UNIT II: Bioenergetics and Carbohydrate Metabolism Chapter 12: Metabolism of Monosaccharides and Disaccharides

I. Overview Glucose is the most common monosaccharide consumed by humans However, two other monosaccharides, fructose and galactose, occur in significant amounts in the diet, and make important contributions to energy metabolism In addition, galactose is an important component of cell structural carbohydrates Figure 12.1 shows the metabolism of fructose and galactose as part of the essential pathways of energy metabolism

Figure 12.1: Galactose and fructose metabolism as part of the essential pathways of energy metabolism (see Figure 8.2, p. 92, for a more detailed view of the overall reactions of metabolism). UDP = uridine diphosphate; P = phosphate. Galactose and fructose metabolism as part of the essential pathways of energy metabolism (see Figure 8.2, p. 92, for a more detailed view of the overall reactions of metabolism). UDP = uridine diphosphate; P = phosphate.

II. Fructose metabolism Approximately 10% of the calories contained in Western diet are supplied by fructose (~ 55 g/day). The major source of fructose is the disaccharide sucrose, which, when cleaved in intestine, releases equimolar amounts of fructose and glucose Fructose is also found as a free monosaccharaide in high- fructose corn syrup (55% fructose/45% glucose, which is used to sweeten most cola drinks), in many fruits, and in honey. Entry of fructose into cells is not insulin-dependent (unlike that of glucose into certain tissues), and in contrast to glucose, fructose does not promote secretion of insulin

II. Fructose metabolism A. Phosphorylation of fructose For fructose to enter pathways of intermediary metabolism, it must first be phosphorylated. This can be accomplished by either hexokinase or fructokinase Hexokinase phosphorylates glucose in all cells of the body, and several additional hexoses can serve as substrates for this enzyme. However, it has a low affinity (high Km) for fructose. Therefore, unless intracellular concentration of fructose becomes unusually high, the normal presence of saturating concentrations of glucose means that little fructose is phosphorylated by hexokinase Fructokinase provides the primary mechanism for fructose phosphorylation (Figure 12.2) It is found in the liver (which processes most of dietary fructose), kidney, and small intestinal mucosa, and converts fructose to fructose 1-phosphate, using ATP as the phosphate donor

Phosphorylation products of fructose and their cleavage Phosphorylation products of fructose and their cleavage. P = phosphate; ADP = adenosine diphosphate. Figure 12.2: Phosphorylation products of fructose and their cleavage. P = phosphate; ADP = adenosine diphosphate.

II. Fructose metabolism B. Cleavage of fructose 1-phosphate Fructose 1-phosphate is not phosphorylated to fructose 1,6- bisphosphate as is fructose 6-phosphate but is cleaved by aldolase B (also called fructose 1-phosphate aldolase) to dihydroxyacetone phosphate (DHAP) and glyceraldehyde. Humans express three aldolases, A, B and C, the products of three different genes. Aldolase A (found in most tissues), aldolase B (in liver, kidney, and small intestine), and aldolase C (in brain) All cleave fructose 1,6-bisphosphate produced during glycolysis to DHAP and glyceraldehyde 3-phosphate, but only aldolase B cleaves fructose 1-phosphate. DHAP can directly enter glycolysis or gluconeogenesis, whereas glyceraldehyde can be metabolized by a number of pathways, as illustrated in Figure 12.3

Summary of fructose metabolism Summary of fructose metabolism. P = phosphate; Pi = inorganic phosphate; NAD(H) = nicotinamide adenine dinucleotide; ADP = adenosine diphosphate. Figure 12.3: Summary of fructose metabolism. P = phosphate; Pi = inorganic phosphate; NAD(H) = nicotinamide adenine dinucleotide; ADP = adenosine diphosphate.

