Conversion of Glucose to Fructose
The conversion of glucose to fructose involves the enzymatic process known as isomerization, where the glucose molecule is rearranged to form fructose. This reaction is catalyzed by the enzyme glucose isomerase, which facilitates the conversion of the aldehyde group in glucose to a ketone group in fructose. This process is important in the production of high-fructose corn syrup and various other industrial applications.
7 Key excerpts on "Conversion of Glucose to Fructose"
eBook - ePub- Raymond S. Ochs(Author)
- 2021(Publication Date)
- CRC Press(Publisher)
...Until the 1980s, most dietary sugar was sucrose, a disaccharide consisting of glucose and fructose. In the last few decades, most sucrose has been replaced by high-fructose corn syrup, a mixture of free glucose and fructose (Box 9.4). A small amount of fructose is metabolized by many cells because some GLUT isoforms transport it and because it is a poor substrate of some hexokinase isozymes. However, most fructose is metabolized through a different pathway that is unique to the liver. In the liver, fructose is transported across the plasma membrane via GLUT2. Intracellular fructose is converted to fructose-1-P, catalyzed by the enzyme fructokinase (Figure 9.31). Next, aldolase B, a liver isozyme, catalyzes the aldol cleavage of fructose-1-P to dihydroxyacetone-P and glyceraldehyde (analogous to the aldolase reaction of glycolysis, Figure 9.11). Glyceraldehyde is a substrate for the triose kinase reaction, catalyzed by a third unique liver enzyme, which converts glyceraldehyde to glyceraldehyde-3-P. From this point forward, the carbon of fructose is metabolized using the usual glycolytic steps. Note that the ATP balance – that is, the difference between ATP input and ATP production – matches that of glucose, despite using different steps (i.e., triose kinase in place of P-fructokinase). FIGURE 9.31 Metabolism of fructose in the liver. Three special liver enzymes allow fructose carbon to enter glycolysis. First, fructokinase catalyzes the conversion of fructose to fructose-1-P. Second, aldolase B catalyzes the splitting of fructose-1-P into dihydroxyacetone-P and glyceraldehyde. Third, glyceraldehyde is a substrate for triose kinase, which catalyzes the formation of glyceraldehyde-P. While fructose metabolism has the same balance of ATP as glucose, the major regulatory step of glycolysis, PFK, is bypassed...
eBook - ePub- Antonio Blanco, Gustavo Blanco(Authors)
- 2017(Publication Date)
- Academic Press(Publisher)
...They can follow this pathway and finally be oxidized to CO 2 and H 2 O to provide energy, or can be derived to gluconeogenesis and form glucose or glycogen (Fig. 14.12). Figure 14.12 Fructose metabolism. An alternative minor route for fructose is its phosphorylation at carbon 6, catalyzed by hexokinase. This enzyme has low affinity for fructose. F-6-P can then be incorporated into the glycolytic pathway. Fructose has synergistic effect with glucose metabolism. It increases affinity of glucokinase for glucose and stimulates its utilization in liver. Fructose entry into the glycolytic pathway bypasses the limiting reactions of glycolysis (formation of G-6-P and F-1,6-bisP). Besides, F-1-P is an allosteric pyruvate kinase activator. This explains why lactate formation is faster from fructose than from glucose. An excessive intake of fructose may cause lactic acidosis. On the other hand, an overload of dietary fructose saturates the glycolytic pathway, which derives glyceraldehyde toward triacylglycerol formation and can consume ATP in hepatocytes, reducing other biosynthetic processes. Also, high fructose intake causes increase of uric acid in blood and urine. There is fructose in human semen, which is used as an energy source for spermatozoa. Fructose is produced from glucose in the seminal vesicles. The process is accomplished in two steps: first, catalyzed by aldol reductase (NADPH dependent), which reduces carbon 1 of glucose and forms sorbitol. In the second, NAD-linked sorbitol dehydrogenase oxidizes the hydroxyl group on sorbitol carbon 2 and produces fructose. Both enzymes of this pathway are also found in the liver. Several genetic diseases related to fructose metabolism have been described. One of them, called fructose intolerance, is due to deficiency of aldolase B. This disorder, which is inherited in an autosomal recessive manner, causes accumulation of F-1-P and fall of intracellular ATP and P i concentrations...
