Fructokinase

Hers has presented evidence which suggests a close relationship between K+, Mg++, and ATP for optimum fructokinase activity. In a reaction mixture containing a low cation concentration, the optimum Mg++ ATP ratio was 0.6. When K+ was added in increasing concentrations, a final concentration of 1 Af K+ yielded the maximum activity of a four to five-fold increase above that of the low cation system. With this high K+ concentration the ratio of Mg++ ATP increased to 1. Hers concluded that fructokinase contains at least two reaction sites, one requiring 

K+ and the other a Mg++-ATP complex. When Na+ replaces K+, the system decreases in activity. 

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Lack of hepatic fructokinase causes essential fructo-suria, and absence of hepatic aldolase B, which cleaves... 

Figure 5 Model of phosphorus (P) deficiency-induced physiological changes associated with the release of P-mobilizing root exudates in cluster roots of white lupin. Solid lines indicate stimulation and dotted lines inhibition of biochemical reaction sequences or mclaholic pathways in response to P deliciency. For a detailed description see Sec. 4.1. Abbreviations SS = sucrose synthase FK = fructokinase PGM = phosphoglueomutase PEP = phosphoenol pyruvate PE PC = PEP-carboxylase MDH = malate dehydrogenase ME = malic enzyme CS = citrate synthase PDC = pyruvate decarboxylase ALDH — alcohol dehydrogenase E-4-P = erythrosc-4-phosphate DAMP = dihydraxyaceConephos-phate APase = acid phosphatase.

The introduction of the invertase from yeast alone was not sufficient. Invertase cleaves sucrose to release the two component sugars, glucose and fructose. While fructose can be readily metabolized by fructokinase in potato tubers, there is insufficient hexokinase activity in developing potato tubers to bring the glucose into intermediary metabolism. Therefore, it was necessary to introduce a second transgene, a bacterial glucokinase, in order to ensure that the hexoses became available for subsequent metabolism.25... 

Genetic deficiency of fructokinase is benign and often detected incidentally when the urine is checked for glucose with a dipstick. Fructose 1-phosphate aldolase deficiency is a severe disease because of accumulation of fructose 1-phosphate in the liver and renal proximal tubules. Table 1-12-4 compares the two conditions. Symptoms are reversed after removing fructose and sucrose from the diet. 

Table 1-12-4. Comparison of Fructokinase and Fructose 1-Pho hate Aldolase Deficiencies...

Figure 11.2 Pathway for conversion of fructose to acetyl-CoA. The enzyme fructokinase phosphorylates fructose to form fructose 1-phosphate. (The enzyme is present only in the liver.) Fructose 1-phosphate is cleaved by aldolase to form glyceraldehyde and dihydroxyacetone phosphate. Glyceraldehyde is phos-phorylated to form glyceraldehyde 3-phosphate, catalysed by the enzyme triokinase. Dihydroxyacetone phosphate is converted to glyceraldehyde 3-phosphate, catalysed by the isomerase. Glyceraldehyde 3-phosphate is converted to pyruvate by the glycolytic reactions (Chapter 6).

Fructose is converted to fructose 6-phosphate by hexo-kinase, which phosphorylates both glucose and fructose. It then is converted to pyruvate via glycolysis, which is converted to acetyl-coenzyme-A in the mitochondria for oxidation by the Krebs cycle. The enzyme that is specific for fructose metabolism, fructokinase, has not been found in... 

Schirmer, T. and Evans, P.R. (1990) Structural basis of the allosteric behaviour of phospho-fructokinase. Nature 343, 140-145. 

ADP NADH Disappearance Pyruvate Kinase and Lactate Dehydrogenase Fructokinase, y-Glutamylcysteine Synthetase, Mannokinase, ATPase ... 

Fructose 6-P NADH Formation Glucosephosphate Isomerase, Glucose-6-P Dehydrogenase Fructokinase ... 

This enzyme [EC 2.7.1.105], also known as phosphofructokinase 2, catalyzes the reaction of ATP with D-fructose 6-phosphate to produce ADP and D-fructose 2,6-bisphos-phate. This enzyme is distinct from that of 6-phospho-fructokinase. It copurifies with fructose-2,6-bisphos-phatase. 

In the liver, the enzyme fructokinase catalyzes the reaction of fructose with ATP to form fmctose 1 -phosphate. 

Essential fructosuria is a benign, asymptomatic condition arising from deficiency of the enzyme fructokinase that causes a portion of fructose to be excreted in the urine. 

Ferulic acid Rt Call Tiss "° °, LP Fructokinase, phospho Rt ... 

For values during an intravenous fructose tolerance test in controls and individuals with deficiencies of fructokinase, fructaldolase, and fructose- 1,6-bisphosphatase, see Steinmann et al. [36]. 

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 ... 

The irreversible phosphorylation reaction catalyzed by phospho fructokinase-1 (PFK-1) is the most important control point and the rate-limiting step of glycolysis (Figure 8.16). PFK-1 is controlled by the available concentrations of the substrates ATP and fructose 6 phosphate, and by regulatory substances described below. 

For fructose to enter the pathways of intermediary metabolism, it must first be phosphorylated (Figure 12.2). This can be accomplished by either hexokinase or fructokinase (also called ketohexo-kinase). Hexokinase phosphorylates glucose in all cells of the body (see p. 96), and several additional hexoses can serve as substrates for this enzyme. However, it has a low affinity (that is, a high Km, see p. 59) for fructose. Therefore, unless the intracellular concentration of fructose becomes unusually high, the normal presence of saturating concentrations of glucose means that little fructose is converted... 

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