II. Fructose metabolism C. Kinetics of fructose metabolism The rate of fructose metabolism is more rapid than that of glucose because the trioses formed from fructose 1-phosphate bypass phosphofructokinase-1, the major rate-limiting step in glycolysis

II. Fructose metabolism D. Disorders of fructose metabolism A deficiency of one of the key enzymes required for the entry of fructose into metabolic pathways can result in either: a benign condition as a result of fructokinase deficiency (essential fructosuria) Or a severe disturbance of liver and kidney metabolism as a result of aldolase B deficiency (hereditary fructose intolerance [HFI]) (Figure 12.3) The first symptoms of HFI appear when a baby is weaned from milk and begins to be fed food containing sucrose or fructose Diagnosis of HFI can be made on the basis of fructose in the urine, enzyme assay using liver cells, or by DNA-based testing Aldolase B deficiency is part of the newborn screening panel. With HFI, sucrose, as well as fructose, must be removed from the diet to prevent liver failure and possible death. Individuals with HFI display an aversion to sweets and, consequently, have an absence of dental caries

Note: There is little mannose in dietary carbohydrates. II. Fructose metabolism E. Conversion of mannose to fructose 6-phosphate Mannose, the C-2 epimer of glucose, is an important component of glycoproteins. Hexokinase phosphorylates mannose, producing mannose 6- phosphate, which, in turn, is reversibly isomerized to fructose 6-phosphate by phosphomannose isomerase. Note: There is little mannose in dietary carbohydrates. Most intracellular mannose is synthesized from fructose or is preexisting mannose produced by the degradation of structural carbohydrates

II. Fructose metabolism F II. Fructose metabolism F. Conversion of glucose to fructose via sorbitol Most sugars are rapidly phosphorylated following their entry into cells. Therefore, they are trapped within the cells, because organic phosphates cannot freely cross membranes without specific transporters. An alternate mechanism for metabolizing a monosaccharide is to convert it to a polyol (sugar alcohol) by the reduction of an aldehyde group, thereby producing an additional hydroxyl group.

Synthesis of sorbitol: II. Fructose metabolism F. Conversion of glucose to fructose via sorbitol Synthesis of sorbitol: Aldose reductase reduces glucose, producing sorbitol (glucitol; Figure 12.4). This enzyme is found in many tissues, including the lens, retina, Schwann cells of peripheral nerves, liver, kidney, placenta, red blood cells, and cells of the ovaries and seminal vesicles. In cells of the liver, ovaries, and seminal vesicles, there is a second enzyme, sorbitol dehydrogenase, which can oxidize the sorbitol to produce fructose (see Figure 12.4). The two reaction pathway from glucose to fructose in the seminal vesicles benefits sperm cells, which use fructose as a major carbohydrate energy source. The pathway from sorbitol to fructose in the liver provides a mechanism by which any available sorbitol is converted into a substrate that can enter glycolysis or gluconeogenesis. hat produce the myelin sheath around neuronal axons

The seminal vesicles secrete a significant proportion of the fluid that ultimately becomes semen Figure 12.4: Sorbitol metabolism. NAD(H) = nicotinamide adenine dinucleotide; NADP(H) = nicotinamide adenine dinucleotide phosphate

Effect of hyperglycemia on sorbitol metabolism: II. Fructose metabolism F. Conversion of glucose to fructose via sorbitol Effect of hyperglycemia on sorbitol metabolism: Because insulin is not required for the entry of glucose into the cells listed in the previous paragraph, large amounts of glucose may enter these cells during times of hyperglycemia (for example, in uncontrolled diabetes). Elevated intracellular glucose concentrations and an adequate supply of reduced nicotinamide adenine dinucleotide phosphate (NADPH) cause aldose reductase to produce a significant increase in the amount of sorbitol, which cannot pass efficiently through cell membranes and, in turn, remains trapped inside the cell (Figure 12.4). This is exacerbated when sorbitol dehydrogenase is low or absent (for example, in retina, lens, kidney, and nerve cells). As a result, sorbitol accumulates in these cells, causing strong osmotic effects and, therefore, cell swelling as a result of water retention. Some of the pathologic alterations associated with diabetes can be attributed, in part, to this phenomenon, including cataract formation, peripheral neuropathy, and microvascular problems leading to nephropathy and retinopathy.