eBook - ePub- Akhlaq A. Farooqui, Tahira Farooqui, Akhlaq A. Farooqui, Tahira Farooqui(Authors)
- 2013(Publication Date)
- Wiley-Blackwell(Publisher)
...In addition, enterocytes in the proximal small bowel express the enzymes required for de novo fatty acid synthesis, and may convert part of fructose into triglycerides, package newly formed triglycerides into chylomicrons, and thus contribute to dyslipidemia. Metabolism of fructose in the gut has been demonstrated in rodents but has not been directly assessed in humans. After sucrose ingestion, glucose and the major portion of fructose are released into the portal vein and are therefore delivered to the liver before gaining access to the systemic circulation. Glucose is taken up by hepatocytes through the hexose carrier GLUT2, and is then converted into glucose-6-phosphate by glucokinase an isoform of hexokinase, the synthesis of which is controlled by insulin in the liver. Glucose-6-phosphate is further metabolized in the glycolytic pathway, in which it is converted into fructose-6-phosphate and trioses-phosphate. Insulin regulates several of the major glycolytic enzymes' activity. One key glycolytic reaction is the conversion of fructose-6-phophate into fructose-1,6 diphosphate. This reaction is controlled by the enzyme phosphofructokinase. The activity of this enzyme is inhibited by ATP and citrate, and hence the overall glycolytic activity is regulated by the energy status of the hepatocyte. As a consequence, only a portion of the portal glucose is metabolized in the liver, and glycolysis is only moderately activated after a glucose meal. Fructose, as glucose, is transported into the hepatocyte by GLUT2. Once in the hepatocyte, however, its metabolism differs markedly from that of glucose. Fructose is rapidly converted into fructose-1-phosphate by a specific enzyme, fructokinase, and fructose-1-phosphate is then directly converted into triose phosphates (DHAP and glyceraldehyde) by a second enzyme, aldolase B. The Km of fructokinase for fructose is low, and the activity of both fructokinase and aldolase B is high...
eBook - ePubMedical Biochemistry
Human Metabolism in Health and Disease
- Miriam D. Rosenthal, Robert H. Glew(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
...4-3). This reversible reaction is catalyzed by glucose 6-phosphate isomerase: The next step in the pathway, which is catalyzed by phosphofructokinase-1 (PFK-1), is irreversible and commits fructose 6-phosphate to glycolysis. PFK-1 also represents the major regulated step in the glycolytic pathway: Next, aldolase A cleaves fructose-1,6-bisphosphate into two three-carbon fragments (called triose phosphates), glyceraldehyde 3-phosphate and dihydroxyacetone phosphate (DHAP): Since aldolase-type reactions are reversible, aldolase A can also participate in the pathway that is essentially the reverse of glycolysis: namely, gluconeogenesis. The liver contains a second aldolase, designated aldolase B, which participates in the metabolism of fructose. Aldolase A and aldolase B are not isozymes. Instead of utilizing two separate pathways for converting each of the trioses from the aldolase A reaction into pyruvate, nature has evolved a more economical strategy that involves the conversion of one of the trioses, dihydroxyacetone phosphate, into the other, glyceraldehyde 3-phosphate. The enzyme that accomplishes this freely reversible interconversion of the two triose phosphates is triosephosphate isomerase: The next step is the only oxidation–reduction reaction of glycolysis: namely, the NAD + -dependent oxidation of glyceraldehyde 3-phosphate to 1,3-bisphos-phoglycerate. This reversible reaction is catalyzed by glyceraldehyde 3-phosphate dehydrogenase: The glyceraldehyde 3-phosphate dehydrogenase reaction couples an oxidation–reduction reaction with a reaction that incorporates inorganic phosphate (Pi) into an organic compound. The overall reaction generates a high-energy phosphate ester linkage whose bond energy is greater than that of the terminal (γ) phosphate of ATP...
eBook - ePub- Jose Perez-Castineira(Author)
- 2020(Publication Date)
- De Gruyter(Publisher)
...It is a major product of plant photosynthesis, playing important biological roles in growth, development, and storage among others [ 13 ]. It is also the most common, almost universal, sweetener, being industrially obtained from sugar cane in tropical and sub-tropical countries, and from sugar-beet in temperate zones. Although competition is increasing from alternative and synthetic sweeteners, worldwide production of sugar in 2017 was around 180 million metric tones and it is expected to rise steadily up to more than 200 million metric tones in the next decade [ 14 ]. The glycosidic bond of sucrose is hydrolyzed by the enzyme sucrase-isomaltase (EC 3.2.1.48), expressed in the human intestinal brush border, and also by extracellular invertases (or β-D-fructofuranosidases, EC 3.2.1.26) present in the microbial flora of the oral cavity [ 15 ]. Mutations in the gene coding for the human sucrase-isomaltase are the cause of congenital sucrase-isomaltase deficiency, a rare disease that has been linked to irritable bowel syndrome [ 16 ]. Maltose, cellobiose, and lactose have two anomeric carbons (one per participating monosaccharide), one of them being involved in the glycosidic bond. In aqueous solutions, hydrolysis of the other hemiacetal group allows the presence of cyclic structures with different configurations as well as an acyclic structure bearing an aldehyde group. As a consequence, these disaccharides exhibit mutarotation and participate in oxidation and reduction reactions through the carbonyl group, being considered reducing sugars. Sucrose, on the contrary, does not show these properties because both anomeric carbons become “fixed” due to their participation in the glycosidic bond...
