Fructose
n., plural: fructoses
[ˈfɹuk.toʊs]
Definition: a ketohexose monosaccharide with a chemical formula of C6H12O6. It is found in fruits and soft drinks.
Table of Contents
Fructose Definition
Fructose is a ketohexose monosaccharide with a chemical formula of C6H12O6. It is the sweetest of all the natural carbohydrates. When combined with glucose, it forms the disaccharide sucrose.
Fructose is one of the three most common monosaccharides. The other two are glucose and galactose. Monosaccharides are the most fundamental type of carbohydrates. They are called simple sugars as opposed to the more complex forms such as oligosaccharides and polysaccharides. Monosaccharides can combine, though, to form complex carbohydrates via glycosidic bonds (glycosidic linkages).
IUPAC name: (3S,4R,5R)-1,3,4,5,6-Pentahydroxyhexan-2-one. Chemical formula: C6H12O6. Synonyms: fruit sugar, l(a)evulose, D-fructofuranose, D-fructose, D-arabino-hexulose.
Terminology
In 1847, French chemist Augustin-Pierre Dubrunfaut 1797 –1881 discovered fructose. The name fructose though was coined by the English chemist William Allen Miller 1817 –1870 in 1857. Miller is also credited as the person who coined the name sucrose in the same year. Etymologically, fructose comes from the Latin fructus (meaning fruit) and –ose (denoting “sugar”).
Properties of Fructose
Fructose is a hexose monosaccharide. It is an organic compound. Its general chemical formula is C6H12O6. The molar mass of fructose is 180.16 g/mol. The melting point is 103°C. It is crystalline, water-soluble, and sweet tasting.
Fructose vs. Glucose vs. Galactose
Fructose, glucose, and galactose are the three most common natural monosaccharides. However, among them, glucose is the most abundant. What is common in them is their chemical formula: C6H12O6. Hence, they are a hexose-type of monosaccharide, owing to the six carbon atoms.
Similar to glucose and mannose, fructose is a reducing sugar. However, fructose is a ketose whereas glucose and galactose are aldoses. Fructose has a reducing group (carbonyl) at carbon 2. This is in contrast to an aldose that has its carbonyl group at carbon 1.
Fructose is the most water-soluble and has the lowest melting point (i.e. 103 °C) among the three. It is also the sweetest not just among the natural monosaccharides but of all the natural carbohydrates. The relative sweetness though decreases as it is heated with increasing temperature.
Similar to glucose, fructose occurs freely in contrary to galactose, which generally does not occur in a free state and is often a constituent of biological compounds. However, free glucose is more common than unbound fructose. Glucose is also more often used metabolically, particularly in energy metabolism. Nonetheless, the three monosaccharides can be directly absorbed during digestion and utilized by the body in different metabolic activities.
These three monosaccharides may enter the glycolytic pathway. However, glucose proceeds directly to glycolysis as opposed to fructose and galactose, which proceed to the glycolytic pathway indirectly. For instance, fructose enters the glycolytic pathway by first going through fructolysis. Galactose, in turn, is converted into glucose primarily through the Leloir pathway.
Common Biological Processes Involving Fructose
Here are some of the most common biological processes where fructose is involved:
- Dehydration synthesis
- Saccharification
- Fructose assimilation
- Fructose catabolism (fructolysis)
- Fructose conversion into glucose
- Polyol pathway
- Glycation
Dehydration synthesis
Through dehydration synthesis, a monosaccharide, such as fructose, binds to another monosaccharide with the release of water and the subsequent formation of a glycosidic bond. The joining of two monosaccharides produces a disaccharide whereas the joining of three to ten monosaccharide units forms an oligosaccharide.
Polysaccharides are produced by the joining of multiple monosaccharides. In this regard, fructose joins with another monosaccharide to form a disaccharide. For instance, sucrose is formed when fructose and glucose molecules are joined together. The two monosaccharides are linked through a glycosidic linkage between C-1 (on the glycosyl subunit) and C-2 (on the fructosyl unit).