III. GALACTOSE METABOLISM The major dietary source of galactose is lactose (galactosyl β- 1,4-glucose) obtained from milk and milk products. Some galactose can also be obtained by lysosomal degradation of complex carbohydrates, such as glycoproteins and glycolipids, which are important membrane components. Like fructose, the transport of galactose into cells is not insulin dependent.

III. GALACTOSE METABOLISM A. Phosphorylation of galactose Like fructose, galactose must be phosphorylated before it can be further metabolized. Most tissues have a specific enzyme for this purpose, galactokinase, which produces galactose 1-phosphate (Figure 12.5). As with other kinases , ATP is the phosphate donor

Figure 12. 5: Metabolism of galactose Figure 12.5: Metabolism of galactose. UDP = uridine diphosphate; UTP = uridine triphosphate; P = phosphate; PPi = pyrophosphate; NADP(H) = nicotinamide adenine dinucleotide phosphate; ADP = adenosine diphosphate.

III. GALACTOSE METABOLISM B. Formation of uridine diphosphate-galactose Galactose 1-phosphate cannot enter the glycolytic pathway unless it is first converted to uridine diphosphate (UDP)- galactose (Figure12.6). This occurs in an exchange reaction, in which UDP-glucose reacts with galactose 1-phosphate, producing UDP-galactose and glucose 1-phosphate (see Figure 12.5). The enzyme that catalyzes this reaction is galactose 1- phosphate uridylyltransferase (GALT).

Figure 12.6: Structure of UDP-galactose. UDP = uridine diphosphate.

III. GALACTOSE METABOLISM C III. GALACTOSE METABOLISM C. Use of uridine diphosphate-galactose as a carbon source for glycolysis or gluconeogenesis For UDP-galactose to enter the mainstream of glucose metabolism, it must first be converted to its C-4 epimer, UDP- glucose, by UDP- hexose 4-epimerase. This “new” UDP-glucose (produced from the original UDP- galactose) can then participate in many biosynthetic reactions as well as being used in the GALT reaction described above (Figure 12.5)

III. GALACTOSE METABOLISM D III. GALACTOSE METABOLISM D. Role of uridine diphosphate-galactose in biosynthetic reactions UDP -galactose can serve as the donor of galactose units in a number of synthetic pathways, including synthesis of lactose, glycoproteins, glycolipids, and glycosaminoglycans. Note: If galactose is not provided by the diet (for example, when it cannot be released from lactose as a result of a lack of β-galactosidase in people who are lactose intolerant), all tissue requirements for UDP-galactose can be met by the action of UDP-hexose 4-epimerase on UDP-glucose, which is efficiently produced from glucose 1-phosphate (Figure 12.5). long unbranched polysaccharides consisting of a repeating disaccharide unit. The repeating unit (except for keratan) consists of an amino sugar (N-acetylglucosamine or N-acetylgalactosamine) along with a uronic sugar (glucuronic acid or iduronic acid) or galactose.

III. GALACTOSE METABOLISM E. Disorders of galactose metabolism Galactose 1-phosphate uridylyltransferase (GALT) is deficient in individuals with classic galactosemia (Figure 12.5). In this disorder, galactose 1-phosphate and, therefore, galactose accumulate. Physiologic consequences are similar to those found in hereditary fructose intolerance, but a broader spectrum of tissues is affected. The accumulated galactose is shunted into side pathways such as that of galactitol production This reaction is catalyzed by aldose reductase, the same enzyme that converts glucose to sorbitol. Treatment requires removal of galactose and lactose from the diet. GALT deficiency is part of the newborn screening panel. Note: A deficiency in galactokinase results in a less severe disorder of galactosemia metabolism, although cataracts are common (Figure 12.5).

Figure 12. 5: Metabolism of galactose Figure 12.5: Metabolism of galactose. UDP = uridine diphosphate; UTP = uridine triphosphate; P = phosphate; PPi = pyrophosphate; NADP(H) = nicotinamide adenine dinucleotide phosphate; ADP = adenosine diphosphate.