eBook - ePubNutritional Biochemistry
From the Classroom to the Research Bench
- Sami Dridi(Author)
- 2022(Publication Date)
- Bentham Science Publishers(Publisher)
...The transaldolase reversibly converts glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate to erythrose 4-phosphate and fructose 6-phosphate. The non-oxidative branch not only replenishes metabolites of the oxidative branch by the reversible reactions but also regulates the flux of glycolysis or gluconeogenesis by providing fructose-6-phosphate and glyceraldehyde 3-phosphate. Conclusion Understanding the biochemistry of carbohydrates and the regulation of energetic molecules like glucose is a critical step to comprehending the pathophysiology of metabolic disorders. The regulation and retour of glucose is a complex process that involves many interconnected biochemical pathways. The objective of this chapter is to provide and summarize the current knowledge about carbohydrate metabolism and to facilitate the understanding of the underlying biochemical pathways. Notes 1 Ketosis is a metabolic state characterized by high levels of ketone bodies in the blood or urine. 2 Anabolic pathways are metabolic pathways that construct (synthesis) molecules from smaller units. 3 Catabolism is the breakdown of complex molecules to form simpler ones. References [1] Bhattacharya M. A historical exploration of Indian diets and a possible link to insulin resistance syndrome. Appetite 2015; 95: 421-54.[ http://dx.doi.org/10.1016/j.appet.2015.07.002 ] [PMID: 26206172 ] [2] Jörgens V, Grüsser M. Happy Birthday, Claude Bernard. Diabetes 2013; 62(7): 2181-2.[ http://dx.doi.org/10.2337/db13-0700 ] [PMID: 23801718 ] [3] Tanner ME. Understanding nature’s strategies for enzyme-catalyzed racemization and epimerization. Acc Chem Res 2002; 35(4): 237-46.[ http://dx.doi.org/10.1021/ar000056y ] [PMID: 11955052 ] [4] Goodman BE. Insights into digestion and absorption of major nutrients in humans. Adv Physiol Educ 2010; 34(2): 44-53.[ http://dx.doi.org/10.1152/advan.00094.2009 ] [PMID: 20522896 ] [5] Wright EM. The intestinal Na+/glucose cotransporter...
eBook - ePubEnzymes in Food Biotechnology
Production, Applications, and Future Prospects
- Mohammed Kuddus(Author)
- 2018(Publication Date)
- Academic Press(Publisher)
...Other applications of inulin-containing wastes are related to producing biofuels and biobased chemicals (Hughes et al., 2017). 26.5 Invertases Invertases are another group of enzymes that are useful in the food industry (EC 3.2.1.26), responsible for sucrose inversion yielding d -glucose and d -fructose (Lincoln and More, 2017), as shown in Fig. 26.3. Fig. 26.3 Sucrose hydrolysis by invertase activity, yielding xmolar amounts of glucose and fructose molecules. 26.5.1 Nomenclature and Classification of Invertases Sucrose (α- d -glucopyranosyl β- d -fructofuranoside) is one of the most abundant products in nature. Not only is it the main compound derived from photosynthesis and the predominant molecule of carbon translocation in most plants, but it also plays a central role in the plant's biological functions and responses to environmental stress (Vargas and Salerno, 2010). In plants, glucose and fructose are involved in signaling pathways in which sucrose concentration functions as a key sensor of the nutritional status of plants. Therefore, invertase plays a key role in the control of cell differentiation and development. Although animals, including man, show a marked preference for diets containing sucrose, their genomes do not code for invertase. Instead, they use a different and unrelated enzyme to hydrolyze sucrose, sucrose-glucosidase (EC 3.2.1.48). The genomes of human intestinal microorganisms such as Bacteriodes thetaiotamicron (Xu, 2003) and Bifidobacterium longum (Schell et al., 2002) have invertase genes, demonstrating that these organisms benefit from the consumption of sucrose by humans. The use of sucrose as a source of carbon and energy depends on the rupture of the α-1-β-2-glucosidic bond by the action of invertases that irreversibly hydrolyze the disaccharide in glucose and fructose...
