate. Crude extracts have a fructose glucose ratio of 0.26 on fractionation with ammonium sulfate between 0.35 and 0.54 saturation, a fructose glucose ratio of 0.148 is observed. Muscle fructokinase is not inhibited by glucose and is saturated by its substrate at unusually high concentrations. The reaction product is not the expected fructose-6-phosphate but rather fructose-1-phosphate. Muscle glucokinase has not as yet been examined to any extent for its specificity. 

Rat liver extracts also contain two highly specific kinases, namely, glucokinase and fructokinase. Glucokinase, in the presence of ATP and Mg++, forms glucose-6-phosphate fructokinase catalyzes the formation of fructose-l-phosphate. Beef Uver fructokinase, like rat fiver fructokinase, is strongly activated by potassium chloride. Sodium and ammonium ions are relatively inert. The affinity of the enzyme for fructose is very high, the Km being lower than 5 X 10 moles per liter. This is in contrast to muscle fructokinase, which has a very weak aflfinity for its subtrate. 

This is a major pathway of fructose entry into glycolysis in the muscles and kidney. In the liver, however, fructose enters by a different pathway. The liver enzyme fructokinase catalyzes the phosphorylation of fructose at C-l rather than C-6 ... 

Hand, S.C., and G.N. Somero (1982). Urea and methylamine effects on rabbit muscle phospho-fructokinase. J. Biol. Chem. 257 734-741. 

A number of other enzymopathic substances (e.g., pyruvate kinase. Chapter 13 and pyrimidine-5 -nucleotidase. Chapter 27), abnormal hemoglobins (Chapter 28), and abnormalities of the erythrocyte cytoskeleton (Chapter 10) may cause hemolytic anemia. Because many enzymes in the red cell are identical to those in other tissues, defects in these enzymes may have pleiotropic effects. Thus, in addition to hemolytic anemia, triose phosphate isomerase deficiency causes severe neuromuscular disease, and phospho-fructokinase deficiency causes a muscle glycogen storage disease (Chapter 13). Mutations that result in decreased enzyme stability are usually most strongly expressed in erythrocytes because of their inability to synthesize proteins. 

Randle has reviewed his concept of a glucose-fatty acid cycle, with some new experimental material. 5 There has been a recent review of gluconeogenesis, with good current references.The control of phospho-fructokinase, one of the important rate limiting enzymes of glycolysis, is still not fully understood. The activity of the enzyme in mammalian muscle is influenced by substrate concentration and by wiiich can acti-... 

In the liver, kidney, and intestine, fructose can be converted to glycolytic/ gluconeogenic intermediates by the actions of three enzymes—fructokinase, aldolase B, and triokinase (also called triose kinase)—as shovra in Figure 24-1. In these tissues, fructose is rapidly phosphorylated to fructose 1-phosphate (FIP) by fructokinase at the expense of a molecule of adenosine triphosphate (ATP). This has the effect of trapping fructose inside the cell. A deficiency in this enzyme leads to the rare but benign condition known as essential fmcto-suria. In other tissues such as muscle, adipose, and red blood cells, hexokinase can phosphorylate fructose to the glycolytic intermediate fmctose 6-phosphate (F6P). 

Fructokinase is found in liver,56,312-315 kidney,32 intestinal mucosa,33,316 adipose tissue,71,72 and lenses,317 but not in heart muscle, skeletal muscle, brain, and seminal vesicles.57... 

The metabolism of fructose occurs principally in the liver and to a lesser extent in the small intestinal mucosa and proximal epithelium of the renal tnbnle, becanse these tissues have both fructokinase and aldolase B. Aldolase exists as several isoforms aldolases A, B, C, and fetal aldolase. Although all of these aldolase isoforms can cleave fructose 1,6-bisphosphate, the intermediate of glycolysis, only aldolase B can also cleave fructose 1-phosphate. Aldolase A, present in muscle and most other tissues, and aldolase C, present in brain, have almost no ability to cleave fructose 1-phosphate. Fetal aldolase, present in the liver before birth, is similar to aldolase C. 

Van Schaftingen, E. Hers, H.-G. Purification and properties of phospho-fructokinase 2/fructose 2,6-bisphosphatase from chicken liver and from pigeon muscle. Eur. J. Biochem., 159, 359-365 (1986)... 

Pyruvate kinase is the enzyme that catalyzes this reaction. Like phospho-fructokinase, it is an allosteric enzyme consisting of four suhunits of two different types (M and L), as we saw with phosphofructokinase. Pyruvate kinase is inhibited hy ATP. The conversion of phosphoenolpyruvate to pyruvate slows down when the cell has a high concentration of ATP—that is to say, when the cell does not have a great need for energy in the form of ATP. Because of the different isozymes of pyruvate kinase found in liver versus muscle, the control of glycolysis is handled differently in these two tissues, which we will look at in detail in Chapter 18. 

Glucose is not the only hexose used for glycolysis— fructose, mannose, and galactose can also enter the glycolytic cycle after phosphorylation. Like glucose, fructose can be used only after phosphorylation in one of three ways [33] (1) phosphorylation to fructose-6-phosphate by hexokinase, (2) phosphorylation to fructose-6-phosphate by a specific fructokinase, and (3) phosphorylation to fructose-1-phosphate by fructokinase (Fig. 1-7). It is well established that the glu-cokinase of liver and muscle can also phosphorylate fructose. Fructose can enter muscle metabolism only in the form of fructose-6-phosphate. This is strikingly different from liver metabolism in which fructose is converted to fructose-1-phosphate by a specific fructokinase. 

A kinase catalyzing the formation of fructose-1-phosphate in muscle has been described. It appears that the mechanism of fructose utilization in muscle is not entirely resolved. Neither the affinity of the hexokinase for fructose nor the marginal activity of fructokinase explains the efficient utilization of fructose by muscle. 

The fate of fructose-1-phosphate is varied—it may be phosphorylated again in the presence of 1-phospho-fructokinase, magnesium, and ATP to yield fructose-1,6-diphosphate and ADP. 1-Phosphofructokinase is found in liver and muscle. Fructose-1,6-diphosphate may then be used by the glycolytic cycle. 

However, it has been found that the hexokinases of different organisms and of different tissues within an organism may differ with respect to the specificity of the enzyme and the reaction catalyzed. For instance, the phosphorylation of glucose in muscle is effected by a specific glucokinase which forms glucose-6-phosphate. This enzyme does not phosphorylate fructose, which has its own transphosphorylase, fructokinase. 

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