IV. LACTOSE SYNTHESIS Lactose is a disaccharide that consists of a molecule of β-galactose attached by a β(1→4) linkage to glucose. Therefore, lactose is galactosyl β(1→4)-glucose. Lactose, known as “milk sugar,” is made by lactating (milk-producing) mammary glands. Therefore, milk and other dairy products are the dietary sources of lactose Lactose is synthesized in the Golgi by lactose synthase (UDP-galactose:glucose galactosyltransferase), which transfers galactose from UDP-galactose to glucose, releasing UDP (Figure 12.7). This enzyme is composed of two proteins, A and B.

Figure 12.7: Lactose synthesis. UDP = uridine diphosphate.

IV. LACTOSE SYNTHESIS Protein B is found only in lactating mammary glands. It is α-lactalbumin, and its synthesis is stimulated by the peptide hormone prolactin. Protein B forms a complex with the enzyme, protein A, changing the specificity of that transferase so that lactose, rather than N-acetyllactosamine, is produced (Figure 12.7).

V. CHAPTER SUMMARY The major source of fructose is sucrose, which when cleaved releases equimolar amounts of fructose & glucose. Entry of Fructose into cells is insulin-independent. Fructose is 1st phosphorylated to F-1-P by fructokinase, & then cleaved by aldolase B to DHAP & glyceraldehyde. These enzymes are found in liver, kidney, & small intestinal mucosa. A deficiency of fructokinase causes a benign condition (fructosuria), but a deficiency of aldolase B causes hereditary fructose intolerance, in which severe hypoglycemia & liver failure lead to death if the amount of Fructose (and therefore, sucrose) in the diet is not severely limited. Mannose, an important component of glycoproteins, is phosphorylated by hexokinase to mannose-6-P, which is reversibly isomerized to F-6-P by phosphomannose isomerase

V. CHAPTER SUMMARY Glucose can be reduced to sorbitol (glucitol) by aldose reductase in many tissues, including the lens, retina, Schwann cells, liver, kidney, ovaries, & seminal vesicles. In cells of liver, ovaries, & seminal vesicles, a 2nd enzyme, sorbitol dehydrogenase, can oxidize sorbitol to produce fructose. Hyperglycemia results in accumulation of sorbitol in those cells lacking sorbitol dehydrogenase. The resulting osmotic events cause cell swelling, & may contribute to the cataract formation, peripheral neuropathy, nephropathy, & retinopathy seen in diabetes The major dietary source of galactose is lactose. The entry of galactose into cells is not insulin-dependent. Galactose is 1st phosphorylated by galactokinase which produces galactose 1-P. this compound is converted to UDP-galactose by galactose 1-phosphate uridyltransferase, with the nucleotide supplied by UDP-glucose.

V. CHAPTER SUMMARY A deficiency of this enzyme causes classic galactosemia. Galactose 1-P accumulates, & excess galactose is converted to galactitol by aldose reductase. This causes liver damage, severe mental retardation, & cataracts. Treatment requires removal of galactose (& therefore, lactose) from the diet. In order for UDP-galactose to enter the mainstream of glucose metabolism, it must be converted to UDP-glucose by UDP- hexose-4-epimerase. This enzyme can also be used to produce UDP-galactose from UDP-glucose when the former is required for synthesis of structural Carbohydrates Lactose is a disaccharide that consists of galactose & glucose. Milk & other dairy products are the dietary sources of lactose.

V. CHAPTER SUMMARY Lactose is synthesized by lactose synthase from UDP- galactose & glucose in the lactating mammary gland. The enzyme has two subunits, protein A (which is a galatosyl transferase) found in most cells where it synthesizes N- acetyllactosamine) & protein B (α-lactalbumin, which is found only in the lactating mammary glands, & whose synthesis is stimulated by the peptide hormone, prolactin). When both subunits are present, the transferase produces lactose