Sucrose occurs in many plants. It is commonly extracted from sugar cane and sugar beet, and processed (refined) to be marketed as common table sugar. It is used as a sweetening agent in food and beverages. Synthetic disaccharide consisting of galactose and fructose has been made available not as a sweetener but for medical and health purposes. It is called lactulose. It is not absorbed by the body but can be metabolized by the gut flora. It is prescribed for use as a laxative, a prebiotic, and a treatment for hyperammonemia.
Fructan, a polymer of fructose, may occur as an oligosaccharide or as a polysaccharide, depending on the length of the fructose chain. Fructan with a shorter chain is called a fructooligosaccharide. They are present in asparagus, leeks, garlic, onions, wheat, artichoke, and grass.
Saccharification
The process wherein complex carbohydrates are degraded into simpler forms is called saccharification. It entails hydrolysis. In humans and other higher animals, this involves enzymes. In a diet containing fructose (e.g. sucrose, fructolipids, etc.), they are broken down into monomeric units through the action of digestive enzymes. One of them is invertase (also called sucrase) released from the small intestine. The enzyme cleaves sucrose by breaking the β-glycosidic bond, thereby, releasing glucose and fructose.
Too much fructose, though, could lead to malabsorption in the small intestine. When this happens, unabsorbed fructose transported to the large intestine could be used in fermentation by the colonic flora. This could lead to gastrointestinal pain, diarrhea, flatulence, or bloating due to the products (e.g. hydrogen gas, carbon dioxide, short-chain fatty acids, organic acids, and trace gases) of fructose metabolism by bacteria.
Fructose assimilation
Fructose that is made available from the digestion of dietary sources is taken up by the intestinal cells (enterocytes) through the proteins called glucose transporters (GluT). GluT5 transporter takes up fructose more effectively than glucose.1 There is no consensus as of this time as to how fructose is absorbed by the enterocytes. Some scientists theorize that it involves passive transport (via facilitated diffusion). Others presume it is by active transport just as it is in the absorption of free glucose molecules by enterocytes.
Fructose leaves the enterocytes and then enters the bloodstream. Unlike blood glucose, fructose in the bloodstream is not regulated by the pancreatic enzymes, insulin, and glucagon. Fructose is then transported into the cells of other tissues by facilitated diffusion using the GluT-mediated transport system (such as by GluT2 and GluT5).
Fructose catabolism
Fructose, together with the other dietary monosaccharides, is transported by the blood into the liver. Fructose reaches the liver via the hepatic portal vein and is taken up by the liver cells. Apart from the liver where fructose is predominantly metabolized, other tissues that metabolize fructose include the testis, kidney, skeletal muscle, fat tissues, brain, and intestine. Fructose is taken in by these cells chiefly by GluT2 and GluT5 transporters.
The catabolism of fructose is called fructolysis (as glucose catabolism is to glycolysis). Fructose is trapped inside the cell, e.g. inside the hepatocyte, when it is phosphorylated into fructose 1-phosphate by the enzyme fructokinase. Fructose 1-phosphate is split by aldolase B into two trioses: (1) dihydroxyacetone phosphate (DHAP) and (2) glyceraldehyde.
The common metabolic fate of DHAP is as follows:
- DHAP is isomerized to glyceraldehyde 3-phosphate (Ga-3-P) by triose phosphate isomerase.
- DHAP is reduced to glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase.
The common metabolic fate of glyceraldehyde is as follows:
- Glyceraldehyde is phosphorylated into Ga-3-P‘by glyceraldehyde kinase.
- Glyceraldehyde is converted into glycerol 3-phosphate by glycerol 3-phosphate dehydrogenase.
Thus, DHAP and Ga-3-P from fructolysis in the hepatocyte may enter:
- Gluconeogenesis, several metabolic pathways lead to gluconeogenesis for glucose formation. One of them is by trioses Ga-3-P (or DHAP) combining to form the hexose, fructose-1,6-bisphosphate. The latter is converted into fructose 6-phosphate by utilizing one water molecule and releasing one phosphate through the enzyme fructose 1,6-bisphosphatase.
Another pathway is the phosphorylation of fructose into fructose-6-phosphate, which, in turn, is converted into glucose-6-phosphate. Glucose-6-phosphate is then hydrolyzed by the enzyme glucose-6-phosphatase to produce glucose and inorganic phosphate. This is a more direct way than the first. - Glycogenesis, where DHAP and Ga-3-P are converted for use in glycogen synthesis
- Glycolysis, where Ga-3-P (or DHAP isomerized to Ga-3-P) enters the second phase of glycolysis to be converted ultimately into pyruvate. Pyruvate may enter the Krebs cycle in the presence of oxygen.
Another pathway is fructose entering a part of glycolysis in a rather direct way. For instance, fructose is phosphorylated into fructose-6-phosphate. Or, fructose-1-phosphate is phosphorylated by phosphofructokinase-1 to fructose-1,6-bisphosphate.
- Free fatty acid synthesis, whereby the accumulating citrate from the Krebs cycle may be removed from the cycle to be transported to the cytosol where it will be converted into acetyl-CoA, to oxaloacetate, and then to malonyl CoA for fatty acid synthesis
- Triglyceride synthesis, where glycerol 3-phosphate from DHAP and Ga-3-P may serve as glycerol backbone for triglyceride. Triglycerides in the liver are incorporated into the very-low-density lipoproteins (VLDL) that are released to peripheral fat and muscle cells for storage.
Fructose conversion into glucose
A huge percentage of dietary fructose is converted in the liver to glucose. One way by which fructose becomes glucose is when fructose is converted into Ga-3-P and DHAP that enters gluconeogenesis (the reverse of glycolysis).
Polyol pathway
Polyol pathway, a two-step process, converts glucose into fructose. The first step is the reduction of glucose to produce sorbitol through the enzyme aldose reductase. The last step is the oxidation of sorbitol to produce fructose through the enzyme sorbitol dehydrogenase.
In bacteria, glucose converted into fructose is catalyzed by glucose isomerase, which is a bacterial enzyme. The discovery of this enzyme led to its use in the industry, particularly in the manufacture of high fructose corn syrup.
Glycation
Glycation is the process of covalently joining a carbohydrate constituent, such as fructose or glucose, to a protein or a lipid molecule. It is non-enzymatic glycosylation.
Metabolic Disorders
Improper metabolism of fructose may result in metabolic disorders. For instance, fructose intolerance is a hereditary disease caused by a defect in the aldolase B gene that codes for the enzyme aldolase B. In the metabolism of fructose, aldolase B cleaves fructose 1-phosphate into glyceraldehyde and DHAP.
Thus, inadequate or absence of aldolase B could lead to the improper catabolism of fructose, and hinder the various metabolic pathways that DHAP and glyceraldehyde take part in. The condition could impair the liver and cause severe damage to it. Another condition is fructosuria (high fructose level in urine), which is caused by an excess of fructose.
This is usually due to a defect in the gene encoding for the enzyme fructokinase. The enzyme is supposed to phosphorylate fructose into fructose 1-phosphate.
Biological Importance and Functions of Fructose
Fructose is one of the most common monosaccharides and plays various biological roles. Fructan, a polymer of fructose, is essential to plants (e.g. grasses, asparagus, leeks, garlic, onion, wheat, except for rice that does not synthesize it). In these plants, it serves as a storage polysaccharide.
Fructose exists in food either as a monosaccharide (free fructose) or as a unit of a disaccharide (sucrose). Sucrose (the common table sugar) is a non-reducing disaccharide that forms when glucose and fructose are linked together by an alpha linkage between carbon 1 of glucose and carbon 2 of fructose.
Sucrose is present in different fruits, vegetables, honey, and other plant-derived food products. When consumed, sucrose comes into contact with the membrane of the small intestine. The enzyme sucrase catalyzes the cleavage of sucrose to yield one glucose unit and one fructose unit, which are then each absorbed by the intestine.
One of the major biological functions of fructose is it acts as an alternative metabolite in providing energy especially when glucose is not sufficient while the metabolic energy demand is high. It can enter glycolysis and produce intermediates for cellular respiration. Fructose also enters other important metabolic pathways, such as glycogen synthesis, triglyceride synthesis, free fatty acid synthesis, and gluconeogenesis. It can also be used during glycation wherein a lipid or a protein is combined with a sugar, such as fructose.
Try to answer the quiz below to check what you have learned so far about fructose.
References
- Catabolism of sugars other than glucose. (2019). Retrieved from http://watcut.uwaterloo.ca/webnotes/Metabolism/OtherSugars.html
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